Climate Change: Winners and Losers
Lesson 4: Desert Belts
Page 1 Page 2: Moving Air Page 3 Review
Moving Air:
Let's
do a couple of experiments. In the first, place a balloon over the
mouth of a water bottle.
In a saucepan, heat some water on the stove
until it is just about boiling. In another pan or bowl, add some water
and ice. Start by placing the bottle with the balloon in the hot
water. What happens to the balloon?
The balloon stands up. The air inside the bottle is expanding because it is warming up.
Now place the bottle in the cold water. What happens?
The balloon collapses. The air contracts as it cools.
In the top photo the volume of air increased as the air warmed up. In
the second photo the volume decreased when it cooled. But even though
the volume changed, the amount of air was the same in both photos--no
air entered or escaped from the bottle. So the amount of air did not
change, but the density changed as the air heated or cooled. This
illustrates a very important point about the air in our atmosphere:
warmer air is less dense than cooler air. Warmer air tends to rise up
into the atmosphere; cooler air tends to sink back down to Earth.
There is one more important point that we need to consider. Look at the
glass jar sitting on the stove next to the saucepan in the
photo below:
The air surrounding the jar is at room temperature. Notice that the jar is completely dry.
But watch what happens to the jar when, after turning off the flame below the saucepan, I hold the jar above the saucepan:
The water in the
saucepan is still very hot, but it is not boiling, and there is no
visible steam. Yet the outside of the jar becomes covered with
moisture. Why is this?
The air above the saucepan has heated up. This warm air is less dense
than the room-temperature surrounding it, so it rises. As it rises, it
carries moisture from the pan up with it. The jar, however, is still at
room temperature, so when the rising, moist air hits the jar, it cools
off. As the air cools, it loses its ability hold moisture, and so the
moisture in the air condenses into tiny water droplets that collect on
the surface of the jar. This illustrates a second key point about the
air in our atmosphere: warmer air can hold more moisture than cooler
air.
Now that we know these two important principles about moving air, let's go back and look at the desert belts.
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