4.31 Air Pressure and Altitude
4.31 Air Pressure and Altitude
- Explain how and why the pressure of the atmosphere changes with increasing altitude.
- Describe some ways that we use air pressure.
Rays of sunlight pour into this old room and reflect off dust particles in the air. Did you ever see dust particles in the
air like these? If so, you may have noticed that they constantly move about at random.
Q: Why do dust particles move randomly in the air?
A: Invisible particles of gases in the air are constantly moving and bumping into them.
Pressure of Gas Particles
Because gas particles in the air—like particles of all fluids—are constantly moving and bumping into things, they
exertpressure. The pressure exerted by the air in the atmosphere is greater close to Earth’s surface and decreases as
you go higher above the surface. You can see this in theFigure5.68.
Q: Denver, Colorado, is called the “mile-high city” because it is located 1 mile (1.6 km) above sea level. What is the
average atmospheric pressure that high above sea level?
A: From theFigure5.68, the average atmospheric pressure 1.6 km above sea level is about 85 kPa.
Explaining Changes in Air Pressure with Altitude
There are two reasons why air pressure decreases as altitude increases: density and depth of the atmosphere.
- Most gas molecules in the atmosphere are pulled close to Earth’s surface by gravity, so gas particles are denser
near the surface. With more gas particles in a given volume, there are more collisions of particles and therefore
greater pressure.