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air. This small, variable percentage of water vapor is essential in ATMOSPHERIC PRESSURE
maintaining life on Earth. It enters the atmosphere by evaporation, The atmosphere exerts a pressure that decreases with increas-
mostly from the ocean, and leaves the atmosphere as rain or snow. ing altitude above the surface. Atmospheric pressure can be
The continuous cycle of evaporation and precipitation is called the understood in terms of two different frames of reference that
hydrologic cycle. The hydrologic cycle is considered in detail in will be useful for different purposes. These two frames of refer-
chapters 23 and 24. ence are called (1) a hydrostatic frame of reference and (2) a
Apart from the variable amounts of water vapor, the relatively molecular frame of reference. Hydrostatics is a consideration of
fixed amounts of nitrogen, oxygen, and argon make up about the pressure exerted by a fluid at rest. In this frame of reference,
99.97 percent of the volume of a sample of dry air. The remaining atmo spheric pressure is understood to be produced by the mass
0.03 percent is mostly carbon dioxide (CO 2 ) and traces of the of the atmosphere being pulled to Earth’s surface by gravity.
inert gases neon, helium, krypton, and xenon, along with less than In other words, atmospheric pressure is the pressure from the
5 parts per million of free hydrogen, methane, and nitrous oxide. weight of the atmosphere above you. The atmosphere is deepest
The carbon dioxide content varies locally near cities from the at Earth’s surface, so the greatest pressure is found at the surface.
combustion of fossil fuels and from the respiration and decay of Pressure is less at higher altitudes because there is less air above
organisms and materials produced by organisms. The overall at- you and the air is thinner. The molecular frame of reference is
mospheric concentration of carbon dioxide is regulated (1) by re- a consideration of the number of molecules (nitrogen, oxygen,
moval from the atmosphere through the pho tosynthesis process of etc.) and the force with which they strike a surface. Air pres-
green plants, (2) by massive exchanges of carbon dioxide between sure in this frame of reference is understood to be the result
the ocean and the atmosphere, and (3) by chemical reactions be- of the composite bombardment of air molecules. Atmospheric
tween the atmosphere and rocks of the surface, primarily limestone. pressure is greatest at Earth’s surface because there are more
The ocean contains some 50 times more carbon dioxide than molecules at lower levels. At higher altitudes in the atmosphere,
the atmosphere in the form of carbonate ions and in the form of a fewer molecules are present per unit volume, so the pressure
dissolved gas. The ocean seems to serve as an equilibrium buffer, they exert is less.
absorbing more if the atmospheric concentration increases and At Earth’s surface (sea level), the atmosphere exerts a force
releasing more if the atmospheric con centration decreases. Lime- of about 10.0 newtons on each square centimeter (14.7 lb/sq in).
stone rocks contain an amount of carbon dioxide that is equal to As you go to higher altitudes above sea level, the pressure rapidly
about 20 times the mass of all of Earth’s present atmosphere. If all decreases with increasing altitude. At an altitude of about 5.6 km
this chemically locked-up carbon dioxide were released, the atmo- (about 3.5 mi), the air pressure is about one-half of what it is at
2
2
sphere would have a concentration of carbon dioxide similar to sea level, about 5.0 newtons/cm (7.4 lb/in ). At 12 km (about
the present atmosphere of Venus. This amount of carbon dioxide 7 mi), the air pressure is about 2.5 newtons/cm (3.7 lb/in ).
2
2
would result in a tremendous increase in the atmospheric pres- Compare this decreasing air pressure at greater elevations to Fig-
sure and temperatures on Earth. Overall, however, equilibrium ure 22.3. Again, you can see that most of the atmosphere is very
exchange processes with the ocean, rocks, and living things regu- close to Earth, and it thins rapidly with increasing altitude. Even
late the amount of carbon dioxide in the atmosphere. Measure- a short elevator ride takes you high enough that the atmospheric
ments have indicated a yearly increase of about 1 part per million pressure on your eardrum is reduced. You equalize the pressure
of carbon dioxide in the atmosphere over the last several decades. by opening your mouth, allowing the air under greater pressure
This increase is believed to be a result of the destruction of tropical inside the eardrum to move through the eustachian tube. This
rainforests along with increased fossil fuel combustion. makes a “pop” sound that most people associate with changes in
In addition to gases and water vapor, the atmosphere con- air pressure.
tains particles of dust, smoke, salt crystals, and tiny solid or liq- Atmospheric pressure is measured by an instrument called
uid particles called aerosols. These particles become suspended a barometer. The mercury barometer was invented in 1643 by
and are dispersed among the molecules of the atmospheric an Italian named Torricelli. He closed one end of a glass tube
gases. Aerosols are produced by combustion, often resulting in and then filled it with mercury. The tube was then placed, open
air pollution. Aerosols are also produced by volcanoes and forest end down, in a bowl of mercury while holding the mercury in
fires. Volcanoes, smoke from combustion, and the force of the the tube with a finger. When Torricelli removed his finger with
wind lifting soil and mineral particles into the air all contribute the open end below the surface in the bowl, a small amount of
to dust particles larger than aerosols in the atmosphere. These mercury moved into the bowl, leaving a vacuum at the top end
larger particles are not suspended as the aerosols are, and they of the tube. The mercury remaining in the tube was supported
soon settle out of the atmosphere as dust and soot. by the atmospheric pressure on the surface of the mercury in the
Tiny particles of salt crystals that are suspended in the atmo- bowl. The pressure exerted by the weight of the mercury in the
sphere come from the mist created by ocean waves and the surf. tube thus balanced the pressure exerted by the atmosphere. At
This mist forms an atmospheric aerosol of seawater that evap- sea level, Torricelli found that atmospheric pressure balanced a
orates, leaving the solid salt crystals suspended in the air. The column of mercury about 76.00 cm (29.92 in) tall (Figure 22.5).
aerosol of salt crystals and dust becomes well mixed in the lower As the atmospheric pressure increases and decreases, the
atmosphere around the globe, playing a large and important role height of the supported mercury column moves up and down.
in the formation of clouds. (For a worked example on this mate- Atmospheric pressure can be expressed in terms of the height of
rial, see the chapter 22 resources on www.mhhe.com/tillery.) such a column of mercury. Public weather reports give the

544 CHAPTER 22 The Atmosphere of Earth 22-4