Geoscience Reference
In-Depth Information
decreased overall since the 1980s. The overall
effect of aerosols on the lower atmosphere is
uncertain; urban pollutants generally warm the
atmosphere through absorption and reduce
solar radiation reaching the surface (see
Chapter 3C). Aerosols may lower the planetary
albedo above a high-albedo desert or snow
surface but increase it over an ocean surface.
Thus, the global role of tropospheric aerosols
is difficult to evaluate, although most authori-
ties now consider it to be one of cooling.
P
=
R
ρ
T
Thus, at any given pressure, an increase in
temperature causes a decrease in density, and vice
versa.
1 Total pressure
Air is highly compressible, such that its lower
layers are much more dense than those above.
Fifty percent of the total mass of air is found below
5km (see
Figure 2.13
), and the average density
decreases from about 1.2kg m
-3
at the surface to
0.7kg m
-3
at 5000m (approximately 16,000ft),
close to the extreme limit of human habitation.
Pressure is measured as a force per unit area.
A force of 10
5
newtons acting on 1m
2
, corresponds
to the Pascal (Pa) which is the Système Inter-
national (SI) unit of pressure. Meteorologists still
commonly use the millibar (mb) unit; 1 millibar
= 10
2
Pa (or 1hPa; h = hecto) (see Appendix 2).
Pressure readings are made with a mercury
barometer, which in effect measures the height of
the column of mercury that the atmosphere is
able to support in a vertical glass tube. The closed
upper end of the tube has a vacuum space and
its open lower end is immersed in a cistern of
B MASS OF THE ATMOSPHERE
Atmospheric gases obey a few simple laws in
response to changes in pressure and temperature.
The first, Boyle's Law, states that, at a constant
temperature, the volume (
V
) of a mass of gas
varies inversely as its pressure (
P
), i.e.,
k
1
P
=
V
(
k
1
is a constant); and the second, Charles's Law,
that, at a constant pressure, volume varies directly
with absolute temperature (
T
) measured in
degrees Kelvin (see Note 2):
V
=
k
2
T
These laws imply that the three qualities of
pressure, temperature and volume are completely
interdependent, such that any change in one of
them will cause a compensating change to occur
in one, or both, of the remainder. The gas laws
may be combined to give the following relation-
ship:
80
50
70
40
60
50
30
40
20
30
PV
=
RmT
where
m
= mass of air, and
R
= a gas constant for
dry air (287 J kg
-1
K
-1
) (see Note 3).
If
m
and
T
are held fixed, we obtain Boyle's
Law; if
m
and
P
are held fixed, we obtain Charles's
Law. Since it is convenient to use density, ρ (=
mass/volume), rather than volume when studying
the atmosphere, we can rewrite the equation in the
form known as the equation of state:
20
10
10
0
0
100
80
60
% total mass of air below
40
20
0
Figure 2.13
The percentage of the total mass of
the atmosphere lying below elevations up to 80km
(50 miles). This illustrates the shallow character of
the earth's atmosphere.
