Five Principal Layers of the Atmosphere

My team came in second in last night's pub quiz at the Liberal Cup. We often come in second, and in fact, coming in second is a recurring theme of my life. But we did that well only because one of my teammates remembered the name of the atmospheric layer closest to the earth.

This is something I learned in Mrs. White's fifth grade science class, and have never needed to know since. After last night's quiz, though, I looked it all up again — and since the atmosphere turns out to have five principal layers, I'm sharing this regained knowledge with you.

1. Troposphere. This was the answer Joe knew, which I did not remember; it is the layer of the atmosphere that starts at the surface of the earth, and extends to about four-and-a-half miles up, at the poles, or ten miles up at the Equator. The troposphere is the densest part of the Earth's atmosphere; since it holds warmth from the planet's surface, it gets colder as altitude increases.

2. Stratosphere. Bruce Springsteen knows: "I spent month-long vacations in the stratosphere/where you know it's really hard to hold your breath." Where the troposphere ends — about 4.3 miles from sea level — humans don't have enough oxygen to live, which is why mountaineers call this the death zone. (The summit of Mt. Everest, at 29,029 feet above sea level, is approximately 5.5 miles high.) The end of the troposphere is called the tropopause, and the stratosphere extends from the tropopause to approximately 32 miles above the Earth's surface. In the stratosphere, temperature increases with altitude, and pressure is only 1/1000th of that at sea level.

3. Mesosphere. The mesosphere starts at the stratopause and goes up to between 50 and 53 miles above the Earth's surface. In the mesosphere, the atmosphere starts cooling down again, and the outer edge of the mesosphere is the coldest place in the atmosphere, with an average temperature of -148F. This level is where most meteors burn up.

4. Thermosphere. As its name suggests, things get hot again in the thermosphere, reaching temperatures of up to 2,730F. This is where the International Space Station orbits. The thermosphere extends from the mesopause (the upper limit of the mesosphere) to as far as 500 miles above the Earth's surface, depending on solar activity. The International Space Station stays between 200 and 240 miles above the Earth.

5. Exosphere. The outer limits of the Earth's atmosphere start at the edge of the thermosphere, which is called not the thermopause (psych!) but the exobase. At lower levels, the atmosphere behaves like a liquid, but in the exosphere, particles — mainly hydrogen and helium — are so far apart that they seldom collide, and shoot out on ballistic trajectories. The exosphere is the dividing line between our atmosphere and the vacuum of space.

Within these five principal layers are other layers, including the ozone layer, which is part of the stratosphere, and the ionosphere, which straddles the thermosphere and the exosphere and affects radio waves. But this is more science than I've done in a very long time, so we'll end today's lesson here, with me wondering how anything survives in conditions of 2,730F — or how anyone even measures that.