Effect of gravity on atmosphere

[jackrabbit]

My ten year old asked the following question - why doesn't gravity pull the atmosphere all the way down to the surface of the earth?

 

[D H]

Have you ever used an air mattress? Your laying down on an air mattress compresses the gas in the mattress somewhat, just enough to support your weight. No part of your body is touching the ground if the mattress is sufficiently inflated. Similarly, gravity does pull the atmosphere down toward the surface of the Earth, just enough to support the weight of all of the air above it.

That atmospheric pressure at sea level is 1 atmosphere or 101.325 kilopascal can be used to estimate the weight and thus the mass of the atmosphere. The Earth's equatorial radius is 6378.1 km while its polar radius is 6356.8 km. Using

• The average of these two numbers, 6367.4 km, as an estimate of the Earth's mean radius
• The standard formula for the surface area of a sphere, $A=4\pi r^2$
• The mean gravitational acceleration at sea level is g=9.80665 m/s²

$$W_{\text{atmos}} \approx \frac{4\pi\,(6367.4\,\text{km})^2\cdot 101.325\,\text{kPa}} {9.80665\,\text{m}/\text{s}^2} = 5.26\cdot 10^{18}\,\text{kg}$$

This is a simple yet fairly accurate estimate of the total mass of the atmosphere.

 

[Drakkith]

I like to think of it like this: Gravity pulls the air down, but the air pushes back up! And so the two reach a point of equilibrium.

 

[johnnymorales]

Well the answer is It Does.

The Earth gravity does pull the atmosphere down to the surface, and holds it.

 

[Dr Lots-o'watts]

If the Earth was flat, all the air would indeed fall and spread all over. But since it is round, air "meets at the other end" and holds itself up.

The density of air is greater at the surface. It is "squashed up" by the air above.

 

[reasonmclucus]

Air molecules move at extremely high speeds which allow them to temporarily move away from the ground. As they move up some of the kinetic energy (i.e., heat) is lost causing them to slow down, but they are also farther from the center of the Earth so the gravitational attraction is lower allowing them to stay up longer. the push of warmed air from the ground helps hold up the colder air until a downdraft allows the cold air to fall. When cold air falls it can create a vacuum which draws up warmed air. In extreme cases this process helps produce tornadoes.