Structure and Composition of the Present-Day Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 46

the pressure exerted by air and solving for the
height gives

100 kgm −1 s −2
1hPa

1,000 hPa
13,558 kg

h = ×

9.81 m
s 2 m 3 ×

= 0.752 m = 29.6 inches ,

indicating that a column of mercury 29.6 inches
(75.2 cm) high exerts as much pressure at the
Earth's surface as a 1,000-hPa column of air
extending between the surface and the top of
the atmosphere.

Soon after Torricelli's discovery, French mathemati-
cian Blaise Pascal (1623-1662) confirmed Torricelli's
theory. He and his brother-in-law, Florin Perier, each
carried a glass tube of mercury, inverted in a bath of
mercury, up the hill of Puy-de-Dome, France. They
recorded the level of mercury at the same time at
different altitudes on the hill, confirming that atmo-
spheric pressure decreases with increasing altitude. In
1663, the Royal Society of London built its own mer-
cury barometer based on Torricelli's model. A more
advanced aneroid barometer wasdeveloped in 1843.
The aneroid barometer measures pressure by gauging
the expansion and contraction of a metal tightly sealed
in a case containing no air.
Air density is the mass of air per unit volume of
air. Because oxygen and nitrogen are concentrated near
the Earth's surface, air density peaks near the surface.
Air density decreases exponentially with increasing alti-
tude. Figure 3.2a,b shows standard profiles of air pres-
sure and density, respectively, and Figure 3.2a indicates
that 50 percent of the atmosphere's mass lies between
sea level and 5.5 km. About 99.9 percent of its mass lies
below about 48 km. The Earth's radius is about 6,378
km. Thus, almost all the Earth's atmosphere lies in a
layer thinner than 1 percent of the radius of the Earth.

Figure 3.1. Experiment with mercury barometer,
conducted by Evangelista Torricelli (1608-1647) and
directed by Blaise Pascal (1623-1662). The third
person is the artist, Ernest Board. Edgar Fahs Smith
Collection, University of Pennsylvania Library.

was called a mercury barometer ,where a barometer
is a device for measuring atmospheric pressure.

3.2. Processes Affecting Temperature
At the Earth's surface, where air density varies only
slightly in time and space, high or low temperature cor-
responds to how hot or cold we feel. However, temper-
ature is really a measure of the average kinetic energy
of an air molecule (energy giving rise to the motion of
air molecules). At a given density of air, we feel hot if
air molecules have a high kinetic energy because many
bombard our skin, and the resulting friction against our
skin converts kinetic energy from the molecules to heat
that we absorb. When molecules have a low kinetic

Example 3.1
To what height must mercury in a barometer
rise to balance an atmospheric pressure of 1,000
hPa, assuming the density of mercury is 13,558
kg m −3 ?

Solution
The pressure (kg m −1 s −2 )exerted by mercury
equals the product of its density (kg m −3 ), gravity
(9.81 m s −2 ), and the height ( h ,inmeters) of the
column of mercury. Equating this pressure with