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What Happens When Altitude Increases?

As you sit back, relax and enjoy your next airline flight, you might find yourself wondering what's going on outside your window. The Earth's atmosphere extends for hundreds of miles, but most of the mass is contained in only the first 50 miles or so---the layers known as the troposphere, stratosphere and mesosphere. As you go further up, the increased altitude has effects on gravity, atmospheric density, temperature and radiation exposure.

Density
- As altitude increases, air density drops sharply. Most of the Earth's atmosphere is relatively close to the surface---the troposphere, where we live and where most human activity takes place, contains 75 percent of the Earth's atmosphere by mass, and nearly all of the water vapor. The decrease in air density at altitude helps explain why commercial airliners tend to fly as high as possible: because there's less air, the effects of atmospheric drag are greatly lessened, meaning planes can fly much more efficiently.
Temperature
- For humans, a more significant aspect of higher altitudes is lower temperatures. In general, for every 1,000 feet of altitude the temperature of the troposphere drops by 3.5 degrees Fahrenheit---so at 30,000 feet, the temperature is more than 100 degrees cooler. They don't let you roll down the windows on an airliner, so human beings don't experience this temperature drop directly. You can see its effects, however, by looking straight up: because cooler air can hold less moisture, water vapor condenses out of the atmosphere at high altitudes, producing clouds.
Gravity
- Gravity keeps you, and the atmosphere, close to the surface, but its effects diminish with distance away from the center of the Earth. Mathematically, gravity decreases by 1 divided by the square of the distance from the Earth's center. Because the Earth's surface is already so far away from the center, the differences are relatively minimal for altitudes along the range of normal human habitation. As a weight loss tactic, therefore, altitude loses out to diet and exercise: if you weigh 150 pounds at sea level, you'd weigh only 3.2 ounces less at the top of Pike's Peak, elevation 14,115 feet.
Radiation
- Beyond giving you something to breathe, the atmosphere also serves a vital role in keeping you safe from background radiation---the cosmic rays that are constantly bombarding the planet. Radiation exposure is commonly measured in sieverts, designed to quantify the effects of ionizing radiation exposure. At sea level, you're exposed to .04 microsieverts per hour; by 30,000 feet, this has grown to 3.25 microsieverts per hour. As a result, commercial airline pilots are in the top 5 percent of all occupations for radiation exposure.

Astronomy