The earth is 25,000 miles around on a great circle. That’s the horizontal scale of our atmosphere. So it is surprising that the scale is massively compressed in the vertical.
If you go up a mile, you’ll notice a difference; up four miles, you’ll need an oxygen mask. And at a mere ten miles above sea level, you’re already nine-tenths of the way through the ocean of air that sustains life on earth.
Using the (arbitrary) point at which 99% of the atmosphere is below, the top of the atmosphere comes less than 20 miles above sea level.
The Earth and the Apple
An average apple is about 225 millimeters around, and its skin is about 3 millimeters thick. If we compare the Earth and its atmosphere to an apple and its skin, the skin of the apple is about 20 times thicker relative to the size of the apple than the atmosphere is to the size of the Earth.
If you were a creature who could only live in the skin of an apple, you wouldn’t want to stray very far in the wrong direction. Similarly, human beings don’t go very far from sea level — even to the tops of high mountains — without oxygen masks or pressurized cabins.
The Mass of Earth’s Atmosphere
The mass of the atmosphere, measured in kilograms (a kilogram is a little more than two pounds) is approximately five followed by 18 zeroes. In other words, a lot. From that perspective, the atmosphere is thin vertically because it is compressed in that direction. Instead of having a modest amount of air spread out miles wide and miles high, we have a heavy concentration of air near the ground and a rapid thinning with height.
The total mass of the atmosphere corresponds to a weight of nearly fifteen pounds on every square inch of surface. That means if you have an average-size head, which by my rough calculation would be about 50 square inches across the top, there would be nearly 800 pounds of atmosphere pushing down on you. So why don’t you weigh nearly a thousand pounds? That’s because the atmosphere is also pushing down on your scale and the scale weighs the difference between the atmosphere-plus-you, and just the atmosphere alone.
Life on Earth
Oxygenation of the atmosphere began about two and a half billion years ago. It took another billion years for the amount of oxygen to reach a level that could sustain breathing animals. At the present time, the atmosphere is stable, with 21% oxygen and most of the rest nitrogen.
The distribution of the atmosphere — most of it within a very few miles of the earth’s surface — is determined by the mass of Earth, the mass of the atmosphere, and the laws of physics. None of these is likely to change much in the near future, so you can eat your apple and breathe easy — as long as you stay near sea level.
Computer Support Group Network. Pressure Altitude Calculator. (2013). Accessed July 8, 2013.
NASA. Earth Fact Sheet. (2013). Accessed July 8, 2013.
James Farquhar, Huiming Bao, Mark Thiemens. Atmospheric Influence of Earth’s Earliest Sulfur Cycle. (2000). Accessed July 8, 2013.
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