The atmosphere is the air that we breathe. It is also the wind that blows through our hair and the clouds that float in the sky. All the weather that takes place is part of the atmosphere. The atmosphere surrounds us at every moment, keeping us alive and warm. It is the part of the Earth system that we are most familiar with; so familiar that we often take it for granted. Without the atmosphere, we would perish within minutes.

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There are a few important properties that have to do with the atmosphere. The first is temperature. That is simply how hot or cold the air is. During the day, the atmosphere is warmed by the sun, so the temperature goes up. At night, after the sun goes down, the atmosphere cools and the temperature drops.

Larger version (Credit: NOAA Central Library)

Another property is called pressure. Pressure is how thick or dense the air is. The atmosphere is several kilometers thick. That means there is a lot of air above us, and it's all pushing down on top of us. On the surface, pressure is usually pretty constant from one place to another. But, often there are small differences, and these differences are what lead to all the weather we experience. When two neighboring regions are at different pressures, they are out of balance with each other. The atmosphere produces winds and storms which try to restore the balance.

The atmosphere is the most dense near the surface because of gravity. As one goes higher, the air gets thinner and thinner. At an altitude of about ten kilometers, the atmosphere is only half as thick as at the surface. That's why air tanks are needed to climb Mt. Everest (nine kilometers high) and commercial airliners must be pressurized (flying at a height of about ten kilometers).

The last important property of the atmosphere is humidity. Humidity is a measure of how much water is in the atmosphere. When water evaporates from lakes and oceans, it turns into a gas and becomes part of the atmosphere. It is this water that sometimes condenses into clouds and becomes rain.