It's a summer morning in the Bahamas (we snow-weary New Englanders like to dream), and you're at the beach catching rays under a hot sun. Slowly, however, clouds form. By the middle of the afternoon, thunder rumbles, and you're running to keep from getting soaked with rain. Why did those clouds have to form to drench your day on the dunes?

With a little help from an adult, you can find the answer in a soda-pop bottle.

Factoid 1: When you are lying on that beach, from head to toe you are enduring 4 to 5 tons of pressure from the air directly above you. And you didn't feel a thing, did you? (P.S. Don't try this with an elephant!) If you draw a square that is one foot on each side, at sea level the pressure on that square from an imaginary column of air rising directly from the page to the top of the atmosphere is slightly more than a ton. This air pressure shrinks as you climb higher into the atmosphere, because there is less air above you.

Factoid 2: Changes in pressure can change the air's temperature.

To see how, find a clear plastic 2-liter soda bottle and a temperature strip from a pet or aquarium supply store. Bend the temperature strip slightly along its length by taping it to a piece of paper slightly shorter than the strip. Put it in the 2-liter bottle, snugly fasten the cap, and squeeze the bottle in the middle by bending it over a table. What happens to the temperature? It rises. Indeed, whenever a gas is squeezed, it heats up. After a minute, let the bottle return to its normal shape, which reduces the pressure. What happens? The air temperature drops again.

It's now time to make that cloud. Warning: It will not be like the white puffs you see in the sky.

Leave the thermometer in the bottle and add a few drops of water. Swirl them around to wet the bottle's sides. Put the cap on the bottle, and let it sit for a few minutes to let the water evaporate into the air inside.

Once again lay the bottle on its side, remove the cap, squeeze the bottle to about half its normal diameter. While you're doing this, have someone light a match. Blow out the match, and while it's still smoking, put it in the bottle. Quickly put the cap back on and twist it tight.

Now, squeeze the bottle again against the edge of the table, release it, and watch what happens to the temperature. After you release the bottle, look carefully at the bottle for changes in visibility. When you see those changes, you've found your cloud. (If you can't see the changes, try squeezing and releasing the bottle under different lights.)

So how does all this relate to what happened on the beach? During the early part of the day, sun warmed the air (you squeezing the bottle to raise the temperature). The air also was picking up large amounts of moisture from the ocean through evaporation (you adding the drops of water to the bottle). The warming air began to rise, it began to rise higher into the atmosphere, where the air pressure was less (you taking the pressure off the bottle). The warm air expanded, which cooled it. As it cooled, the moisture it carried condensed around tiny dust particles in the air (smoke from the match). Voila! Clouds, rain, and a spoiled day at the beach.

These effects not only happen on a local level. They also happen on a larger scale, fueled by big areas of high and low pressure in the atmosphere. Like water in a pan, the amount of Earth's atmosphere doesn't change much these days. But when you swirl the water in the pan with a spoon, the water piles up in some spots and develops depressions in others.

The atmosphere does much the same. ''Mixed'' by constantly rising warm air at the equator (which sinks again at the poles) and by Earth's rotation, our envelope of air develops hills and valleys - areas of high pressure and low pressure. The air piled up in a zone of high pressure wants to flatten out. So it tries to fill the low pressure ''valleys.''

This generates winds that tend to push air in the valleys higher into the atmosphere. This air rises, cools, and moisture condenses into clouds. That's why weathercasters on TV point to the big L's (for low pressure) on the weather map and talk about storms, while the big H's bring generally clear skies.

And all from a cloud in a bottle!

* This project is part of the American Meteorological Society's Project ATMOSPHERE. For more information, call the AMS at 202-466-5728.

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