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4.31: Air Pressure and Altitude

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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 exert pressure . 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 the Figure below .

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 the graph above, 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.
  • The depth (distance from top to bottom) of the atmosphere is greatest at sea level and decreases at higher altitudes. With greater depth of the atmosphere, more air is pressing down from above. Therefore, air pressure is greatest at sea level and falls with increasing altitude. On top of Mount Everest, which is the tallest mountain on Earth, air pressure is only about one-third of the pressure at sea level.

How We Use Air Pressure

The pressure of air in the atmosphere allows us to do many things, from sipping through a straw to simply breathing. You can see in the Figures below and below how we use air pressure in both of these ways.

When you first suck on a straw, you remove air from the straw, so the air pressure in the straw is lower that the air pressure on the surface of the drink. A fluid always flows from an area of higher pressure to an area of lower pressure, so the drink moves up the straw and into your mouth.

Q : Can you think of other ways that air pressure is useful?

A : You can see more examples of ways we use air pressure by watching the video at this URL:

http://www.youtube.com/watch?v=ZG29OJyZpyA&feature=related

Summary

  • The pressure exerted by the air in the atmosphere is greatest at Earth’s surface and falls as altitude increases. The reason is that density and depth of the atmosphere are greatest at sea level and decline with increasing altitude.
  • The pressure of air in the atmosphere allows us to do many things, from sipping through a straw to breathing.

Vocabulary

  • pressure : Result of force acting on a given area.

Practice

Select any five U.S. cities and at the first URL below, find their elevations (in meters) above sea level. If more than one elevation is given for a city, choose just one. At the second URL below, find the atmospheric pressure (in kPa) at those elevations (altitudes). Then make a graph of the atmospheric pressures of the five cities. Use the type of graph that best shows how the cities differ in atmospheric pressure. Be sure to label both axes of your graph.

http://www.worldatlas.com/aatlas/infopage/elvation.htm

http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

Review

  1. Describe how air pressure in the atmosphere changes from sea level to the top of the atmosphere.
  2. What factors explain why atmospheric pressure changes as altitude increases?
  3. Explain how differences in air pressure allow us to breathe.

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Difficulty Level:

At Grade

Grades:

7 , 8

Date Created:

Nov 01, 2012

Last Modified:

Sep 15, 2014
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