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Introduction to Thunderstorms

It is estimated that there are as many as 40,000 thunderstorm occurrences each day world-wide. This translates into an astounding 14.6 million occurrences annually! The United States certainly experiences its share of thunderstorm occurrences..

It is in this part of the country that warm, moist air from the Gulf of Mexico and Atlantic Ocean (which we will see later are necessary ingredients for thunderstorm development) is most readily available to fuel thunderstorm development.

Ingredients for a Thunderstorm

All thunderstorms require three ingredients for their formation:

  • Moisture,
  • Instability, and
  • a lifting mechanism.

Sources of moisture

Typical source of moisture for thunderstorms are the oceans. However, water temperature plays a large role in how much moisture is added to the atmosphere.

Recall from the Ocean Section that warm ocean currents occur along east coasts of continents with cool ocean currents occur along west coasts. Evaporation is higher in warm ocean currents and therefore puts more moisture into the atmosphere as compared to the cold ocean currents at the same latitude.

Therefore, in the southeastern U.S. the warm water from the two moisture sources (Atlantic Ocean and Gulf of Mexico) helps explain why there is much more precipitation in that region as compared to the same latitude in Southern California.

Instability

Air is considered unstable if it continues to rise when given a nudge upward (or continues to sink if given a nudge downward). An unstable air mass is characterized by warm moist air near the surface and cold dry air aloft.

In these situations, if a bubble or parcel of air is forced upward it will continue to rise on its own. As this parcel rises it cools and some of the water vapor will condense forming the familiar tall cumulonimbus cloud that is the thunderstorm.

Sources of Lift (upward)

Typically, for a thunderstorm to develop, there needs to be a mechanism which initiates the upward motion, something that will give the air a nudge upward. This upward nudge is a direct result of air density.

Some of the sun's heating of the earth's surface is transferred to the air which, in turn, creates different air densities. The propensity for air to rise increases with decreasing density. This is difference in air density is the main source for lift and is accomplished by several methods.

Differential Heating

The sun's heating of the earth's surface is not uniform. For example, a grassy field will heat at a slower rate than a paved street. A body of water will heat slower than the nearby landmass.

This will create two adjacent areas where the air is of different densities. The cooler air sinks, pulled toward the surface by gravity, forcing up the warmer, less dense air, creating thermals.

Fronts, Drylines and Outflow Boundaries

Fronts are the boundary between two air masses of different temperatures and therefore different air densities. The colder, more dense air behind the front lift warmer, less dense air abruptly. If the air is moist thunderstorms will often form along the cold front.

Drylines are the boundary between two air masses of different moisture content and divides warm, moist air from hot, dry air. Moist air is less dense than dry air. Drylines therefore act similarly to fronts in that the moist, less dense air is lifted up and over the drier, more dense air.

 

The air temperature behind a dryline is often much higher due to the lack of moisture. That alone will make the air less dense but the moist air ahead of the dryline has an even lower density making it more buoyant. The end result is air lifted along the dryline forming thunderstorms. This is common over the plains in the spring and early summer.

 

Outflow boundaries are a result of the rush of cold air as a thunderstorm moves overhead. The rain-cooled, more dense, air acts as a "mini cold front", called an outflow boundary. Like fronts, this boundary lifts warm moist air and can cause new thunderstorms to form.

Terrain
As air encounters a mountain it is forced up because of the terrain. Upslope thunderstorms are common in the Rocky Mountain west during the summer.

Life Cycle of a Thunderstorm

The building block of all thunderstorms is the thunderstorm cell. The thunderstorm cell has a distinct life-cycle that lasts about 30 minutes.

The Towering Cumulus Stage

A cumulus cloud begins to grow vertically, perhaps to a height of 20,000 feet (6 km). Air within the cloud is dominated by updraft with some turbulent eddies around the edges.

 

The storm has considerable depth, often reaching 40,000 to 60,000 feet (12 to 18 km). Strong updrafts and downdrafts coexist. This is the most dangerous stage when tornadoes, large hail, damaging winds, and flash flooding may occur.

 

The downdraft cuts off the updraft. The storm no longer has a supply of warm moist air to maintain itself and therefore it dissipates. Light rain and weak outflow winds may remain for a while during this stage, before leaving behind just a remnant anvil top.

Learning Lesson: How much water is in that cloud?

 

How much water is in that cloud?

Overview

The updrafts in thunderstorms can be extremely strong. The stronger the updraft, the more weight of rain and hail that can be supported. This experiment will show that cotton balls, like clouds, hold a tremendous amount of water. In nature, once the weight of the water is more than can be supported by the updraft, the water falls as rain. Using cotton balls the students will learn of the high water capacity in clouds.

Procedure

  1. Divide the students into pairs. Distribute one cotton ball, one eyedropper, and one cup of water to each pair.
  2. Have one student hold the cotton ball and one the eyedropper. (For best results, the student with the cotton ball should hold it over the cup of water by pinching a small portion of the cotton ball between his/her thumb and index finger.)
  3. Explain the purpose is to put as many drops of water into the cotton ball as possible. The cotton ball will be full (saturated) when water begins to drip from the bottom.
  4. Before they begin however, ask for estimates of the number of drops they think it will take to saturate the cotton ball. Write their estimates on the chalk board.
  5. Have the students count every drop and stop counting when water begins to drop from the bottom of the cotton ball. During the experiment the students should not leave the eyedropper in one position but move it around to ensure they have as much water as possible in the cotton ball.
  6. Record their results on the chalk board.

Discussion

Typically, the original estimates will be low (10-30 drops). Often, the first estimate sets the general area around where most of the remaining estimates will occur. However, some students will throw out a "wild" answer (100, 150, etc.).

The results often surprise the students when they discover the cotton ball holds much more water than they thought. When done properly, using the smallest drops possible and completely saturating the cotton ball, more than 200 drops of water will be contained within the cotton ball.

Since the results can vary widely, ask the students which answer was the "correct" one. The correct answer, of course, is that ALL results are correct. Ask the students why the results vary. The three main reasons are...

  1. Drop sizes were different,
  2. Cotton balls are not exactly alike, and
  3. Some students did not move the eyedropper around to saturate the cotton ball.

This is what also occurs in nature. Drop sizes are different in thunderstorms based partly upon the strength of the updraft. Although the processes involved in making a thunderstorm are similar, no two clouds are exactly the same. Also, the amount of moisture in the clouds varies.

For example, thunderstorms occasionally develop over forest fires. While they may look like rain producers, the moisture is limited so much that often these clouds produce little, if any, rain. More times than not, all they do is start more fires due to lightning.

When too much rain falls too quickly, flash flooding occurs. The National Weather Service issues Flash Flood Warnings to alert you to the dangers of the rapidly rising waters.

Live weatherwise

  • When a flash flood warning is issued for your area or the moment you first realize that a flash flood is imminent, act quickly to save yourself. You may have only seconds.
  • Get out of areas subject to flooding. This includes dips, low spots, canyons, washes, etc.
  • Avoid already flooded and high velocity flow areas. Do not try to cross a flowing stream on foot where water is above your knees.
  • If driving, know the depth of the water in a dip before crossing. The road bed may not be intact under the water.
  • If the vehicle stalls, abandon it immediately and seek higher ground. Rapidly rising water may engulf the vehicle and its occupants and sweep them away.
  • If you come to an area that is covered with water, you will not know the depth of the water or the condition of the ground under the water. This is especially true at night, when your vision is more limited. Play it smart, play it safe. Whether driving or walking, any time you come to a flooded road, TURN AROUND, DON'T DROWN!

Types of Thunderstorms

Ordinary Cell

As the name implies, there is only one cell with this type of thunderstorm. Also called a "pulse" thunderstorm, the ordinary cell consist of a one time updraft and one time downdraft. In the towering cumulus stage, the rising updraft will suspend growing raindrops until the point where the weight of the water is greater than what can be supported.

At which point, drag of air from the falling drops begins to diminish the updraft and, in turn, allow more raindrops to fall. In effect, the falling rain turns the updraft into a downdraft. With rain falling back into the updraft, the supply of rising moist air is cut-off and the life of the single cell thunderstorm is short.

They are short lived and while hail and gusty wind can develop, these occurrences are typically not severe. However, if atmospheric conditions are right and the ordinary cell is strong enough, there is the potential for more than one cell to form and can include microburst winds (usually less than 70 mph/112 km/h) and weak tornadoes.

Multi-cell Cluster

Although there are times when a thunderstorm consists of just one ordinary cell that transitions through its life cycle and dissipates without additional new cell formation, thunderstorms often form in clusters with numerous cells in various stages of development, merging together.

While each individual thunderstorm cell, in a multi-cell cluster, behaves as a single cell, the prevailing atmospheric conditions are such that as the first cell matures, it is carried downstream by the upper level winds with a new cell forming upwind of the previous cell to take its place.

The speed at which the entire cluster of thunderstorms move downstream can make a huge difference in the amount of rain any one place receives. There are many times where the individual cell moves downstream but addition cells forming on the upwind side of the cluster and move directly over the path of the previous cell.

The term for this type of pattern when viewed by radar is "training echoes". Training thunderstorms produce tremendous rainfall over relatively small areas leading to flash flooding.

Sometimes the atmospheric condition are such that new cell growth is quite vigorous. They form so fast that each new cell develops further and further upstream giving the appearance of the thunderstorm cluster is stationary or is moving backwards, against the upper level wind.

Tremendous rainfall amounts can be produced over very small areas by back-building thunderstorms. In 1972, 15" (380 mm) fell in six hours over parts of Rapid City, SD due to back-building storms

Multi-cell Line (Squall Line) 

Sometimes thunderstorms will form in a line which can extend laterally for hundreds of miles. These "squall lines" can persist for many hours and produce damaging winds and hail.

Updrafts, and therefore new cells, continually re-form at leading edge of system with rain and hail following behind. Individual thunderstorm updrafts and downdrafts along the line can become quite strong, resulting in episodes of large hail and strong outflow winds which move rapidly ahead of system.

While tornadoes occasionally form on the leading edge of squall lines they primarily produce "straight-line" wind damage.

This is damage as a result of the shear force of the down draft from a thunderstorm spreading horizontally as it reaches the earth's surface.

Long-lived strong squall lines after called "derechos" (Spanish for 'straight'). Derechos can travel many hundreds of miles and can produce considerable widespread damage from wind and hail. Learn more about derechos.

Often along the leading edge of the squall line is a low hanging arc of cloudiness called the shelf cloud.

This appearance is a result of the rain cooled air spreading out from underneath the squall line acts as a mini cold front. The cooler dense air forces the warmer, less dense air, up. The rapidly rising air cools and condenses creating the shelf cloud.

Supercell Thunderstorms

Supercell thunderstorms are a special kind of single cell thunderstorm that can persist for many hours.

They are responsible for nearly all of the significant tornadoes produced in the U.S. and for most of the hailstones larger than golf ball size. Supercells are also known to produce extreme winds and flash flooding.

Supercells are highly organized storms characterized by updrafts that can attain speeds over 100 mph (160 km/h) and are able to produce giant hail with strong or even violent tornadoes. Downdrafts produced by these storms can produce downbursts/outflow winds in excess of 100 mph (160 km/h), posing a high threat to life and property.

The most ideal conditions for supercells occur when the winds are veering or turning clockwise with height. For example, in a veering wind situation the winds may be from the south at the surface and from the west at 15,000 feet (4,500 meters). This change in wind speed and direction produces storm-scale rotation, meaning the entire cloud rotates, which may give a striated or corkscrew appearance to the storm's updraft.

Dynamically, all supercells are fundamentally similar. However, they often appear quite different visually from one storm to another depending on the amount of precipitation accompanying the storm and whether precipitation falls adjacent to, or is removed from, the storm"s updraft.

Based on their visual appearance, supercells are often divided into three groups;

  • Rear Flank Supercell - Low precipitation (LP),
  • Classic (CL), or
  • Front Flank Supercell - High precipitation (HP).

In low precipitation supercells the updraft is on the rear flank of the storm providing a barber pole or corkscrew appearance to the cloud. Precipitation is sparse or well removed from the updraft and/or often is transparent.

Also, large hail is often difficult to discern visually. With the lack of precipitation no "hook" seen on Doppler radar.

The majority of supercells fall in the "classic" category. The classic supercell will have a large, flat updraft base with striations or banding seen around the periphery of the updraft. Heavy precipitation falls adjacent to the updraft with large hail likely and has the potential for strong, long-lived tornadoes.

High precipitation supercells will have...

  • the updraft on the front flank of the storm
  • precipitation that almost surrounds updraft at times
  • the likelihood of a wall cloud (but it may be obscured by the heavy precipitation)
  • tornadoes that are potentially wrapped by rain (and therefore difficult to see), and
  • extremely heavy precipitation with flash flooding.

Beneath the supercell, the rotation of the storm is often visible as well. The is visible as a lowered, rotating cloud, called a Wall Cloud, forms below the rain-free base and/or below the main storm tower updraft. Wall clouds are often located on the trailing flank of the precipitation.

The wall cloud is sometimes a precursor to a tornado. If a tornado were to form, it would usually do so within the wall cloud.

With some storms, such as high precipitation supercells, the wall cloud area may be obscured by precipitation or located on the leading flank of the storm.

Wall clouds associated with potentially severe storms can:

  • Be a persistent feature that lasts for 10 minutes or more
  • Have visible rotation
  • Appear with lots of rising or sinking motion within and around t

Thunderstorm Hazards - Hail

Hail is precipitation that is formed when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere. Hail can damage aircraft, homes and cars, and can be deadly to livestock and people. One of the people killed during the March 28, 2000 tornado in Fort Worth was killed when struck by grapefruit-size hail.

While Florida has the most thunderstorms, New Mexico, Colorado, and Wyoming usually have the most hail storms. Why? The freezing level in the Florida thunderstorms is so high, the hail often melts before reaching the ground.

Hailstones grow by collision with supercooled water drops. (Supercooled drops are liquid drops surrounded by air that is below freezing which is a common occurrence in thunderstorms.) There are two methods by which the hailstone grows, wet growth and dry growth, and which produce the "layered look" of hail.

In wet growth, the hailstone nucleus (a tiny piece of ice) is in a region where the air temperature is below freezing, but not super cold. Upon colliding with a supercooled drop the water does not immediately freeze around the nucleus.

Instead liquid water spreads across tumbling hailstones and slowly freezes. Since the process is slow, air bubbles can escape resulting in a layer of clear ice.

With dry growth, the air temperature is well below freezing and the water droplet immediately freezes as it collides with the nucleus. The air bubbles are "frozen" in place, leaving cloudy ice.

Strong updrafts create a rain-free area in supercell thunderstorms. Meteorologists call this area a WER which stands for "weak echo region".

This term, WER, comes from an apparently rain free region of a thunderstorm which is bounded on one side AND above by very intense precipitation indicted by a strong echo on radar.

This rain-free region is produced by the updraft and is what suspends rain and hail aloft producing the strong radar echo.

  1. The hail nucleus, buoyed by the updraft is carried aloft by the updraft and begins to grow in size as it collides with supercooled raindrops and other small pieces of hail.
  2. Sometimes the hailstone is blown out of the main updraft and begins to fall to the earth.
  3. If the updraft is strong enough it will move the hailstone back into the cloud where it once again collides with water and hail and grows. This process may be repeated several times.
  4. In all cases, when the hailstone can no longer be supported by the updraft it falls to the earth. The stronger the updraft, the larger the hailstones that can be produced by the thunderstorm.

 

Multi-cell thunderstorms produce many hail storms but usually not the largest hailstones. The reason is that the mature stage in the life cycle of the multi-cell is relatively short which decreases the time for growth.

 

However, the sustained updraft in supercell thunderstorms support large hail formation by repeatedly lifting the hailstones into the very cold air at the top of the thunderstorm cloud.

The stronger the updraft the larger the hailstone can grow. In all cases, the hail falls when the thunderstorm's updraft can no longer support the weight of the ice.

How strong does the updraft need to be for the various sizes of hail? The table provides the approximate speed for each size.

Updrafts in Action

Overview

Rain and hail will be suspended by the updraft inside a thunderstorm until the weight of the hail and water can no longer be supported. Usually, the stronger the updraft in a thunderstorm, the more intense the storm and the larger the size of hail that can be produced. Suspending a ping pong ball in the stream of air supplied by a hair dryer will demonstrates how hail is supported in thunderstorms.

Procedure

Point the nozzle of the hair drier up and turn the power on HI. Place the ping pong ball in the stream of air. The ping pong ball will be suspended by the air. Slowly tilt the hair dryer (to the left or right) until the ball falls.

Repeat the demonstration but add a second ping pong ball. Depending upon power of the hair dryer, both ping pong balls will be suspended. Occasionally, the balls will swap their order as they bounce around in the air stream.

Discussion

The ping pong ball remains in the stream of air due to lower pressure created around the surface of the ball. The effect is called the Bernoulli Principle named after Daniel Bernoulli, an eighteenth-century Swiss scientist, who discovered that as the velocity of a fluid increases, its pressure decreases.

Bernoulli's principle can be seen most easily through the use of a venturi tube (see figure left). A venturi tube is simply a tube which is narrower in the middle than it is at the ends.

When the fluid passing through the tube reaches the narrow part, it speeds up. According to Bernoulli's principle, it then should exert less pressure.

This low pressure effect also can be seen around the ping pong ball albeit in a different way. Instead of a narrowing in the center as in the venturi tube, the narrowing takes place around the perimeter of the ping pong ball (see figure right). In effect, there is an area of low pressure immediately adjacent to the ball.

The pressure is higher in the air outside of the stream created by the hair drier. The result is the ping pong ball bouncing from side-to-side as it reaches the edge of the flowing air and is pushed back into the region of low pressure.

You can now repeat the experiment and this time have the students notice the back-and-fourth oscillation of the ball as it tries to fall out of the stream but is push inward. Another way of seeing this is inward push.

Updrafts are responsible for the thunderstorms we experience. Generally the stronger the updraft, the stronger the thunderstorm.

While we cannot predict if you will experience a thunderstorm on any particular day, we can know the area where thunderstorms are possible. If atmospheric conditions are such that the thunderstorms may become severe, the National Weather Service will issue a SEVERE THUNDERSTORM or TORNADO WATCH.

A WATCH, issued by the Storm Prediction Center in Norman, OK, is used when the risk of a severe thunderstorms and/or tornadoes has increased significantly, but its occurrence, location, and/or timing is still uncertain.

It is intended to provide enough lead time so that those who need to set their plans in motion can do so. These watches are issued by county.

The National Weather Service defines a severe thunderstorm as one having wind speed 58 mph (93 km/h) or greater, and/or hail size of 1" (2.5 cm) or larger.

Each watch is numbered sequentially, beginning with number 1 for the first issuance of each calendar year and contains...

  • hail size (in inches),
  • turbulence (for aviation community),
  • surface wind speed in knots,
  • maximum height of thunderstorm tops (in hundreds of feet),
  • estimated direction and speed of thunderstorm movement, and
  • a discussion of the meteorological reasoning that support the watch issuance and forecast for severe weather.

Live weatherwise

The most important safety rule is to known what is happening weather-wise so you will not be caught unaware in a hazardous situation. At the beginning of each day...

  1. Learn if you need to be aware of hazardous weather that might threaten you. You can do this by listening to the NOAA Weather Radio or check out the day's forecast at www.weather.gov.
  2. Check the Convective Outlooks to discover where thunderstorms are most likely to occur.
  3. Periodically during the day, recheck the forecast to learn of any updates or advisories.

If a WATCH is issued for your area, listen carefully to the message. This message will tell you the type of threat you can expected from severe thunderstorms. If hazardous weather approaches your location, seek sturdy shelter.

 

Sizing Up Hail

Overview

SKYWARN is a concept developed in the early 1970s that was intended to promote a cooperative effort between the National Weather Service and communities. The emphasis of the effort is often focused on the storm spotter, an individual who takes a position near their community and reports wind gusts, hail size, rainfall, and cloud formations that could signal a developing tornado.

Focusing on hail, based upon samples picked at random the student learn to estimate the size of hail.

Procedure

Choose a student to select balls from the box. Once the ball is selected have the student tell the class the number on the ball. The students should write that number on their paper.

Pass the ball, row by row, around the class allowing the students to hold it and estimate the ball's diameter in inches. Once the entire class has written their estimates, place it aside.

Repeat the procedure with the next ball chosen from the box. (you can save time by allowing several balls to be passed through the classroom at the same time.) Once the last estimate has been made, tell the students which number ball represented which size.

Discussion

Take a poll of the class asking their results of their estimates. For example, hold up a 1" ball and ask...

  • How many students had the correct estimate?
  • How many estimated the ball was greater than 1"? If so, by how much?
  • How many estimated the ball was less than 1"? If so, by how much?

Again holding up the 1" ball say the National Weather Service defines a severe thunderstorm as one containing hail size of 1" (2.5 cm) or larger (and/or any wind speed 58 mph (93km/h) or greater).

Live weatherwise

When the National Weather Service issues a severe thunderstorm warning it means a severe thunderstorm is occurring, is imminent, or has a high probability of occurring. The warning will contain...

  • County or counties affected by the severe weather event,
  • Warning expiration time,
  • Location and direction of storm movement,
  • Locations in the path of the storm, and
  • Additional information and/or call-to-action statement(s).

Remember, due to the nature of a thunderstorm's size, there may be a severe thunderstorm warning in effect for your county but you may experience mostly blue skies. Know where the storm is in relation to your location and which direction it is moving.

Just because a thunderstorm may not be severe that does not mean cause damage. A thunderstorm can be deadly due to lightning alone. If you can hear thunder, you are close enough to be struck by lightning. Seek shelter indoors immediately and remain indoors until 30 minutes after the last thunder is heard.

Thunderstorm Hazards - Damaging Wind

Damaging wind from thunderstorms is much more common than damage from tornadoes. In fact, many confuse damage produced by "straight-line" winds and often erroneously attribute it to tornadoes.

The source for damaging winds is well understood and it begins with the downdraft. As air rises, it will cool to the point of condensation where water vapor forms tiny water droplets, comprising the cumulus cloud we see.

Near the center of the updraft, the particles begin to collide and coalescence forming larger droplets. This continues until the rising air can no longer support the ever increasing size of water drops.

Once the rain drops begin to fall friction causes the rising air to begin to fall towards the surface itself. Also, some of the falling rain will evaporate. Through evaporation heat energy is removed from the atmosphere cooling the air associated with the precipitation.

As a result of the cooling, the density of the air increases causing it to sink toward the earth. The downdraft also signifies the end of the convection with the thunderstorm and its subsequent decrease.

When this dense rained-cooled air reaches the surface it spreads out horizontally with the leading edged of the cool air forming a gust front. The gust front marks the boundary of a sharp temperature decrease and increase in wind speed. The gust front can act as a point of lift for the development of new thunderstorm cells or cut off the supply of moist unstable air for older cells.

Downbursts are defined as strong winds produced by a downdraft over a horizontal area up to 6 miles (10 kilometers). Downbursts are further subdivided into microbursts and macrobursts.

Microbursts and Macrobursts

A microburst is a small downburst with an outflow less than 2 miles (4 kilometers) in horizontal diameter and last for only 2-5 minutes. Despite their small size, microbursts can produce destructive winds up to 168 mph (270 km/h). Also, they create hazardous conditions for pilots and have been responsible for several disasters. For example...

  1. As aircraft descend into the airport they follow an imagery line called the "glide slope" (solid light blue line) to the runway.
  2. Upon entering the microburst, the plane encounters a "headwind", an increase in wind speed over the aircraft. The stronger wind creates additional lift causing the plane to rise above the glide slope. To return the plane to the proper position, the pilot lowers the throttle to decrease the plane's speed thereby causing the plane to descend.
  3. As the plane flies through to the other side of the microburst, the wind direction shifts and is now a "tailwind" as it is from behind the aircraft. This decreases the wind over the wing reducing lift. The plane sinks below the glide slope.
  4. However, the "tailwind" remains strong and even with the pilot applying full throttle trying to increase lift again, there may be little, if any, room to recover from the rapid descent causing the plane to crash short of the runway.

Since the discovery of this effect in the early to mid 1980's, pilots are now trained to recognize this event and take appropriate actions to prevent accidents. Also, many airports are now equipped with equipment to detect microbursts and warn aircraft of their occurrences.

A macroburst is larger than a microburst with a horizontal extent more than 2 miles (4 km) in diameter. Also a macroburst is not quite a strong as a microburst but still can produce winds as high as 130 mph (210 km/h). Damaging winds generally last longer, from 5 to 20 minutes, and produce tornado-like damage up to an EF-3 scale.

In wet, humid environments, macrobursts and microbursts will be accompanied by intense rainfall at the ground. If the storm forms in a relatively dry environment, however, the rain may evaporate before it reaches the ground and these downbursts will be without precipitation, known as dry microbursts.

In the desert southwest, dust storms are a rather frequent occurrence due to downbursts. The city of Phoenix, AZ typically has 1-3 dust storms each summer due to the cooler dense air spreading out from thunderstorms.

On July 5, 2011, a massive dust storm resulted in widespread areas of zero or near zero visibility in Phoenix. The wind that produced this storm was generated by downbursts from thunderstorms with winds up to 70 mph (110 km/h).

Heat Bursts

Dry downbursts are responsible for a rare weather event called "Heat Bursts". Heat bursts usually occur at night, are associated with decaying thunderstorms, and are marked by gusty, and sometimes damaging, winds combined a sharp increase in temperature and a sharp decrease in dew point (humidity).

The process of creating a dry microburst begins higher in the atmosphere for heat bursts. A pocket of cool air aloft forms during the evaporation process as for any downburst. But as the precipitation falls it evaporates before reaching the ground. The cool dense air sinks by the pull of gravity but since there is no rain drops to absorb heat, the air then warms due to compression.

In fact, it can become quite hot and very dry. Temperatures generally rise 10 to 20 degrees in a few minutes and have been known to rise to over 120F (49C) and remain in place for several hours before returning to normal. One such heat burst occurred in Wichita, KS on June 9, 2011.

Derechos

If the atmospheric conditions are right, widespread and long-lived windstorms, associated with a band of rapidly moving showers or thunderstorms, can result. The word "derecho" is of Spanish origin, and means straight ahead. A derecho is made up of a "family of downburst clusters" and by definition must be at least 240 miles in length. Learn more about derechos.

Thunderstorm Hazards - Tornadoes

A tornado is a violently rotating (usually counterclockwise in the northern hemisphere) column of air descending from a thunderstorm and in contact with the ground. Although tornadoes are usually brief, lasting only a few minutes, they can sometimes last for more than an hour and travel several miles causing considerable damage. Tornadoes are the #3 most hazardous aspect of thunderstorms (#2 is lightning).

In a typical year around 1200 tornadoes will strike the United States. The peak of the tornado season is April through June with more tornadoes striking the central United States than any other place in the world. This area of the country has been nicknamed "tornado alley."

Wind Shear

Most tornadoes are spawned from supercell thunderstorms. Supercell thunderstorms are characterized by a persistent rotating updraft and form in environments of strong vertical wind shear. Wind shear is the change in wind speed and/or direction with height.

Directional wind shear is the change in wind direction with height. In the image below, the view is looking north. The wind near the surface is blowing from the southeast to the northwest.

As the elevation increases the direction veers (changes direction in a clock-wise motion) becoming south, then southwest, and finally, west.

Speed shear is the change in wind speed with height. In the illustration below, the wind is increasing with height. This tends to create a rolling affect to the atmosphere and is believed to be a key component in the formation of mesocyclones which can lead to tornadoes.

Strong vertical shear is the combination of a veering directional shear and strong speed shear and is the condition that is most supportive of supercells.

Directional Shear
Wind direction changes with height
Speed Shear
Wind speed changes with height.

The updraft lifts the rotating column of air created by the speed shear. This provides two different rotations to the supercell; cyclonic or counter clockwise rotation and an anti-cyclonic of clockwise rotation.

The directional shear amplifies the cyclonic rotation and diminishes the anti-cyclonic rotation (the rotation on the right side of the of the updraft in the illustration - located right).

All that remains is the cyclonic rotation called a mesocyclone. By definition a supercell is a rotating thunderstorm.

When viewed from the top (left image), the counter-clockwise rotation of the mesocyclone gives the supercell its classic "hook" appearance when seen by radar. As the air rises in the storm, it becomes stretched and more narrow with time.

The exact processes for the formation of a funnel are not known yet. Recent theories suggest that once a mesocyclone is underway, tornado development is related to the temperature differences across the edge of downdraft air wrapping around the mesocyclone.

However, mathematical modeling studies of tornado formation also indicate that it can happen without such temperature patterns; and in fact, very little temperature variation was observed near some of the most destructive tornadoes in history on May 3, 1999 in Oklahoma.

The Tornado Itself

The funnel cloud of a tornado consists of moist air. As the funnel descends the water vapor within it condenses into liquid droplets. The liquid droplets are identical to cloud droplets yet are not considered part of the cloud since they form within the funnel.

The descending funnel is made visible because of the water droplets. The funnel takes on the color of the cloud droplets, which is white.

Due to the air movement, dust and debris on the ground will begin rotating, often becoming several feet high and hundreds of yards wide.

After the funnel touches the ground and becomes a tornado, the color of the funnel will change. The color often depends upon the type of dirt and debris is moves over (red dirt produces a red tornado, black dirt a black tornado, etc.).

Tornadoes can be thin and rope-like.

Tornadoes can last from several seconds to more than an hour but most last less than 10 minutes. The size and/or shape of a tornado is no measure of its strength.

Occasionally, small tornadoes do major damage and some very large tornadoes, over a quarter-mile wide, have produced only light damage.

The tornado will gradually lose intensity. The condensation funnel decreases in size, the tornado becomes tilted with height, and it takes on a contorted, rope-like appearance before it completely dissipates. Learn more about tornadoes from the NWS Storm Prediction Center's FAQ.

The Enhanced F-Scale

 

EF-Scale wind speeds
EF
scale
Class Wind speed Description
mph km/h
EF0 weak 65-85 105-137 Gale
EF1 weak 86-110 138-177 Moderate
EF2 strong 111-135 178-217 Significant
EF3 strong 136-165 218-266 Severe
EF4 violent 166-200 267-322 Devastating
EF5 violent > 200 > 322 Incredible
The Fujita (F) Scale was originally developed by Dr. Tetsuya Theodore Fujita to estimate tornado wind speeds based on damage left behind by a tornado. An Enhanced Fujita (EF) Scale, developed by a forum of nationally renowned meteorologists and wind engineers, makes improvements to the original F scale. This EF Scale has replaced the original F scale, which has been used to assign tornado ratings since 1971.

The original F scale had limitations, such as a lack of damage indicators, no account for construction quality and variability, and no definitive correlation between damage and wind speed. These limitations may have led to some tornadoes being rated in an inconsistent manner and, in some cases, an overestimate of tornado wind speeds.

The EF Scale takes into account more variables than the original F Scale did when assigning a wind speed rating to a tornado. The EF Scale incorporates 28 damage indicators (DIs) such as building type, structures, and trees. For each damage indicator, there are 8 degrees of damage (DOD) ranging from the beginning of visible damage to complete destruction of the damage indicator. The original F Scale did not take these details into account.

For example, with the EF Scale, an F3 tornado will have estimated wind speeds between 136 and 165 mph (218 and 266 km/h), whereas with the original F Scale, an F3 tornado has winds estimated between 162-209 mph (254-332 km/h).

The wind speeds necessary to cause "F3" damage are not as high as once thought and this may have led to an overestimation of some tornado wind speeds.

There is still some uncertainty as to the upper limits of the strongest tornadoes so F5 ratings do not have a wind speed range. Wind speed estimations for F5 tornadoes are left open ended and assigned wind speeds greater than 200 mph (322 km/h).

This video is from January 7, 2008 when a tornado crossed the Chicago and Northwestern Railroad and blew 12 moving railroad cars off the tracks near the town of Lawrence, Il.

The train enters the rain from the parent thunderstorm around 0:33. The wind begins to pick up around 0:53.

At 1:04 a gray blur can be seen, most likely the tornado striking the train several cars back with domino effect pulling the last car off the track at 1:09. The remaining cars on the track slam into the front part of the train.

Thunderstorm Hazards - Flash Floods

Except for heat related fatalities, more deaths occur from flooding than any other hazard. Why? Most people fail to realize the power of water. For example, six inches of fast-moving flood water can knock you off your feet.

While the number of fatalities can vary dramatically with weather conditions from year to year, the national 30-year average for flood deaths is 127. That compares with a 30-year average of 73 deaths for lightning, 68 for tornadoes and 16 for hurricanes.

National Weather Service data also shows:

  • Nearly half of all flash flood fatalities are vehicle-related,
  • The majority of victims are males, and
  • Flood deaths affect all age groups.

Most flash floods are caused by slow moving thunderstorms, thunderstorms that move repeatedly over the same area or heavy rains from tropical storms and hurricanes. These floods can develop within minutes or hours depending on the intensity and duration of the rain, the topography, soil conditions and ground cover.

Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of 30 feet or more. Furthermore, flash flood-producing rains can also trigger catastrophic mud slides.

Occasionally, floating debris or ice can accumulate at a natural or man-made obstruction and restrict the flow of water. Water held back by the ice jam or debris dam can cause flooding upstream. Subsequent flash flooding can occur downstream if the obstruction should suddenly release.

TURN AROUND, DON'T DROWN

Turn Around, Don't Drown

Each year, more deaths occur due to flooding than from any other thunderstorm related hazard. Why? The main reason is people underestimate the force and power of water. Many of the deaths occur in automobiles as they are swept downstream. Of these deaths, many are preventable, but foolish people drive around the barriers in place that warn you the road is flooded.

Whether you are driving or walking, if you come to a flooded road, Turn Around...Don't Drown!. You will not know the depth of the water nor will you know the condition of the road under the water.

Of the three deaths which occurred as a result of the Fort Worth tornado, March 28, 2000, one death was due to flooding. The man who drowned was a passenger in a car with his girlfriend, the driver. They approached a low spot with water flowing over the road due to very heavy rain. Flooding was a common occurrence at this location with heavy rains and the danger was well marked.

As the driver drove her car into the water she became frightened as the water rose higher and higher around her vehicle. She backed out to higher ground. The passenger said the water was NOT too deep and he would prove it by walking across to the other side. He never made it.

Follow these safety rules.

  • Monitor the NOAA Weather Radio, or your favorite news source for vital weather related information.
  • If flooding occurs, get to higher ground. Get out of areas subject to flooding. This includes dips, low spots, canyons, washes etc.
  • Avoid already flooded and high velocity flow areas. Do not attempt to cross flowing streams. If you enter a flowing stream and the water gets above you knee, TURN AROUND, DON'T DROWN.
  • If driving be aware that the road bed may not be intact under flood waters. Turn around and go another way. NEVER drive through flooded roadways! If your vehicle stalls, leave it immediately and seek higher ground. Rapidly rising water may engulf the vehicle and sweep you and your occupants away.
  • Do not camp or park your vehicle along streams and washes, particularly during threatening conditions.
  • Be especially cautious at night when it is harder to recognize flood dangers.

We say "Follow these safety rules" and most folks say "Yeah, yeah, whatever. It's not going to happen to me." A simple Internet search of flash flood victims (and some seen from inside the vehicle) will show you the reason to just TURN AROUND, DON'T DROWN.

Staying Ahead of the Storms

Severe weather rarely happens without any warning. While we will never be able to pinpoint when and where severe weather will develop, we can identify broader areas with the potential for the development of severe weather. It is your responsibility to check the weather forecast, which may be often several times daily, to see if you are, or will be, under a risk of severe weather.

The weather office charged with monitoring and forecasting the potential for severe weather over the 48 continental United States is the Storm Prediction Center (SPC) located in Norman, OK. Use the information provided by SPC to give you early critical information concerning the threat of severe weather in your locale.

Convective Outlooks

Convective Outlooks consist of a narrative and a graphic depicting severe thunderstorm threats across the continental United States. The outlook narratives are written in technical language, intended for sophisticated weather users, and provide the meteorological reasoning for the risk areas.

This product also provides explicit information regarding the timing, the greatest severe weather threat and the expected severity of the event. The graphics that accompany the narratives provide vital information to help plan your day.

The convective outlook graphics display up to six different color categories to reflect the six likelihood of occurrences and/or increased severity of a severe weather event(s). The four convective outlooks issued (Day 1, Day 2, Day 3 and Days 4-8) are...

Day 1 This is the risk of severe weather today through early tomorrow morning. Day 1 forecasts are issued five times daily; 06z (around midnight), 13z (around sunrise), 1630z (mid-morning), 20z (mid-afternoon), and 01z (early evening). This is the forecast you will see on SPC's frontpage. 
Day 2 Day 2 continues from the ending of Day 1 (tomorrow morning) for the next 24 hours. These are issued twice daily; 07z (around midnight) and 1730z (around noon).
Day 3 This is the forecast for the subsequent 24 hours. Day 3 forecasts are issued daily by 0830z on standard time and 0730z on daylight time (after midnight).
Days 4-8 A severe weather area depicted in the days four through eight period. It is issued at 10z daily (early morning) and indicates a 15% or 30% or higher probability for severe thunderstorms (e.g. a 15% or 30% chance that a severe thunderstorm will occur within 25 miles of any point).

Following are the meanings of the colors used in convective outlooks.

General Thunderstorms

The light green shading depicts a 10% or higher probability of non-severe or near severe thunderstorms during the valid period. However, just remember that a thunderstorm producing " hail and wind gusts to 55 mph wind is officially a NON-severe storm but can still produce damage. So, just because you may be in an area of "general thunderstorms", you need to keep alert for the possibility of rapidly changing weather conditions.

Severe Category 1 - Marginal Risk

The dark green shading area indicates a marginal (MRGL) risk of severe thunderstorms during the forecast period. This means a...

2% probability or greater tornado probability
OR
probability for severe hail (≥1" / ≥2.4cm) OR severe wind. (≥58 mph / ≥93 km/h).

Severe Category 2 - Slight

The yellow shaded area indicates a slight (SLGT) risk of severe thunderstorms during the forecast period. This means a...

5% probability or greater tornado probability
OR
15% probability for severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability of hail 2" (4.8 cm) or greater in diameter
OR
wind gusts 75 mph (120 km/h) or greater .

Severe Category 3 - Enhanced

The orange shaded area indicates an enhanced (ENH) risk of severe thunderstorms during the forecast period. This means a...

10% probability for any tornado WITH OR WITHOUT 10% or greater probability of an EF2+ tornado
OR
15% probability for any tornado
OR
30% severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability of hail 2" (4.8 cm) or greater in diameter, or wind gusts 75 mph or greater
OR
45% probability of severe hail or wind.

Severe Category 4 - Moderate

The red shaded area indicates a moderate (MDT) risk of severe thunderstorms are expected. This means a...

15% tornado probability AND 10% or greater probability of an EF2+ tornado
OR
30% probability for any tornado
OR
45% severe wind probability AND 10% or greater probability of a wind gusts 75 mph (120 km/h) or greater
OR
45% severe hail probability AND 10% or greater probability of hail 2" (4.8 cm) or greater in diameter
OR
60% severe wind probability
OR
60% severe hail probability WITH OR WITHOUT 10% or greater probability of hail 2" (4.8 cm) or greater in diameter.

Severe Category 5 - High

The fuschia shaded area indicates a high (HIGH) risk of severe thunderstorms are expected. This means a...

30% tornado probability AND 10% or greater probability of an EF2+ tornado
OR
45% or greater probability for any tornado WITH OR WITHOUT 10% or greater probability of an EF2+ tornado
OR
60% severe wind probability AND a 10% or greater probability of a wind gust 75 mph (120 km/h) or greater.

These are the official definitions. The reason for the "AND's", "OR's" and "WITH OR WITHOUT's" is the atmosphere is complicated with many different conditions that can occur. For example, there will be times when the number of severe weather events will be high but the overall intensities will not necessarily be extreme. Conversely, there may only be one or two severe events expected but the intensity of the event(s) will be extremely high.

Therefore, below is a simplified version of the official definitions.

Still Confused?!? Just know that the greater the threat (from Slight to High), the greater the risk for severe weather which could be either in number of events or intensity or both.

The following are current severe weather outlooks from the Storm Prediction Center (click to enlarge - takes you to the SPC website)

Today
Day 1 Outlook - click to enlarge
Tomorrow
Day 2 Outlook - click to enlarge
Day 3
Day 3 Outlook - click to enlarge
Days 4-8
Days 4-8 Outlook - click to enlarge

Public Severe Weather Outlooks

The Public Severe Weather Outlooks (PWO) are issued when a potentially significant or widespread tornado outbreak is expected. This plain-language forecast is typically issued 12-24 hours prior to the event and is used to alert National Weather Service field offices and other weather customers concerned with public safety of a rare, dangerous situation.

The Public Severe Weather Outlook is reserved for for all high risks and for moderate risks with a strong risk for tornadoes and/or widespread damaging winds. The SPC issues about 30 PWOs each year.

Mesoscale Discussions

When conditions appear favorable for severe storm development, SPC issues a Mesoscale Discussion (MCD), normally 1 to 3 hours before issuing a weather watch.

SPC also puts out MCDs for mesoscale aspects of hazardous winter weather events including heavy snow, blizzards and freezing rain. MCDs are also issued on occasion for heavy rainfall or convective trends.

The MCD basically describes what is currently happening, what is expected in the next few hours, the meteorological reasoning for the forecast, and when/where SPC plans to issue the watch (if dealing with severe thunderstorm potential). Severe thunderstorm MCDs provide you with extra lead time on the severe weather development.

Severe Weather Watches

When conditions become favorable for severe thunderstorms and tornadoes to develop, SPC usually issues a severe thunderstorm or tornado watch.

Tornadoes can occur in either type of watch, but tornado watches are issued when conditions are especially favorable for tornadoes. Severe thunderstorm watches are blue with tornado watches in red.

Watches are large areas, 20,000 to 40,000 square miles, and are issued by county. They are numbered sequentially (the count is reset at the beginning of each year). A typical watch has a duration of about four to six hours but it may be canceled, replaced, or re-issued as required. A watch is NOT a warning, and should not be interpreted as a guarantee that there will be severe weather!

When a watch is issued, stay alert for changing weather conditions and possible warnings. Any warnings will be issued by your local NWS Weather Forecast Office.

When a severe weather watch is issued close to your location but does not include your county, you should still remain alert.

The atmosphere rarely follows straight lines, and thunderstorms do not always remain within the man-made boundaries we create around them. When SPC feels confident about the possibility of severe weather in a specific area, they try to issue a watch at least one hour prior the onset of severe weather.

In some instances the phrase "THIS IS A PARTICULARLY DANGEROUS SITUATION" will headline a watch (called a PDS watch). The "particularly dangerous situation" wording is used in rare situations when long-lived, strong and violent tornadoes are possible.

PDS watches are issued when, in the opinion of the forecaster, the likelihood of significant events is boosted by very volatile atmospheric conditions.Usually this decision is based on a number of atmospheric clues and parameters, so the decision to issue a PDS watch is subjective.

There are no hard threshold or criteria. PDS watches are most often issued in association with "high risk" convective outlooks.

Introduction to Lightning

Lightning is one of the oldest observed natural phenomena on earth. At the same time, it also is one of the least understood. While lightning is simply a gigantic spark of static electricity (the same kind of electricity that sometimes shocks you when you touch a doorknob), scientists do not have a complete grasp on how it works, or how it interacts with solar flares impacting the upper atmosphere or the earth's electromagnetic field.

Lightning has been seen in volcanic eruptions, extremely intense forest fires, surface nuclear detonations, heavy snowstorms, and in large hurricanes. However, it is most often seen in thunderstorms. In fact, lightning (and the resulting thunder) is what makes a storm a thunderstorm.

At any given moment, there can be as many as 2,000 thunderstorms occurring across the globe. This translates to more than 14.5 MILLION storms each year. NASA satellite research indicated these storms produce lightning flashes about 40 times a second worldwide.

This is a change from the commonly accepted value of 100 flashes per second which was an estimate from 1925. Whether it is 40, 100, or somewhere in between, we live on an electrified planet.

Annual number of lightning flashes based on observations from NASA satellites - From High Resolution Full Climatology https://lightning.nsstc.nasa.gov/data/data_lis-otd-climatology Very large version | KMZ file for Google Earth.

How Lightning is Created

The conditions needed to produce lightning have been known for some time. However, exactly how lightning forms has never been verified so there is room for debate.

Leading theories focus around separation of electric charge and generation of an electric field within a thunderstorm. Recent studies also indicate that ice, hail, and semi-frozen water drops known as graupel are essential to lightning development. Storms that fail to produce large quantities of ice usually fail to produce lightning.

Forecasting when and where lightning will strike is not yet possible and most likely never will be. But by educating yourself about lightning and learning some basic safety rules, you, your family, and your friends can avoid needless exposure to the dangers of one of the most capricious and unpredictable forces of nature.

Separated charges in a thunderstorm

Charge Separation

Thunderstorms have very turbulent environments. Strong updrafts and downdrafts occur with regularity and within close proximity to each other. The updrafts transport small liquid water droplets from the lower regions of the storm to heights between 35,000 and 70,000 feet, miles above the freezing level.

Meanwhile, downdrafts transport hail and ice from the frozen upper regions of the storm. When these collide, the water droplets freeze and release heat. This heat in turn keeps the surface of the hail and ice slightly warmer than their surrounding environment, and a "soft hail", or "graupel" forms.

When this graupel collides with additional water droplets and ice particles, a critical phenomenon occurs: Electrons are sheared off of the ascending particles and collect on the descending particles. Because electrons carry a negative charge, the result is a storm cloud with a negatively charged base and a positively charged top.

Field Generation

The electric field within a thunderstorm

In the world of electricity, opposites attract and insulators inhibit. As positive and negative charges begin to separate within the cloud, an electric field is generated between its top and base. Further separation of these charges into pools of positive and negative regions results in a strengthening of the electric field.

However, the atmosphere is a very good insulator that inhibits electric flow, so a TREMENDOUS amount of charge has to build up before lightning can occur. When that charge threshold is reached, the strength of the electric field overpowers the atmosphere's insulating properties, and lightning results.

The electric field within the storm is not the only one that develops. Below the negatively charged storm base, positive charge begins to pool within the surface of the earth (see image right).

This positive charge will shadow the storm wherever it goes, and is responsible for cloud-to-ground lightning. However, the electric field within the storm is much stronger than the one between the storm base and the earth's surface, so most lightning (~75-80%) occurs within the storm cloud itself.

 

How Lightning Develops Between The Cloud And The Ground

A moving thunderstorm gathers another pool of positively charged particles along the ground that travel with the storm (image 1).

As the differences in charges continue to increase, positively charged particles rise up taller objects such as trees, houses, and telephone poles.

A channel of negative charge, called a "stepped leader" will descend from the bottom of the storm toward the ground (image 2).

It is invisible to the human eye, and shoots to the ground in a series of rapid steps, each occurring in less time than it takes to blink your eye. As the negative leader approaches the ground, positive charge collects in the ground and in objects on the ground.

This positive charge "reaches" out to the approaching negative charge with its own channel, called a "streamer" (image 3).

When these channels connect, the resulting electrical transfer is what we see as lightning. After the initial lightning stroke, if enough charge is leftover, additional lightning strokes will use the same channel and will give the bolt its flickering appearance.  

Tall objects such as trees and skyscrapers are commonly struck by lightning. Mountains also make good targets. The reason for this is their tops are closer to the base of the storm cloud.

Remember, the atmosphere is a good electrical insulator. The less distance the lightning has to burn through, the easier it is for it to strike.

However, this does not always mean tall objects will be struck. It all depends on where the charges accumulate. Lightning can strike the ground in an open field even if the tree line is nearby.

The Lightning Process: Keeping in Step

Lightning can be divided into two types:
  • Flashes with at least one channel connecting the cloud to the ground, known as "cloud-to-ground" discharges (CG); and
  • Flashes with no channel to ground, known as "in-cloud" (IC), "cloud-to-cloud" (CC), or "cloud-to-air" (CA).

The lightning process is more or less the same for both types.

Step 1

The stepped leader.

A typical CG lightning strike initiates inside the storm. Under the influences of the electric field between the cloud and the ground, a very faint, negatively charged channel called a "stepped leader" emerges from the storm base and propagates toward the ground in a series of steps about 160 feet (50 meters) in length and 1 microsecond (0.000001 seconds) in duration.

In what can be loosely described as an "avalanche of electrons", the stepped leader usually branches out in many directions as it approaches the ground, carrying an EXTREMELY strong electric potential: about 100 MILLION volts with respect to the ground and about 5 coulombs of negative charge.

Between each step there is a pause of about 50 microseconds, during which the stepped leader "looks" around for an object to strike. If none is "seen", it takes another step, and repeats the process until it "finds" a target.

It takes the stepped leader about 50 milliseconds (1/20th of a second) to reach its full length, though this number varies depending on the length of its path. Studies of individual strikes have shown that a single leader can be comprised of more than 10,000 steps!

 

Step 2

Stepped leader inducing streamers.

As the stepped leader approaches the ground, its strong, negative charge repels all negative charge within the immediate strike zone of the earth's surface, while attracting vast amounts of positive charge. The influx of positive charge into the strike zone is so strong that the stepped leader actually induces electric channels up from the ground known as "streamers".

When one of these positively charged streamers connects with a negatively charged stepped leader (anywhere from 100 to 300 feet (30 to 100 meters) above the surface of the earth), the following steps occur in less than 100 microseconds.

 

Step 3

Connection is made with the ground.

The electric potential of the stepped leader is connected to the ground and the negative charge starts flowing DOWN the established channel.

 

Step 4

Connection is made with the ground.

An electric current wave, called a "return stroke", then shoots UP the channel producing a brilliant pulse. It only takes the current about 1 microsecond to reach its peak value, which averages around 30,000 amperes.

This "return stroke" is more than 99% of a lightning bolt's luminosity and is what we see as lightning. The stroke actually travels FROM the ground INTO the cloud, but because the strike takes place so quickly, to the unaided eye is appears the opposite is true.

 

Step 5

The return stroke, what we see when lightning flashes. Dart leader generally uses the same channel created by the stepped leader.

An electric current wave, called a "return stroke", shoots UP the channel as a brilliant pulse. Behind the wave front, electric charge flows up the channel and produces a ground current. It takes the current about 1 microsecond to reach its peak value, which averages around 30,000 amperes.

The "return stroke" produces more than 99% of a lightning bolt's luminosity and is what we see as lightning. The stroke actually travels FROM the ground INTO the cloud, but because the strike takes place so quickly, to the unaided eye is appears the opposite is true.

After the return stroke ceases flowing up the channel, there is a pause of about 20 to 50 milliseconds. After that, if enough charge is still available within the cloud, another leader can propagate down to the ground. This leader is called a "dart leader" because it uses the channel already established by the stepped leader and therefore has a continuous path.

Dart leaders give lightning its flickering appearance and normally are not branched like the initial stepped leader. Not every lightning flash will produce a dart leader because a sufficient charge to initiate one must be available within about 100 milliseconds of the initial stepped leader.

The dart leader carries additional electric potential to the ground and induces a new streamer from the ground. The dart leader's peak current is usually less than the initial stepped leader and its return stroke has a shorter duration than the initial return stroke. As additional dart leaders are produced, their peak currents and return stroke durations continue to decrease.

Dart leaders and their return strokes don't necessarily have to use the same cloud-to-ground channel that was burned by initial stepped leader. If a dart leader takes a different path to the ground, the lightning will appear to dance from one spot to another. This is known as "forked lightning".

The combination of each leader (stepped and dart) and their subsequent return strokes is known collectively as a "stroke". All strokes that use the same channel constitute a single "flash". A flash can be made up of a single stroke, or tens of strokes. (The highest number of strokes ever recorded in a single cloud-to-ground flash is 47!)

Notice, in the greatly slowed down animation , the top part of a lightning flash is not connected to anything physical in the cloud, as the cloud itself is not conductive. What happens is the lightning channel branches out inside the cloud in a tree-like shape, and draws free electrons to it (the negative charge in the lower half of the thunderstorm).

The Sound of Thunder

Regardless of whether lightning is positive or negative, thunder is produced the same way. Thunder is the acoustic shock wave resulting from the extreme heat generated by a lightning flash.

Lightning can be as hot as 54,000F (30,000C), a temperature that is five times hotter than the surface of the sun! When lightning occurs, it heats the air surrounding its channel to that same incredible temperature in a fraction of a second.

Like all gases, when air molecules are heated, they expand. The faster they are heated, the faster their rate of expansion. But when air is heated to 54,000F (30,000C) in a fraction of a second, a phenomenon known as "explosive expansion" occurs. This is where air expands so rapidly that it compresses the air in front of it, forming a shock wave similar to a sonic boom. Exploding fireworks produce a similar result.

When lightning strikes, a shock wave is generated at each point along the path of the lightning bolt. (The above illustrations show only four points.) When the shock wave is first created there is a sharp boundary associated with it.
The initial sound reaches the ear with a loud bang, crack or snap.
As shock waves propagate away from the path of the lightning bolt, they are distorted becoming stretched and elongated. The sound is more muted. Then other shock waves from more distance locations arrive to the listener. Shock waves emanating along the lightning bolt's path, arriving to the listener's ear at the same time, enhance the intensity of the sound.
At large distances from the center, the shock wave (thunder) can be many miles across. The shock wave is greatly elongated. To the listener, it is the combination of the millions of shock waves that gives thunder the continuous booming/rumbling sound we hear.

Determining distance to a Thunderstorm

Overview

Thunder is a result of the rapid expansion of super heated air caused by the extremely high temperature of lightning. As the lightning bolt passes through the air, the air expands faster than the speed of sound generating a "sonic boom".

Since the sonic boom is created along the path of the lightning bolt, in effect, millions of sonic booms are created, which we hear as a rumble.

Thunder from a nearby lightning strike will have a very sharp crack or loud bang, whereas thunder from a distant strike will have a continuous rumble. The primary reason for this is that the sound shock wave modifies as it passes through the atmosphere.

Sound travels roughly 750 mph (1,200 km/h), or approximately one mile every 5 seconds (one kilometer every 3 seconds). The speed actually varies greatly with the temperature, but the thumb rule of 5 seconds per mile (3 seconds per kilometer) is a good approximation.

Through a series of examples, the student will be able to determine the distance to a lightning strike.

TOTAL TIME 10 minutes
SUPPLIES Flashlight. Optional: thunder sound files (see below); camera flash
PRINTED/AV MATERIAL None
TEACHER PREPARATION

None unless you would like to use the sound files. You can download the following thunder sounds to a computer or smartphone. The sounds are in mp3 format.

Very Sharp Thunder (161k), Sharp Thunder (201k), Close Rumble (237k), Far Rumble (246k)

SAFETY FOCUS Lightning safety

Procedure

  • Instruct the students about thunder and why it occurs. Ensure they know sound travels about one mile every five seconds (three kilometers every three seconds). Instruct the student that they can approximate "seconds" by counting "One-Mississippi", "Two-Mississippi", "Three-Mississippi", etc.
  • Have the student look at the end of the flashlight and instruct them to begin counting once they see it light up.
  • Rapidly turn the flashlight on and off.
  • After you count five seconds, either say "BOOM" or play one of the sharp thunder sounds.
  • Have the students divide the time from the first light to hearing the sound by 5 seconds to determine the distance in miles from the lightning bolt.
  • Repeat the procedure but wait ten seconds between flashing the light and playing the sound.
  • Repeat the procedure but wait 15 seconds between flashing the light and playing the sound.
  • Repeat the procedure several more times but vary the time from flash to sound (two seconds, 14 seconds, etc.). Remember, the longer the time between flash and sound, the farther away the lightning is so use the thunder sounds (distant rumbles) that, by themselves, are an indication of distance.

Discussion

Each time you do the procedure there will be some variability in the student's results due to inconsistent counting of the seconds. However, you will quickly be able to understand the student's grasp of the concept by inquiring how many seconds they counted. For more accurate results, have the student use the second hand of watches or use stop watches.

For advanced students, during the next thunderstorm, have the class record the local time (in hours, minutes, and seconds) and direction of up to 20 cloud-to-ground lightning strikes and the time thunder was heard. Then have the student compare their results with each other.

On a map of your local area, plot the student's homes and by triangulation, determine the location of the strikes based upon the time and direction of occurrence at each dwelling. (DO NOT have the students contact one another during a thunderstorm unless it is by cell or cordless phone. Some people have died while using the phone when lightning struck a nearby telephone pole.)

Live weatherwise

Lightning kills an average of 49 people in the United States each year, and hundreds more are severely injured. Many of these tragedies can be avoided. Finishing the game, getting a tan, or completing a work shift are not worth death or crippling injury.

  • All thunderstorms produce lightning and are dangerous. Lightning kills more people each year than tornadoes and hurricanes combined.
  • Lightning can strike more than 25 miles (40 km) away from any rainfall. Many deaths from lightning occur ahead of the storm because people wait until the last minute before seeking shelter.
  • Lightning can strike well beyond the audible range of thunder. If you hear thunder, the thunderstorm is close enough that lightning could strike your location at any moment.
  • Lightning injuries can lead to permanent disabilities or death. On average, 20% of strike victims die; 70% of survivors suffer serious long term effects.
  • Look for dark cloud bases and increasing wind. Every flash of lightning is dangerous, even the first. Head to safety before that first flash. If you hear thunder, head to safety!
  • NO PLACE outdoors is safe during a lightning storm. If lightning is seen or thunder is heard, or if dark clouds are gathering overhead, quickly move indoors or into a hard-topped vehicle and remain there until 30 minutes after the final clap of thunder. Listen to forecasts and warnings through NOAA Weather Radio or your local TV and radio stations. If lightning is forecast, plan an alternate activity or know where you can take cover quickly.
What to do!

The best thing you can do is stop your outdoor activity and move indoors or get in a hardtop automobile (not a convertible). Don't wait for rain to begin before you act. Once indoors, do not use corded telephones unless it is an emergency as the phone line is the leading cause of indoor lightning injuries in the United States. Lightning can travel long distances in both phone and electrical wires, particularly in rural areas.

 

In addition, the temperature of the atmosphere affects the thunder sound you hear as well as how far away you can hear it.

Sound waves move faster in warm air than they do in cool air. Typically, the air temperature decreases with height. When this occurs, thunder will normally have an audible range up to 10 miles (16 km).

However, when the air temperature increases with height, called an inversion, sound waves are refracted (bent back toward the earth) as they move due to their faster motion in the warmer air. Normally, only the direct sound of thunder is heard. But refraction can add some additional sound, effectively amplifying the thunder and making it sound louder.

This is more common in the winter as thunderstorms develop in the warm air above a cooler surface air mass.

If the lightning in these "elevated thunderstorms" remains above the inversion, then most of the thunder sound also remains above the inversion. However, many of the sound waves from cloud-to-ground strikes remain below the inversion giving thunder a much louder impact.

 

Lightning Safety

Lightning is the MOST UNDERRATED weather hazard. On average, only floods kill more people. Lightning makes every single thunderstorm a potential killer, whether the storm produces one single bolt or ten thousand bolts.

In the United States, lightning routinely kills more people each year than tornadoes or hurricanes. Tornadoes, hail, and wind gusts get the most attention, but only lightning can strike outside the storm itself. Lightning is the first thunderstorm hazard to arrive and the last to leave.

Lightning is one of the most capricious and unpredictable characteristics of a thunderstorm. Because of this, no one can guarantee an individual or group absolute protection from lightning. However, knowing and following proven lightning safety guidelines can greatly reduce the risk of injury or death. Remember, YOU are ultimately responsible for your personal safety, and should take appropriate action when threatened by lightning.

Where to Go

The safest location during a thunderstorm is inside a large enclosed structure with plumbing and electrical wiring. These include shopping centers, schools, office buildings, and private residences.

If lightning strikes the building, the plumbing and wiring will conduct the electricity more efficiently than a human body. If no buildings are available, then an enclosed metal vehicle such as an automobile, van, or school bus makes a decent alternative.

Where NOT to Go

Not all types of buildings or vehicles are safe during thunderstorms. Buildings which are NOT SAFE (even if they are "grounded") have exposed openings. These include beach shacks, metal sheds, picnic shelters/pavilions, carports, and baseball dugouts. Porches are dangerous as well.

Convertible vehicles offer no safety from lightning, even if the top is "up". Other vehicles which are NOT SAFE during lightning storms are those which have open cabs, such as golf carts, tractors, and construction equipment.

What To Do

Once inside a sturdy building, stay away from electrical appliances and plumbing fixtures. As an added safety measure, stay in an interior room.

If you are inside a vehicle, roll the windows up, and avoid contact with any conducting paths leading to the outside of the vehicle (e.g. radios, CB's, ignition, etc.).

What NOT to Do

Lightning can travel great distances through power lines, especially in rural areas. Do not use electrical appliances, ESPECIALLY corded telephones unless it is an emergency (cordless and cell phones are safe to use).

Computers are also dangerous as they usually are connected to both phone and electrical cords. Do not take a shower or bath or use a hot tub.

Frequently Asked Questions

  1. What is the difference between a thundershower and a thunderstorm?
  2. What are my chances of being struck by lightning?
  3. What should I do if I'm caught out in the open during a thunderstorm and no shelter is nearby?
  4. Shouldn't I lie flat on the ground to get as low as possible?
  5. How do I avoid having to use the Lightning Crouch?
  6. Does lightning travel from the cloud to the ground, or from the ground to the cloud?
  7. If lightning travels from the ground into the cloud, why do photographs show branches of lightning descending from the cloud?
  8. How far away from the storm center can lightning strike?
  9. Can lightning strike me while I'm indoors?
  10. Can I use my cell phone or cordless phone during a storm?
  11. Can I be struck by lightning if I wear rubber soled shoes?
  12. I am in a boat on the open water. How can I protect myself from lightning?
  13. Should I install a lightning protection system on my home or business?
 
What is the difference between a thundershower and a thunderstorm?

Technically, there is none. In general, the term "thundershower" tends to denote a fairly weak storm with light to moderate rainfall and low levels of lightning activity. However, there are no defined parameters that distinguish between a thundershower and a thunderstorm.

In fact, in order to avoid confusion, we in the National Weather Service do not use the term "thundershower". If a shower is strong enough to produce lightning, even just one single bolt, it's called a thunderstorm. Top

What are my chances of being struck by lightning?
This is a seemingly simple question, but there is no single answer that fits everyone. The odds of being struck vary from person to person because they depend on several factors. The most significant are:
  1. Geographical location and climatology
  2. Diurnal and annual climatology
  3. Personal lifestyle/hobbies

Where there is a lot of lightning, there is an increased chance of being struck. The central Florida peninsula from Tampa Bay to Cape Canaveral has the highest lightning concentration in the United States. More than 90% of the lightning in this area occurs between May and October, between the hours of noon and midnight.

During this time of day and year, people in Central Florida who spend a large portion of their lives outdoors (e.g. construction workers, park rangers, golfers, campers etc.) are more likely to be struck than anywhere else in the country. On the other hand, thunderstorms are uncommon in the Pacific northwest, and are virtually unheard of during the winter months.

People in this region who spend much of their lives indoors (e.g. shopkeepers, librarians, bowlers, billiard players, etc.) might win the lottery before they were struck by lightning. It is impossible to assign one single probability to every person in every situation.

The general odds from the National Weather Service Lightning Safety site...

Estimated population as of 2017 325,000,000
Number of deaths reported: 30 Number of injuries reported: 270 (total) 300
Odds of being struck by lightning in a given year (estimated total deaths + injuries) 1/1,083,000
Odds of being struck in your lifetime (Est. 80 years) 1/13,500
Odds you will be affected by someone being struck (Ten people affected for every one struck) 1/1,350
Top
What should I do if I'm caught out in the open during a thunderstorm and no shelter is nearby?

There are NO SAFE PLACES outdoors during a lightning storm. Don't kid yourself--you are NOT safe outside. Following these tips will not prevent you from being struck by lightning, but may slightly lessen the odds.

If camping, hiking, etc., far from a safe vehicle or building, avoid open fields, the top of a hill or a ridge top. Keep your site away from tall, isolated trees or other tall objects. If you are in a forest, stay near a lower stand of trees.

If you are camping in an open area, set up camp in a valley, ravine or other low area. Remember, a tent offers NO protection from lightning. If you are camping and your vehicle is nearby, run to it before the storm arrives.

Stay away from water, wet items such as ropes and metal objects, such as fences and poles. Water and metal are excellent conductors of electricity. The current from a lightning flash will easily travel for long distances.

Top
Shouldn't I lie flat on the ground to get as low as possible?

NO! Lying flat on the ground was once thought to be the best course of action, but this advice is now decades out of date.

When lightning strikes the earth, it branches out along the ground. The lightning bolt can be fatal up to 100 feet away from the point of strike.

These currents fan out from the strike center in a tendril pattern, so in order to minimize your chance of being struck, you have to minimize BOTH your height AND your body's contact with the earth's surface. Top

How do I avoid having to use the Lightning Crouch?

We don't recommend the crouch because it will not significantly lower your risk of being killed or injured from a nearby lightning strike.

Be aware of your situation and PLAN AHEAD. If you going to be involved in an outdoor activity, make sure you know what the forecast is, ESPECIALLY if you live in a lightning prone area. If storms are forecast, have a plan of action that you can enact quickly to reduce your chances of being struck. Top

Does lightning travel from the cloud to the ground, or from the ground to the cloud?

An entire lightning strike employs both upward and downward moving forces. However, the return stroke of a lightning bolt travels FROM THE GROUND INTO THE CLOUD and accounts for more that 99% of the luminosity of a lightning strike. What we SEE as lightning does indeed travel from the ground into the cloud. Top

If lightning travels from the ground into the cloud, why do photographs show branches of lightning descending from the cloud?

In photographs, it may APPEAR that lightning is descending from the cloud to the ground, but in reality, the return stroke is so brilliant that as it travels up the strike channel, it illuminates all of the branches of the stepped leader that did not connect with a streamer. Top

How far away from the storm center can lightning strike?

Almost all lightning will occur within 10 miles of its parent thunderstorm, but it CAN strike much farther than that. Lightning detection equipment has confirmed bolts striking almost 50 miles away. Top

Can lightning strike me while I'm indoors?
YES! If a bolt strikes your house or a nearby power line, it CAN travel into your house through the plumbing or the electric wiring! If you are using any electrical appliances or plumbing fixtures (INCLUDING telephones and computers), and a storm is overhead, you are putting yourself at risk! FACT: About 4-5% of people struck by lightning are struck while talking on a corded telephone. Top
 
Can I use my cell phone or cordless phone during a storm?

Yes. These are safe to use because there is no direct path between you and the lightning. Avoid using a corded telephone unless it's an emergency. Top

Can I be struck by lightning if I wear rubber soled shoes?

Absolutely! While rubber is an electric insulator, it's only effective to a certain point. The average lightning bolt carries about 30,000 amps of charge, has 300 million volts of electric potential, and is about 50,000F.

These amounts are several orders of magnitude HIGHER than what humans use on a daily basis and can burn through ANY insulator (even the ceramic insulators on power lines!)

Besides, the lightning bolt may just have traveled many miles through the atmosphere, which is a good insulator. Your " (or less) of rubber will make no difference. Top

I am in a boat on the open water. How can I protect myself from lightning?

The vast majority of lightning injuries and deaths on boats occur on small boats with NO cabin. It is crucial to listen to weather information when you are boating.

If thunderstorms are forecast, do not go out. If you are out and cannot get back to land and safety, drop anchor and get as low as possible.

Large boats with cabins, especially those with lightning protection systems properly installed, or metal marine vessels are relatively safe. Remember to stay inside the cabin and away from any metal surfaces. Stay off the radio unless it is an emergency! Top

Should I install a lightning protection system on my home or business?

It depends. Do you have electrically sensitive equipment and do you think your building may be struck? Contrary to some popular beliefs, lightning protection systems DO NOT prevent lightning.

Instead, they actually bank on the assumption that your building will be struck. They help mitigate damage by giving the lightning a preferred pathway into the ground, not unlike a flood spillway system.