Geoscience Reference
In-Depth Information
i.e. the rate of change of pressure (
p
) with height (
z
) is
dependent on gravity (
g
) multiplied by the air density
(ρ). With increasing height, the drop in air density
causes a decline in this rate of pressure decrease. The
temperature of the air also affects this rate, which is
greater for cold dense air (see Chapter 7A.1). The rela-
tionship between pressure and height is so significant
that meteorologists often express elevations in millibars:
1000 mb represents sea-level, 500 mb about 5500 m
and 300 mb about 9000 m. A conversion nomogram
for an idealized (standard) atmosphere is given in
Appendix 2.
2 Vapour pressure
At any given temperature there is a limit to the density
of water vapour in the air, with a consequent upper limit
to the vapour pressure, termed the
saturation vapour
pressure
(
e
s
). Figure 2.14A illustrates how
e
s
increases
with temperature (the Clausius-Clapeyron relation-
ship), reaching a maximum of 1013 mb (1 atmosphere)
at boiling-point. Attempts to introduce more vapour
into the air when the vapour pressure is at saturation
produce condensation of an equivalent amount of
vapour. Figure 2.14B shows that whereas the saturation
vapour pressure has a single value at any temperature
above freezing-point, below 0°C the saturation vapour
pressure above an ice surface is lower than that above a
Figure 2.14
Plots of saturation vapour pressure as a function of
temperature (i.e. the dew-point curve). (A) The semi-logarithmic
plot. (B) shows that below 0°C the atmospheric saturation vapour
pressure is less with respect to an ice surface than with respect
to a water drop. Thus condensation may take place on an ice
crystal at lower air humidity than is necessary for the growth of
water drops.
80
50
70
40
60
50
30
40
20
supercooled water surface. The significance of this will
be discussed in Chapter 5D.1.
Vapour pressure (
e
) varies with latitude and season
from about 0.2 mb over northern Siberia in January to
over 30 mb in the tropics in July, but this is not reflected
in the pattern of surface pressure. Pressure decreases at
the surface when some of the overlying air is displaced
horizontally, and in fact the air in high-pressure areas is
generally dry owing to dynamic factors, particularly
vertical air motion (see Chapter 7A.1), whereas air in
low-pressure areas is usually moist.
30
20
10
10
0
0
100
80
60
40
20
0
% TOTAL MASS OF AIR BELOW
Figure 2.13
The percentage of the total mass of the atmosphere
lying below elevations up to 80 km (50 miles). This illustrates the
shallow character of the earth's atmosphere.
