Why Does the Air Keep Getting Colder As We Go Higher In Altitude?

The air does keep getting colder, by an average of about 3.6 degrees Fahrenheit for every thousand feet (6.5 degrees Celsius per kilometer) , up to around 33,000 feet (10,000 meters) above sea level.

That’s just a bit higher than the cruising altitude of large commercial jet aircraft. You may have heard the airliner’s captain try to impress you when flying at that altitude by announcing that the temperature outside your flimsy-looking window was something like 40 degrees below zero Fahrenheit (−40 degrees Celsius). Good thing the window is double-pane-insulated plastic.

Above about 33,000 feet (10,000 meters), you’re in the stratosphere, where the air stops getting colder as you go higher; it stays roughly constant at about −55 degrees Fahrenheit (−48 degrees Celsius) for the next 12 miles (20 kilometers) or so, and then starts getting warmer. Above the stratosphere, the temperature does a couple of other flip-flops, getting first colder and then warmer again.

What’s going on?

For one thing, the air has somewhat different chemical compositions at different altitudes. The heavier molecules (carbon dioxide, argon) tend to settle out toward the bottom of the atmosphere, while the lighter ones (helium, neon) tend to rise to the top of the pile. Because those different molecules absorb the sun’s energies in different ways, they may heat up differently.

The stratosphere, for example, is where most of the ozone molecules live. Ozone absorbs a lot of the sun’s ultraviolet (very short-wave) radiation, which heats it up and makes the stratosphere warmer than it otherwise would be. Earth’s atmosphere is really quite a complicated system.

Beyond the atmosphere? You’ve heard that the temperature in outer space is extremely cold, haven’t you? Well, it isn’t.

It’s Not the Cold, It’s the Humidity

I’ve often heard people say that it’s too cold to snow. Is there ever any truth to that?

It’s true that when it’s very cold it won’t snow, but the statement is misleading. Once the temperature gets below freezing and other conditions are right for snow, the will-it-or-won’t-it question is purely a matter of the availability of water vapor.

In most cases, in order for it to snow there must first be tiny droplets of liquid water in the air that can freeze into snowflakes. But when the accessible supply of water is very cold, it strongly prefers to stay where it is, namely in the liquid form, so it doesn’t contribute much water vapor to the air. Thus, at very low temperatures there just isn’t enough water in the air to form those tiny droplets that could freeze and fall as snow.

Of course, if it has been very cold for some time, most of the local water supplies will be inaccessible for vapor production anyway because they’re frozen.

In those National Geographic pictures of blinding, whiteout blizzards in the Antarctic, it’s not snowing, it’s blowing. Very strong winds are blowing around loose, already-fallen snow. And when did that already-fallen snow fall? During periods of milder temperatures (but still below freezing), when water vapor was more abundant.