Everybody knows that hot air rises and cool air falls, which is why it's always a bit puzzling to hear the captain's first announcements on a commercial flight. After you get the obligatory "thanks for flying" speech, the captain announces that the plane has reached cruising altitude of, say, 35,000 feet and the weather outside is something like -55 degrees Celsius (that's about -67 Fahrenheit). If hot air rises, shouldn't it be super warm when you're up that high. That's what you'd think, but there are a few scientific forces at work making the air temperature fall as your plane cruises higher.
To understand why air cools as you go up in elevation, you have to know a few things about "atmospheric science," the study of the blanket of air covering the Earth. The first thing you need to know is that the air in Earth's atmosphere is made up of transparent, compressible gases. While you may not be able to see them, they still have mass. They're made up of tiny units called molecules. Gas molecules are spaced pretty far apart, making gases much less dense than liquids or solids. Since gas molecules have a lot of space in which to move around freely, they do just that. This space between molecules explains why air is able to expand and contract dramatically.
Pressure is responsible for determining how the molecules in air move around. When pressure is high, gas contracts. But when pressure is low, gas is free to expand. And when it does, the molecules within it move more slowly.
The last thing you need to know is that temperature is really a measure of how fast molecules are moving within the air. The faster they move, the higher the temperature. On the ground, pressure is higher because gravity is constantly pulling everything -- including air -- toward the Earth's surface. Since there's more pressure closer to the ground, air moves faster and is warmer down there. At high elevations, pressure is low and air moves slower.
You should also know that it's not just pressure that affects air temperature. Solar radiation bouncing off the Earth also makes an impact. The farther away you get from the warm ground, the cooler the air gets. Now you can understand why the air outside your plane gets colder at 35,000 feet: It's pressure and radiation at play.
It's still true that hot air rises and cool air falls. But that's a whole different can of worms.
It's easy to notice a difference in air temperature based on altitude; it might be 81 degrees Fahrenheit (27 degrees Celsius) in the city on a mild late summer day, but 68 degrees Fahrenheit (20 degrees Celsius) on the mountaintop just a few miles outside of town.
The reason for this is that the Earth absorbs some radiation from the sun and emits its own radiation. Solar radiation weakens as it passes through the atmosphere toward Earth, and terrestrial radiation is absorbed by the water and carbon dioxide in the air. Because of this, the further up you go, the less radiation is still in the air, resulting in colder temperatures despite your increased proximity to the sun.
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