Why does the atmospheric temperature drop with increased altitude, when we're actually moving closer to the sun?

by | May 29, 2009 02:00 PM ET

You Asked:

Why does the atmospheric temperature drop with increased altitude, when we're actually moving closer to the sun? --- Sethu, Chennai, India

Marshall Answered:

Let's say you get in a small airplane and you fly from sea level to 10,000 feet. If it is daylight outside, it is true that you are now 10,000 feet closer to the sun. But proportionally speaking, you aren't really that much closer because the sun is 92 million miles away from the earth. 10,000 feet (roughly 2 miles) in insignificant compared to 92 million miles.

The much bigger effect is coming from atmospheric cooling. Let's say you were to take a balloon with you on your airplane ride. At sea level the balloon is 1 foot in diameter and the air pressure is 14.4 PSI. At 10,000 feet the air pressure has dropped to 10 PSI and the balloon has gotten significantly bigger.

When air expands like that, it cools down. In our atmosphere, air cools by about 6 degrees C per 1,000 meters (or 4 degrees F per 1,000 feet). When you get to 10,000 feet, it is 40 degrees F cooler than it was on the ground. When you are flying in a passenger jet at 32,000 feet, the temperature outside the airplane is -50 degrees F.

This temperature change is important to the weather. The sun warms the earth during the day. Warm air near the earth rises. As it rises it cools, and the cooling causes water vapor in the air to condense. This is where clouds and rain come from.

For more info see: How earth works.

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