Weather Basics

Air Pressure

The number of molecules in the
atmosphere decreases with height.

The atoms and molecules that make up the various layers in the atmosphere are constantly moving in random directions. Despite their tiny size, when they strike a surface they exert a force on that surface in what we observe as pressure.

Each molecule is too small to feel and only exerts a tiny bit of force. However, when we sum the total forces from the large number of molecules that strike a surface each moment, then the total observed pressure can be considerable.

Air pressure can be increased (or decreased) one of two ways. First, simply adding molecules to any particular container will increase the pressure. A larger number of molecules in any particular container will increase the number of collisions with the container's boundary which is observed as an increase in pressure.

A good example of this is adding (or subtracting) air in an automobile tire. By adding air, the number of molecules increase as well a the total number of the collisions with the tire's inner boundary. The increased number of collisions forces the tire's pressure increase to expand in size.

The second way of increasing (or decreasing) is by the addition (or subtraction) of heat. Adding heat to any particular container can transfer energy to air molecules. The molecules therefore move with increased velocity striking the container's boundary with greater force and is observed as an increase in pressure.

Since molecules move in all directions, they can even exert air pressure upwards as they smash into object from underneath. In the atmosphere, air pressure can be exerted in all directions.

In the International Space Station, the density of the air is maintained so that it is similar to the density at the earth's surface. Therefore, the air pressure is the same in the space station as the earth's surface (14.7 pounds per square inch).

Back on Earth, as elevation increases, the number of molecules decreases and the density of air therefore is less, meaning a decrease in air pressure. In fact, while the atmosphere extends more than 15 miles (24 km) up, one half of the air molecules in the atmosphere are contained within the first 18,000 feet (5.6 km).

Because of this decrease in pressure with height, it makes it very hard to compare the air pressure at ground level from one location to another, especially when the elevations of each site differ. Therefore, to give meaning to the pressure values observed at each station, we convert the station air pressures reading to a value with a common denominator.

The common denominator we use is the sea-level elevation. At observation stations around the world the air pressure reading, regardless of the observation station elevation, is converted to a value that would be observed if that instrument were located at sea level.

The two most common units in the United States to measure the pressure are "Inches of Mercury" and "Millibars". Inches of mercury refers to the height of a column of mercury measured in hundredths of inches. This is what you will usually hear from the NOAA Weather Radio or from your favorite weather or news source. At sea level, standard air pressure is 29.92 inches of mercury.

Millibars comes from the original term for pressure "bar". Bar is from the Greek "báros" meaning weight. A millibar is 1/1000th of a bar and is approximately equal to 1000 dynes (one dyne is the amount of force it takes to accelerate an object with a mass of one gram at the rate of one centimeter per second squared). Millibar values used in meteorology range from about 100 to 1050. At sea level, standard air pressure in millibars is 1013.2. Weather maps showing the pressure at the surface are drawn using millibars.

How temperature effects the height of pressure.

Although the changes are usually too slow to observe directly, air pressure is almost always changing. This change in pressure is caused by changes in air density, and air density is related to temperature.

Warm air is less dense than cooler air because the gas molecules in warm air have a greater velocity and are farther apart than in cooler air. So, while the average altitude of the 500 millibar level is around 18,000 feet (5,600 meters) the actual elevation will be higher in warm air than in cold air.

Crunch Time

Overview

Pressure is not only a matter of altitude but also is dependent upon the temperature. As the temperature increases so does the pressure. The molecules and atoms that comprise the air we breath gain energy as they absorb heat. That increase in energy results in faster moving atoms which we observe as an increase in energy.

The opposite occurs when the temperature decreases. As the molecules lose energy, their motion is decreased and we observe a decrease in pressure. The students will see a plastic 2-liter bottle crushed by the normal atmospheric pressure in the room by this decrease in pressure.

Procedure

Place two cups of hot tap water into each two 2-liter bottle.
Place your thumb over each bottle opening and shake. This ensures the air inside the bottle is warmed.
Pour the water out of each bottle and screw a bottle cap on only one of the two bottles.
Stand both bottles side-by-side and observe over the next five minutes.

Discussion

The bottle that was capped will eventually begin to collapse. This is a result of the cooling air inside that bottle. The air cools because the molecules and atoms inside the bottle lose energy as they collide with the bottle side that is exposed to the cooler surrounding air.

As their energy decrease so does their velocity and therefore the pressure decreases. Since the pressure inside the bottle decreases, the force of the air outside the bottle begins to crush the bottle.

However the uncapped bottle remains unchanged. As the air cools inside, the drier outside air flows in to take up the space thereby keeping the pressure the same both inside and outside of the bottle.

Live Weatherwise

There was a time the National Weather Service advised, when a tornado was approaching, to open a window in the house to equalized the pressure inside and out. The thought was high pressure trapped inside a house caused to blowup due a rapid fall of pressure outside resulting from the tornadic winds.

Research has shown the pressure difference is only about 10% and houses can handle this as there are vents in bathrooms and kitchens to relieve the pressure. What destroys a house is debris slammed into the structure by the force of the wind.

Therefore, if a tornado approaches your house, LEAVE THE WINDOWS ALONE! The tornado will open them for you as debris hits the glass. Instead seek shelter in the interior of your dwelling, on the lowest floor, away from windows, in order to place as much protection between you and flying debris possible.

The H's represent the location of the area of highest pressure
The L's represent the position of the lowest pressure.

The most basic change in pressure is the twice daily rise and fall in due to the heating from the sun. Each day, around 4 a.m./p.m. the pressure is at its lowest and near its peak around 10 a.m./p.m. The magnitude of the daily cycle is greatest near the equator decreasing toward the poles.

On top of the daily fluctuations are the larger pressure changes as a result of the migrating weather systems. These weather systems are identified by the blue H's and red L's seen on weather maps.

Measure the Pressure - The "Wet" Barometer

Overview

The amount of air over us is constantly changing. As a result, the weight of that air, called pressure, is constantly changing. These changes in air pressure are indications of changes in our weather. We measure this change using a device called barometer (bar-meter or measurer).

This first barometer was created by Evangelista Torricelli in 1643. Torricelli was actually trying to discover the reason that water would rise no more than 33 feet up a tube though the use of a suction pump. He had first built a water barometer, but it required a glass tube 60 feet long.

Aware that mercury was 14 times heavier than water, he constructed a tube only 35 inches long. Filing the tube with mercury and inverting the tube into a bowl of mercury caused mercury in the tube to drop to a level around 30 inches and creating a vacuum at the top of the tube.

The top of the mercury column was observed to fluctuate by a few percent, due mainly to what we now know to be fluctuations in atmospheric pressure. This is because as the column of air directly above the barometer pushes on a dish containing mercury, it is forced up a tube.

The stronger the downward push, the higher the pressure and therefore the higher the mercury rises in the tube. This is where the units "Inches of Mercury" are derived.

Procedure

Place the ruler in the glass and tape it to one side. (Make sure the numbers are visible.)
Tape the plastic tube onto the ruler in the glass. (Make sure the tube is not touching the bottom of the glass.)
Fill the glass about half way with water. Add a drop or two of food coloring and mix thoroughly.
Using the tube like a straw, draw some water about 2/3^rds into the straw.
Using your tongue, trap the water in the tube then cap the end of the tube with model clay or chewing gum.
Record the height of the water in the tube.
At the same time every day, for the next 5-10 days, record the height of the water in the tube, paying close attention to the change in the weather as the water level changes.

Discussion

What the students have constructed is a water barometer (also known as "storm glass") and these types of barometers date from the 17th century. The actual change in pressure will occur too slow for direct observation. Usually, only for a 24 hour period will the change in pressure be most noticeable.

As a storm approaches, the mass of air around your location decreases. Therefore, the pressure decreases as well. After a cold front passes your location, higher pressure moves in and the students will see the pressure rise.

Over and above the pressure changes associated with storms, there are four daily pressure fluctuations in the atmosphere. These diurnal changes are due to the sun heating the atmosphere. The amplitudes of this daily change depend upon the latitude, season, and altitude.

The changes are greatest at the equator, decreasing toward the poles where it becomes zero. Also, the higher the altitude, the greater the daily change.

Live Weatherwise

Rapid falls in pressure can indicate the approach of severe thunderstorms. A Severe thunderstorm WARNING is an urgent announcement that a severe thunderstorm has been reported or is imminent and warns you to take cover. Severe thunderstorm warnings are issued by local National Weather Service offices.

The strong wind gusts of severe thunderstorms can damage buildings, knock down trees, and create a hazard due to wind-blown debris. Therefore:

Seek shelter but avoid trees as these are targets for lightning.
If indoors, stay away from windows and go to the safest location on the lowest level of your home.
When boating, always stay tuned to the latest weather reports and return to a safe harbor before the strong winds arrive.

How are changes in weather related to changes in pressure?
From his vantage point in England in 1848, Rev. Dr. Brewer wrote in his A Guide to the Scientific Knowledge of Things Familiar the following about the relation of pressure to weather:

The FALL of the barometer (decreasing pressure)

In very hot weather, the fall of the barometer denotes thunder. Otherwise, the sudden falling of the barometer denotes high wind.
In frosty weather, the fall of the barometer denotes thaw.
If wet weather happens soon after the fall of the barometer, expect but little of it.
In wet weather if the barometer falls expect much wet.
In fair weather, if the barometer falls much and remains low, expect much wet in a few days, and probably wind.
The barometer sinks lowest of all for wind and rain together; next to that wind, (except it be an east or north-east wind).

The RISE of the barometer (increasing pressure)

In winter, the rise of the barometer presages frost.
In frosty weather, the rise of the barometer presages snow.
If fair weather happens soon after the rise of the barometer, expect but little of it.
In wet weather, if the mercury rises high and remains so, expect continued fine weather in a day or two.
In wet weather, if the mercury rises suddenly very high, fine weather will not last long.
The barometer rises highest of all for north and east winds; for all other winds it sinks.

The barometer UNSETTLED (unsteady pressure)

If the motion of the mercury be unsettled, expect unsettled weather.
If it stands at "MUCH RAIN" and rises to "CHANGEABLE" expect fair weather of short continuance.
If it stands at "FAIR" and falls to "CHANGEABLE", expect foul weather.
Its motion upwards, indicates the approach of fine weather; its motion downwards, indicates the approach of foul weather.

These pressure observations hold true for many other locations as well but not all of them. Storms that occur in England, located near the end of the Gulf Stream, bring large pressure changes. In the United States, the largest pressure changes associated with storms will generally occur in Alaska and northern half of the continental U.S. In the tropics, except for tropical cyclones, there is very little day-to-day pressure change and none of the rules apply.

The "Dry" Barometer

Overview

Barometers using mercury are heavy and fragile. The idea of "dry" barometer was conceived by Gottfried Wilhelm Leibniz around 1700. The idea was to detect pressure changes using sealed bellows. The first working version of an aneroid (without water) barometer was built in 1843 by French scientist Lucien Vidie.

This made the barometer very portable and it became commonly use meteorological instrument. It was still calibrated to the mercurial barometer with readings in inches of mercury.

Even as late at the 1990s, National Weather Service offices still calibrated and verified the accuracy of the aneroid barometer with the mercurial barometer. Using simple items the student will make a device for indicating air pressure changes, called an aneroid barometer.

Procedure

Cover the top of the coffee can tightly with the plastic wrap, using the rubber band to hold it in place. (The cover should be a taut, airtight fit.)
Position the straw so that it lays across two thirds of the cover with the remaining length of the straw suspended over air. Tape in place.
Fold one short end of the index card, about one inch from that end, at a 90° angle. Tape the folded end of the index card to the can behind the straw in such a way that allows you to make marks on the card every day.
Record the level of the straw onto the card.
For the next 10 days, at the same time each day, record the level of the straw while paying close attention to how changes in the weather affect the straw's level.

Discussion

The basic function of an aneroid barometer

What the students have constructed is similar to an aneroid barometer. It is the most common type of barometer for home use.

The aneroid cell volume is very sensitive to changes in atmospheric pressure as it expands and contracts as air pressure decreases or increases. Attached to the aneroid cell is a lever indicating the air pressure. In this case, the aneroid cell is the coffee can.

In this barometer, high pressure in the atmosphere will weigh more the pressure inside the can at the time the barometer was constructed. That added weight will force the plastic wrap into the can, causing the straw tip to rise, indicating higher pressure.

The opposite will occur when low pressure is in the area. The decrease in weight of air on top of the can will help cause the plastic wrap to rise, therefore lowering the straw tip.

Today, even with sensitive electronic sensors having replaced the metal aneroid cells in most barometers, those electronic sensors still need to be calibrated to ensure their accuracy. For that calibration, we still use mercurial barometers.

Live Weatherwise

One measure of the severity of a thunderstorm is the wind speed. In addition to the size of hail, the National Weather Service defines a severe thunderstorm as one containing wind speed of 58 mph (50 kts / 93 km/h) or greater.

The force of all of the molecules moving at 58 mph (50 kts / 93 km/h), or more, can create hazardous weather conditions such a blowing down phone and power lines, trees, and make driving hazardous. When the National Weather Service issues a Severe Thunderstorm Warning it means a thunderstorm with wind gusts to 58 mph (50 kts / 93 km/h) or greater and/or hail size of 1" (2.5 cm) or greater is occur or about to occur near you.

Discuss severe thunderstorm safety with your family. Everyone should know what to do in case all family members are not together. Discussing disaster response ahead of time helps reduce fear and lets everyone know what to do should a severe thunderstorm occur.

Postpone outdoor activities if thunderstorms are likely. Many people take shelter from the rain, but most people struck by lightning are not in the rain! Postponing activities is your best way to avoid being caught in a dangerous situation.

Heavy Air

To show that air has weight, the air is removed from one of two balanced balloons throwing the balance off.

Procedure

Inflate two balloons so they are the same size.
Tape one balloon to each end of the yard/meter stick.
Tie a string to the center of the stick and adjust it so the stick balances when held by the string. Tape the sting in place to prevent it from slipping.
Ask the students, "If one end were heavier, would the heavier end move up or down?"
Carefully deflate the other balloon. Try poking the balloon with a pin in its neck to prevent the balloon from tearing apart as it pops.
Let both balloons hag freely on the yard/meter stick. Ask the students to explain what happens to the balance.

Discussion

Air is all around us. This air is composed of atoms and molecules. Despite their small size, the quantity of atoms and molecules exert weight on us known as pressure. Since our bodies are designed to live in this environment, we do not notice the pressure.

Since the inflated balloon now weighs more than the deflated one (due to the air inside of the balloon) it will sink creating an imbalance. Now, imagine the weight of air if that balloon were now 15 miles (24 km) tall.

That is actually what is occurring at this moment in your classroom. When we measure air pressure with a barometer, we are measuring the weight of a column of air 15 miles (24 km) high directly over us.

Live Weatherwise

The weight of molecules also affects the weather. One measure of the severity of a thunderstorm is the wind speed. In addition to the size of hail, the National Weather Service defines a severe thunderstorm as one containing wind speed of 58 mph (50 kts / 93 km/h) or greater.

The weight of all of the molecules in wind of 58 mph (50 kts / 93 km/h) is the force that can create hazardous weather conditions such a blowing down phone and power lines, trees, and make driving hazardous. When the National Weather Service issues a Severe Thunderstorm Warning it means a thunderstorms with wind gusts to 58 mph (50 kts / 93 km/h) or greater and/or hail size of 1" (2.5 cm) or greater is occur or about to occur near you.

Discuss severe thunderstorms with your family. Everyone should know what to do in case all family members are not together. Preparing for a disaster ahead of time helps reduce fear and lets everyone know how to respond during a severe thunderstorm.

Take an American Safety Council or American Heart Association first aid and CPR course to learn how to treat burns and how to give rescue breathing and administer CPR. Everyone should know how to respond, because severe weather can strike almost anywhere in the country.

Postpone outdoor activities if thunderstorms are likely. Many take shelter from the rain, but most who are struck by lightning are not in the rain! Postponing activities is your best way to avoid being caught in a dangerous situation.

A Pressing Engagement

Overview

We typically do not "feel" atmospheric air pressure. Why? Since air surrounds our bodies, and all things, the pressure, as a result of the air, is applied equally on all sides. For example, if someone holds an 8½x11" sheet of paper by their hand at arms length, the weight of the air directly above the sheet is over 1,300 pounds.

Obviously the paper does not weight that much. Why? That same pressure (14.7 pounds per square inch) is also pressing up on the bottom side of the paper. The equal pressure on all sides cancel each other out so all that is left is the weight of the material that comprises the paper.

Since we do not normally "feel" air pressure, the student will see how the effect of the air pressure on two sheets of paper.

Procedure

Lay a ruler on a table with about 3" (8 cm) hanging over the edge.
Lay a sheet of printer paper on the part of the ruler in direct contact with the table.
Press the paper against the table until it is flat as possible.
Press down on part of the ruler hanging over the edge.
Repeat the above steps except replace the printer paper with a large sheet of opened newspaper in the second step.

Discussion

The student will discover the newspaper was much harder to lift than the printer paper. As the ruler lifted the printer paper, air rush in under the rising paper and thereby quickly allowed the air pressure to equalize on all sides. Essentially, the weight of the air above the paper had no effect on the difficulty in lifting the paper.

As the ruler lifted the newspaper, the edges of the newspaper remained in contact with the desk. Very little air was allowed to rush in and equalize the pressure on the bottom side of the newspaper. Since there is less air below the paper the pressure is less as well. Now the weight of all the air above the paper now becomes more evident.

Live Weatherwise

Take an American Red Cross first aid and CPR course to learn how to treat burns and how to give rescue breathing and administer CPR. Everyone should know how to respond, because severe weather can strike almost anywhere in the country.

Going with the Flow

Overview

Bernoulli's principle states that in fluid flow, an increase in velocity occurs simultaneously with decrease in pressure. The students will discover that the faster air moves (air acting as a fluid), the lower the pressure becomes within that flow of air. They will see this effect blowing between two soda cans.

Procedure

Lay the two cans parallel to each other, about one inch apart, near the edge of a level surface.
Put your face down near the surface and blow lengthwise between the two cans.
It will take some trial and effort but eventually the two cans will roll together.
Another way of demonstration is by suspending two cans with string about an inch apart and having the student blow between them.

Discussion

The affect is Bernoulli's principle in action, named after the Dutch/Swiss mathematician/scientist Daniel Bernoulli. By blowing between the two cans, you are making the air between them move faster than the surrounding air (which is basically calm). The cans roll together as the higher pressure surrounding the two cans (away from the air flow) pushes the cans together toward the region of lower pressure.

Live Weatherwise

There is no such thing as guaranteed safety inside a tornado. Freak accidents happen; and the most violent tornadoes can level and blow away almost any house and its occupants. Extremely violent EF5 tornadoes are very rare, though. Most tornadoes are actually much weaker and can be survived.

Prevention and practice before the storm: At home, have a family tornado plan in place, based on the kind of dwelling you live in and the safety tips below. Know where you can take shelter in a matter of seconds, and practice a family tornado drill at least once a year. Have a pre-determined place to meet after a disaster.

Flying debris is the greatest danger in tornadoes; so store protective coverings (e.g., mattress, sleeping bags, thick blankets, etc) in or next to your shelter space, ready to use on a few seconds' notice. When a tornado watch is issued, think about the drill and check to make sure all your safety supplies are handy. Turn on local TV, radio or NOAA Weather Radio and stay alert for warnings.

Forget about the old notion of opening windows to equalize pressure; the tornado will blast open the windows for you! If you are out shopping, know the locations the store's bathrooms, storage rooms or other interior shelter areas away from windows, and the quickest to get there should a tornado strike.

All administrators of schools, shopping centers, nursing homes, hospitals, sports arenas, stadiums, mobile home communities and offices should have a tornado safety plan in place, with easy-to-read signs posted to direct everyone to a safe, close by shelter area.

Schools and office building managers should regularly run well-coordinated drills. If you are planning to build a house, especially east of the Rockies, consider an underground tornado shelter or an interior "safe room

The Transfer of Heat Energy

The heat source for our planet is the sun. Energy from the sun is transferred through space and through the earth's atmosphere to the earth's surface. Since this energy warms the earth's surface and atmosphere, some of it is or becomes heat energy. There are three ways heat is transferred into and through the atmosphere:

radiation
conduction
convection

Radiation

If you have stood in front of a fireplace or near a campfire, you have felt the heat transfer known as radiation. The side of your body nearest the fire warms, while your other side remains unaffected by the heat. Although you are surrounded by air, the air has nothing to do with this transfer of heat. Heat lamps, that keep food warm, work in the same way. Radiation is the transfer of heat energy through space by electromagnetic radiation.

Most of the electromagnetic radiation that comes to the earth from the sun is invisible. Only a small portion comes as visible light. Light is made of waves of different frequencies. The frequency is the number of instances that a repeated event occurs, over a set time. In electromagnetic radiation, its frequency is the number of electromagnetic waves moving past a point each second.

Our brains interpret these different frequencies into colors, including red, orange, yellow, green, blue, indigo, and violet. When the eye views all these different colors at the same time, it is interpreted as white. Waves from the sun which we cannot see are infrared, which have lower frequencies than red, and ultraviolet, which have higher frequencies than violet light. It is infrared radiation that produce the warm feeling on our bodies.

Most of the solar radiation is absorbed by the atmosphere and much of what reaches the earth's surface is radiated back into the atmosphere to become heat energy. Dark colored objects, such as asphalt, absorb radiant energy faster that light colored objects. However, they also radiate their energy faster than lighter colored objects.

Melts in your bag, not in your hands

Overview

The earth receives its heat from the sun in the form of radiation. Many in the animal kingdom lay out in the sun to absorb this form of energy to warm their bodies. This form of energy is vital to life on this planet. The student will learn how the sun transfers heat to the earth through radiation.

Procedure

Place one piece of chocolate in a bag, seal it and label it with an 'A'.
Do the same with the second piece of chocolate but label that bag with a 'B'.
Take both bags outside and place bag 'A' in the sun and bag 'B' in the shade. Suspend bag 'A' in such a way to ensure is not touching the ground or located near a wall to limit any transfer of heat by convection or conduction. (If this experiment is done indoors, place bag 'A' in the window, exposed to the sun and keep bag 'B' in the shade.)
20 minutes later, inspect the chocolate in both bags.
Ask the students to explain any change in consistency of the chocolate.

Discussion

Depending on how hot it is, the chocolate in the shade may also be softened or even partially melted. However, the chocolate in bag 'A' will be more melted. The bulk of the heating that takes place in bag 'A' is from direct solar radiation. This radiation is what causes objects, such as the metal on automobiles, to become hot. Radiation also causes sunburns.

Live Weatherwise

Melanoma is the most serious type of skin cancer, and accounts for more than 75% of the deaths due to skin cancer. In addition to skin cancer, sun exposure can cause premature aging of the skin, wrinkles, cataracts, and other eye problems.

When you play outdoors, there are five important steps you can take to protect against UV radiation and skin cancer:

Cover up. Wear clothing to protect as much of your skin as possible. Wear clothing that does not transmit visible light. To determine if clothing will protect you, try this test: Place your hand between the fabric and a light source. If you can see your hand through the fabric, the garment offers little protection against sun exposure.
Use a sunscreen. Experts recommend products with a Sun Protection Factor, or SPF, of at least 15. The SPF number represents the level of sunburn protection provided by the sunscreen. Products labeled "broad spectrum" block both UVB and UVA radiation. Both UVA and UVB contribute to skin cancer.
Wear a hat. A wide-brimmed hat is ideal because it protects the neck, ears, forehead, nose and scalp. A baseball cap provides some protection for the front and top of the head, but not for the back of the neck or the ears where skin cancers commonly develop.
Wear sunglasses. UV-absorbent sunglasses can help protect your eyes from sun damage. Ideal sunglasses do not have to be expensive, but they should block 99 to 100 percent of UVA and UVB radiation. Check the label to make sure they do. Darker glasses are not necessarily the best. UV protection comes from an invisible chemical applied to the lenses, not from the color or darkness of the lenses.
Limit direct sun exposure. UV rays are most intense when the sun is high in the sky, between 10 a.m. and 4 p.m. If you are unsure about the sun's intensity, take the shadow test: If your shadow is shorter than you, the sun's rays are the strongest. Seek shade whenever possible.

Remember: There is NO SAFE WAY TO TAN, including the use of tanning beds.

Conduction

Conduction is the transfer of heat energy from one substance to another or within a substance. Have you ever left a metal spoon in a pot of soup being heated on a stove? After a short time the handle of the spoon will become hot.

This is due to transfer of heat energy from molecule to molecule or from atom to atom. Also, when objects are welded together, the metal becomes hot (the orange-red glow) by the transfer of heat from an arc.

This is called conduction and is a very effective method of heat transfer in metals. However, air conducts heat poorly.

Convection

Convection is the transfer of heat energy in a fluid. This type of heating is most commonly seen in the kitchen when you see liquid boiling.

Air in the atmosphere acts as a fluid. The sun's radiation strikes the ground, thus warming the rocks. As the rock's temperature rises due to conduction, heat energy is released into the atmosphere, forming a bubble of air which is warmer than the surrounding air. This bubble of air rises into the atmosphere. As it rises, the bubble cools with the heat contained in the bubble moving into the atmosphere.

As the hot air mass rises, the air is replaced by the surrounding cooler, more dense air, what we feel as wind. These movements of air masses can be small in a certain region, such as local cumulus clouds, or large cycles in the troposphere, covering large sections of the earth. Convection currents are responsible for many weather patterns in the troposphere

Weather Basics

Home	Air Pressure The number of molecules in the atmosphere decreases with height. The atoms and molecules that make up the various layers in the atmosphere are constantly moving in random directions. Despite their tiny size, when they strike a surface they exert a force on that surface in what we observe as pressure. Each molecule is too small to feel and only exerts a tiny bit of force. However, when we sum the total forces from the large number of molecules that strike a surface each moment, then the total observed pressure can be considerable. Air pressure can be increased (or decreased) one of two ways. First, simply adding molecules to any particular container will increase the pressure. A larger number of molecules in any particular container will increase the number of collisions with the container's boundary which is observed as an increase in pressure. A good example of this is adding (or subtracting) air in an automobile tire. By adding air, the number of molecules increase as well a the total number of the collisions with the tire's inner boundary. The increased number of collisions forces the tire's pressure increase to expand in size. The second way of increasing (or decreasing) is by the addition (or subtraction) of heat. Adding heat to any particular container can transfer energy to air molecules. The molecules therefore move with increased velocity striking the container's boundary with greater force and is observed as an increase in pressure. Since molecules move in all directions, they can even exert air pressure upwards as they smash into object from underneath. In the atmosphere, air pressure can be exerted in all directions. In the International Space Station, the density of the air is maintained so that it is similar to the density at the earth's surface. Therefore, the air pressure is the same in the space station as the earth's surface (14.7 pounds per square inch). Back on Earth, as elevation increases, the number of molecules decreases and the density of air therefore is less, meaning a decrease in air pressure. In fact, while the atmosphere extends more than 15 miles (24 km) up, one half of the air molecules in the atmosphere are contained within the first 18,000 feet (5.6 km). Because of this decrease in pressure with height, it makes it very hard to compare the air pressure at ground level from one location to another, especially when the elevations of each site differ. Therefore, to give meaning to the pressure values observed at each station, we convert the station air pressures reading to a value with a common denominator. The common denominator we use is the sea-level elevation. At observation stations around the world the air pressure reading, regardless of the observation station elevation, is converted to a value that would be observed if that instrument were located at sea level. The two most common units in the United States to measure the pressure are "Inches of Mercury" and "Millibars". Inches of mercury refers to the height of a column of mercury measured in hundredths of inches. This is what you will usually hear from the NOAA Weather Radio or from your favorite weather or news source. At sea level, standard air pressure is 29.92 inches of mercury. Millibars comes from the original term for pressure "bar". Bar is from the Greek "báros" meaning weight. A millibar is 1/1000th of a bar and is approximately equal to 1000 dynes (one dyne is the amount of force it takes to accelerate an object with a mass of one gram at the rate of one centimeter per second squared). Millibar values used in meteorology range from about 100 to 1050. At sea level, standard air pressure in millibars is 1013.2. Weather maps showing the pressure at the surface are drawn using millibars. How temperature effects the height of pressure. Although the changes are usually too slow to observe directly, air pressure is almost always changing. This change in pressure is caused by changes in air density, and air density is related to temperature. Warm air is less dense than cooler air because the gas molecules in warm air have a greater velocity and are farther apart than in cooler air. So, while the average altitude of the 500 millibar level is around 18,000 feet (5,600 meters) the actual elevation will be higher in warm air than in cold air. Crunch Time Overview Pressure is not only a matter of altitude but also is dependent upon the temperature. As the temperature increases so does the pressure. The molecules and atoms that comprise the air we breath gain energy as they absorb heat. That increase in energy results in faster moving atoms which we observe as an increase in energy. The opposite occurs when the temperature decreases. As the molecules lose energy, their motion is decreased and we observe a decrease in pressure. The students will see a plastic 2-liter bottle crushed by the normal atmospheric pressure in the room by this decrease in pressure. Procedure Place two cups of hot tap water into each two 2-liter bottle. Place your thumb over each bottle opening and shake. This ensures the air inside the bottle is warmed. Pour the water out of each bottle and screw a bottle cap on only one of the two bottles. Stand both bottles side-by-side and observe over the next five minutes. Discussion The bottle that was capped will eventually begin to collapse. This is a result of the cooling air inside that bottle. The air cools because the molecules and atoms inside the bottle lose energy as they collide with the bottle side that is exposed to the cooler surrounding air. As their energy decrease so does their velocity and therefore the pressure decreases. Since the pressure inside the bottle decreases, the force of the air outside the bottle begins to crush the bottle. However the uncapped bottle remains unchanged. As the air cools inside, the drier outside air flows in to take up the space thereby keeping the pressure the same both inside and outside of the bottle. Live Weatherwise There was a time the National Weather Service advised, when a tornado was approaching, to open a window in the house to equalized the pressure inside and out. The thought was high pressure trapped inside a house caused to blowup due a rapid fall of pressure outside resulting from the tornadic winds. Research has shown the pressure difference is only about 10% and houses can handle this as there are vents in bathrooms and kitchens to relieve the pressure. What destroys a house is debris slammed into the structure by the force of the wind. Therefore, if a tornado approaches your house, LEAVE THE WINDOWS ALONE! The tornado will open them for you as debris hits the glass. Instead seek shelter in the interior of your dwelling, on the lowest floor, away from windows, in order to place as much protection between you and flying debris possible. . The H's represent the location of the area of highest pressure The L's represent the position of the lowest pressure. The most basic change in pressure is the twice daily rise and fall in due to the heating from the sun. Each day, around 4 a.m./p.m. the pressure is at its lowest and near its peak around 10 a.m./p.m. The magnitude of the daily cycle is greatest near the equator decreasing toward the poles. On top of the daily fluctuations are the larger pressure changes as a result of the migrating weather systems. These weather systems are identified by the blue H's and red L's seen on weather maps.
Lesson 1
Lesson 2
Lesson 3
Lesson 4
Lesson 5








		Measure the Pressure - The "Wet" Barometer Overview The amount of air over us is constantly changing. As a result, the weight of that air, called pressure, is constantly changing. These changes in air pressure are indications of changes in our weather. We measure this change using a device called barometer (bar-meter or measurer). This first barometer was created by Evangelista Torricelli in 1643. Torricelli was actually trying to discover the reason that water would rise no more than 33 feet up a tube though the use of a suction pump. He had first built a water barometer, but it required a glass tube 60 feet long. Aware that mercury was 14 times heavier than water, he constructed a tube only 35 inches long. Filing the tube with mercury and inverting the tube into a bowl of mercury caused mercury in the tube to drop to a level around 30 inches and creating a vacuum at the top of the tube. The top of the mercury column was observed to fluctuate by a few percent, due mainly to what we now know to be fluctuations in atmospheric pressure. This is because as the column of air directly above the barometer pushes on a dish containing mercury, it is forced up a tube. The stronger the downward push, the higher the pressure and therefore the higher the mercury rises in the tube. This is where the units "Inches of Mercury" are derived. Procedure Place the ruler in the glass and tape it to one side. (Make sure the numbers are visible.) Tape the plastic tube onto the ruler in the glass. (Make sure the tube is not touching the bottom of the glass.) Fill the glass about half way with water. Add a drop or two of food coloring and mix thoroughly. Using the tube like a straw, draw some water about 2/3^rds into the straw. Using your tongue, trap the water in the tube then cap the end of the tube with model clay or chewing gum. Record the height of the water in the tube. At the same time every day, for the next 5-10 days, record the height of the water in the tube, paying close attention to the change in the weather as the water level changes. Discussion What the students have constructed is a water barometer (also known as "storm glass") and these types of barometers date from the 17th century. The actual change in pressure will occur too slow for direct observation. Usually, only for a 24 hour period will the change in pressure be most noticeable. As a storm approaches, the mass of air around your location decreases. Therefore, the pressure decreases as well. After a cold front passes your location, higher pressure moves in and the students will see the pressure rise. Over and above the pressure changes associated with storms, there are four daily pressure fluctuations in the atmosphere. These diurnal changes are due to the sun heating the atmosphere. The amplitudes of this daily change depend upon the latitude, season, and altitude. The changes are greatest at the equator, decreasing toward the poles where it becomes zero. Also, the higher the altitude, the greater the daily change.
	Live Weatherwise Rapid falls in pressure can indicate the approach of severe thunderstorms. A Severe thunderstorm WARNING is an urgent announcement that a severe thunderstorm has been reported or is imminent and warns you to take cover. Severe thunderstorm warnings are issued by local National Weather Service offices. The strong wind gusts of severe thunderstorms can damage buildings, knock down trees, and create a hazard due to wind-blown debris. Therefore: Seek shelter but avoid trees as these are targets for lightning. If indoors, stay away from windows and go to the safest location on the lowest level of your home. When boating, always stay tuned to the latest weather reports and return to a safe harbor before the strong winds arrive. How are changes in weather related to changes in pressure? From his vantage point in England in 1848, Rev. Dr. Brewer wrote in his A Guide to the Scientific Knowledge of Things Familiar the following about the relation of pressure to weather: The FALL of the barometer (decreasing pressure) In very hot weather, the fall of the barometer denotes thunder. Otherwise, the sudden falling of the barometer denotes high wind. In frosty weather, the fall of the barometer denotes thaw. If wet weather happens soon after the fall of the barometer, expect but little of it. In wet weather if the barometer falls expect much wet. In fair weather, if the barometer falls much and remains low, expect much wet in a few days, and probably wind. The barometer sinks lowest of all for wind and rain together; next to that wind, (except it be an east or north-east wind). The RISE of the barometer (increasing pressure) In winter, the rise of the barometer presages frost. In frosty weather, the rise of the barometer presages snow. If fair weather happens soon after the rise of the barometer, expect but little of it. In wet weather, if the mercury rises high and remains so, expect continued fine weather in a day or two. In wet weather, if the mercury rises suddenly very high, fine weather will not last long. The barometer rises highest of all for north and east winds; for all other winds it sinks. The barometer UNSETTLED (unsteady pressure) If the motion of the mercury be unsettled, expect unsettled weather. If it stands at "MUCH RAIN" and rises to "CHANGEABLE" expect fair weather of short continuance. If it stands at "FAIR" and falls to "CHANGEABLE", expect foul weather. Its motion upwards, indicates the approach of fine weather; its motion downwards, indicates the approach of foul weather. These pressure observations hold true for many other locations as well but not all of them. Storms that occur in England, located near the end of the Gulf Stream, bring large pressure changes. In the United States, the largest pressure changes associated with storms will generally occur in Alaska and northern half of the continental U.S. In the tropics, except for tropical cyclones, there is very little day-to-day pressure change and none of the rules apply.
	The "Dry" Barometer Overview Barometers using mercury are heavy and fragile. The idea of "dry" barometer was conceived by Gottfried Wilhelm Leibniz around 1700. The idea was to detect pressure changes using sealed bellows. The first working version of an aneroid (without water) barometer was built in 1843 by French scientist Lucien Vidie. This made the barometer very portable and it became commonly use meteorological instrument. It was still calibrated to the mercurial barometer with readings in inches of mercury. Even as late at the 1990s, National Weather Service offices still calibrated and verified the accuracy of the aneroid barometer with the mercurial barometer. Using simple items the student will make a device for indicating air pressure changes, called an aneroid barometer. Procedure Cover the top of the coffee can tightly with the plastic wrap, using the rubber band to hold it in place. (The cover should be a taut, airtight fit.) Position the straw so that it lays across two thirds of the cover with the remaining length of the straw suspended over air. Tape in place. Fold one short end of the index card, about one inch from that end, at a 90° angle. Tape the folded end of the index card to the can behind the straw in such a way that allows you to make marks on the card every day. Record the level of the straw onto the card. For the next 10 days, at the same time each day, record the level of the straw while paying close attention to how changes in the weather affect the straw's level. Discussion The basic function of an aneroid barometer What the students have constructed is similar to an aneroid barometer. It is the most common type of barometer for home use. The aneroid cell volume is very sensitive to changes in atmospheric pressure as it expands and contracts as air pressure decreases or increases. Attached to the aneroid cell is a lever indicating the air pressure. In this case, the aneroid cell is the coffee can. In this barometer, high pressure in the atmosphere will weigh more the pressure inside the can at the time the barometer was constructed. That added weight will force the plastic wrap into the can, causing the straw tip to rise, indicating higher pressure. The opposite will occur when low pressure is in the area. The decrease in weight of air on top of the can will help cause the plastic wrap to rise, therefore lowering the straw tip. Today, even with sensitive electronic sensors having replaced the metal aneroid cells in most barometers, those electronic sensors still need to be calibrated to ensure their accuracy. For that calibration, we still use mercurial barometers. Live Weatherwise One measure of the severity of a thunderstorm is the wind speed. In addition to the size of hail, the National Weather Service defines a severe thunderstorm as one containing wind speed of 58 mph (50 kts / 93 km/h) or greater. The force of all of the molecules moving at 58 mph (50 kts / 93 km/h), or more, can create hazardous weather conditions such a blowing down phone and power lines, trees, and make driving hazardous. When the National Weather Service issues a Severe Thunderstorm Warning it means a thunderstorm with wind gusts to 58 mph (50 kts / 93 km/h) or greater and/or hail size of 1" (2.5 cm) or greater is occur or about to occur near you. Discuss severe thunderstorm safety with your family. Everyone should know what to do in case all family members are not together. Discussing disaster response ahead of time helps reduce fear and lets everyone know what to do should a severe thunderstorm occur. Postpone outdoor activities if thunderstorms are likely. Many people take shelter from the rain, but most people struck by lightning are not in the rain! Postponing activities is your best way to avoid being caught in a dangerous situation.
	Heavy Air To show that air has weight, the air is removed from one of two balanced balloons throwing the balance off. Procedure Inflate two balloons so they are the same size. Tape one balloon to each end of the yard/meter stick. Tie a string to the center of the stick and adjust it so the stick balances when held by the string. Tape the sting in place to prevent it from slipping. Ask the students, "If one end were heavier, would the heavier end move up or down?" Carefully deflate the other balloon. Try poking the balloon with a pin in its neck to prevent the balloon from tearing apart as it pops. Let both balloons hag freely on the yard/meter stick. Ask the students to explain what happens to the balance. Discussion Air is all around us. This air is composed of atoms and molecules. Despite their small size, the quantity of atoms and molecules exert weight on us known as pressure. Since our bodies are designed to live in this environment, we do not notice the pressure. Since the inflated balloon now weighs more than the deflated one (due to the air inside of the balloon) it will sink creating an imbalance. Now, imagine the weight of air if that balloon were now 15 miles (24 km) tall. That is actually what is occurring at this moment in your classroom. When we measure air pressure with a barometer, we are measuring the weight of a column of air 15 miles (24 km) high directly over us. Live Weatherwise The weight of molecules also affects the weather. One measure of the severity of a thunderstorm is the wind speed. In addition to the size of hail, the National Weather Service defines a severe thunderstorm as one containing wind speed of 58 mph (50 kts / 93 km/h) or greater. The weight of all of the molecules in wind of 58 mph (50 kts / 93 km/h) is the force that can create hazardous weather conditions such a blowing down phone and power lines, trees, and make driving hazardous. When the National Weather Service issues a Severe Thunderstorm Warning it means a thunderstorms with wind gusts to 58 mph (50 kts / 93 km/h) or greater and/or hail size of 1" (2.5 cm) or greater is occur or about to occur near you. Discuss severe thunderstorms with your family. Everyone should know what to do in case all family members are not together. Preparing for a disaster ahead of time helps reduce fear and lets everyone know how to respond during a severe thunderstorm. Take an American Safety Council or American Heart Association first aid and CPR course to learn how to treat burns and how to give rescue breathing and administer CPR. Everyone should know how to respond, because severe weather can strike almost anywhere in the country. Postpone outdoor activities if thunderstorms are likely. Many take shelter from the rain, but most who are struck by lightning are not in the rain! Postponing activities is your best way to avoid being caught in a dangerous situation.
	A Pressing Engagement Overview We typically do not "feel" atmospheric air pressure. Why? Since air surrounds our bodies, and all things, the pressure, as a result of the air, is applied equally on all sides. For example, if someone holds an 8½x11" sheet of paper by their hand at arms length, the weight of the air directly above the sheet is over 1,300 pounds. Obviously the paper does not weight that much. Why? That same pressure (14.7 pounds per square inch) is also pressing up on the bottom side of the paper. The equal pressure on all sides cancel each other out so all that is left is the weight of the material that comprises the paper. Since we do not normally "feel" air pressure, the student will see how the effect of the air pressure on two sheets of paper.
	Procedure Lay a ruler on a table with about 3" (8 cm) hanging over the edge. Lay a sheet of printer paper on the part of the ruler in direct contact with the table. Press the paper against the table until it is flat as possible. Press down on part of the ruler hanging over the edge. Repeat the above steps except replace the printer paper with a large sheet of opened newspaper in the second step. Discussion The student will discover the newspaper was much harder to lift than the printer paper. As the ruler lifted the printer paper, air rush in under the rising paper and thereby quickly allowed the air pressure to equalize on all sides. Essentially, the weight of the air above the paper had no effect on the difficulty in lifting the paper. As the ruler lifted the newspaper, the edges of the newspaper remained in contact with the desk. Very little air was allowed to rush in and equalize the pressure on the bottom side of the newspaper. Since there is less air below the paper the pressure is less as well. Now the weight of all the air above the paper now becomes more evident. Live Weatherwise The weight of molecules also affects the weather. One measure of the severity of a thunderstorm is the wind speed. In addition to the size of hail, the National Weather Service defines a severe thunderstorm as one containing wind speed of 58 mph (50 kts / 93 km/h) or greater. The weight of all of the molecules in wind of 58 mph (50 kts / 93 km/h) is the force that can create hazardous weather conditions such a blowing down phone and power lines, trees, and make driving hazardous. When the National Weather Service issues a Severe Thunderstorm Warning it means a thunderstorms with wind gusts to 58 mph (50 kts / 93 km/h) or greater and/or hail size of 1" (2.5 cm) or greater is occur or about to occur near you. Discuss severe thunderstorms with your family. Everyone should know what to do in case all family members are not together. Preparing for a disaster ahead of time helps reduce fear and lets everyone know how to respond during a severe thunderstorm. Take an American Red Cross first aid and CPR course to learn how to treat burns and how to give rescue breathing and administer CPR. Everyone should know how to respond, because severe weather can strike almost anywhere in the country. Postpone outdoor activities if thunderstorms are likely. Many take shelter from the rain, but most who are struck by lightning are not in the rain! Postponing activities is your best way to avoid being caught in a dangerous situation.
	Going with the Flow Overview Bernoulli's principle states that in fluid flow, an increase in velocity occurs simultaneously with decrease in pressure. The students will discover that the faster air moves (air acting as a fluid), the lower the pressure becomes within that flow of air. They will see this effect blowing between two soda cans. Procedure Lay the two cans parallel to each other, about one inch apart, near the edge of a level surface. Put your face down near the surface and blow lengthwise between the two cans. It will take some trial and effort but eventually the two cans will roll together. Another way of demonstration is by suspending two cans with string about an inch apart and having the student blow between them. Discussion The affect is Bernoulli's principle in action, named after the Dutch/Swiss mathematician/scientist Daniel Bernoulli. By blowing between the two cans, you are making the air between them move faster than the surrounding air (which is basically calm). The cans roll together as the higher pressure surrounding the two cans (away from the air flow) pushes the cans together toward the region of lower pressure. Live Weatherwise There is no such thing as guaranteed safety inside a tornado. Freak accidents happen; and the most violent tornadoes can level and blow away almost any house and its occupants. Extremely violent EF5 tornadoes are very rare, though. Most tornadoes are actually much weaker and can be survived. Prevention and practice before the storm: At home, have a family tornado plan in place, based on the kind of dwelling you live in and the safety tips below. Know where you can take shelter in a matter of seconds, and practice a family tornado drill at least once a year. Have a pre-determined place to meet after a disaster. Flying debris is the greatest danger in tornadoes; so store protective coverings (e.g., mattress, sleeping bags, thick blankets, etc) in or next to your shelter space, ready to use on a few seconds' notice. When a tornado watch is issued, think about the drill and check to make sure all your safety supplies are handy. Turn on local TV, radio or NOAA Weather Radio and stay alert for warnings. Forget about the old notion of opening windows to equalize pressure; the tornado will blast open the windows for you! If you are out shopping, know the locations the store's bathrooms, storage rooms or other interior shelter areas away from windows, and the quickest to get there should a tornado strike. All administrators of schools, shopping centers, nursing homes, hospitals, sports arenas, stadiums, mobile home communities and offices should have a tornado safety plan in place, with easy-to-read signs posted to direct everyone to a safe, close by shelter area. Schools and office building managers should regularly run well-coordinated drills. If you are planning to build a house, especially east of the Rockies, consider an underground tornado shelter or an interior "safe room
	The Transfer of Heat Energy The heat source for our planet is the sun. Energy from the sun is transferred through space and through the earth's atmosphere to the earth's surface. Since this energy warms the earth's surface and atmosphere, some of it is or becomes heat energy. There are three ways heat is transferred into and through the atmosphere: radiation conduction convection Radiation If you have stood in front of a fireplace or near a campfire, you have felt the heat transfer known as radiation. The side of your body nearest the fire warms, while your other side remains unaffected by the heat. Although you are surrounded by air, the air has nothing to do with this transfer of heat. Heat lamps, that keep food warm, work in the same way. Radiation is the transfer of heat energy through space by electromagnetic radiation. Most of the electromagnetic radiation that comes to the earth from the sun is invisible. Only a small portion comes as visible light. Light is made of waves of different frequencies. The frequency is the number of instances that a repeated event occurs, over a set time. In electromagnetic radiation, its frequency is the number of electromagnetic waves moving past a point each second. Our brains interpret these different frequencies into colors, including red, orange, yellow, green, blue, indigo, and violet. When the eye views all these different colors at the same time, it is interpreted as white. Waves from the sun which we cannot see are infrared, which have lower frequencies than red, and ultraviolet, which have higher frequencies than violet light. It is infrared radiation that produce the warm feeling on our bodies. Most of the solar radiation is absorbed by the atmosphere and much of what reaches the earth's surface is radiated back into the atmosphere to become heat energy. Dark colored objects, such as asphalt, absorb radiant energy faster that light colored objects. However, they also radiate their energy faster than lighter colored objects. Melts in your bag, not in your hands Overview The earth receives its heat from the sun in the form of radiation. Many in the animal kingdom lay out in the sun to absorb this form of energy to warm their bodies. This form of energy is vital to life on this planet. The student will learn how the sun transfers heat to the earth through radiation. Procedure Place one piece of chocolate in a bag, seal it and label it with an 'A'. Do the same with the second piece of chocolate but label that bag with a 'B'. Take both bags outside and place bag 'A' in the sun and bag 'B' in the shade. Suspend bag 'A' in such a way to ensure is not touching the ground or located near a wall to limit any transfer of heat by convection or conduction. (If this experiment is done indoors, place bag 'A' in the window, exposed to the sun and keep bag 'B' in the shade.) 20 minutes later, inspect the chocolate in both bags. Ask the students to explain any change in consistency of the chocolate. Discussion Depending on how hot it is, the chocolate in the shade may also be softened or even partially melted. However, the chocolate in bag 'A' will be more melted. The bulk of the heating that takes place in bag 'A' is from direct solar radiation. This radiation is what causes objects, such as the metal on automobiles, to become hot. Radiation also causes sunburns. Live Weatherwise Melanoma is the most serious type of skin cancer, and accounts for more than 75% of the deaths due to skin cancer. In addition to skin cancer, sun exposure can cause premature aging of the skin, wrinkles, cataracts, and other eye problems. When you play outdoors, there are five important steps you can take to protect against UV radiation and skin cancer: Cover up. Wear clothing to protect as much of your skin as possible. Wear clothing that does not transmit visible light. To determine if clothing will protect you, try this test: Place your hand between the fabric and a light source. If you can see your hand through the fabric, the garment offers little protection against sun exposure. Use a sunscreen. Experts recommend products with a Sun Protection Factor, or SPF, of at least 15. The SPF number represents the level of sunburn protection provided by the sunscreen. Products labeled "broad spectrum" block both UVB and UVA radiation. Both UVA and UVB contribute to skin cancer. Wear a hat. A wide-brimmed hat is ideal because it protects the neck, ears, forehead, nose and scalp. A baseball cap provides some protection for the front and top of the head, but not for the back of the neck or the ears where skin cancers commonly develop. Wear sunglasses. UV-absorbent sunglasses can help protect your eyes from sun damage. Ideal sunglasses do not have to be expensive, but they should block 99 to 100 percent of UVA and UVB radiation. Check the label to make sure they do. Darker glasses are not necessarily the best. UV protection comes from an invisible chemical applied to the lenses, not from the color or darkness of the lenses. Limit direct sun exposure. UV rays are most intense when the sun is high in the sky, between 10 a.m. and 4 p.m. If you are unsure about the sun's intensity, take the shadow test: If your shadow is shorter than you, the sun's rays are the strongest. Seek shade whenever possible. Remember: There is NO SAFE WAY TO TAN, including the use of tanning beds. Conduction Conduction is the transfer of heat energy from one substance to another or within a substance. Have you ever left a metal spoon in a pot of soup being heated on a stove? After a short time the handle of the spoon will become hot. This is due to transfer of heat energy from molecule to molecule or from atom to atom. Also, when objects are welded together, the metal becomes hot (the orange-red glow) by the transfer of heat from an arc. This is called conduction and is a very effective method of heat transfer in metals. However, air conducts heat poorly. Convection Convection is the transfer of heat energy in a fluid. This type of heating is most commonly seen in the kitchen when you see liquid boiling. Air in the atmosphere acts as a fluid. The sun's radiation strikes the ground, thus warming the rocks. As the rock's temperature rises due to conduction, heat energy is released into the atmosphere, forming a bubble of air which is warmer than the surrounding air. This bubble of air rises into the atmosphere. As it rises, the bubble cools with the heat contained in the bubble moving into the atmosphere. As the hot air mass rises, the air is replaced by the surrounding cooler, more dense air, what we feel as wind. These movements of air masses can be small in a certain region, such as local cumulus clouds, or large cycles in the troposphere, covering large sections of the earth. Convection currents are responsible for many weather patterns in the troposphere