Geoscience Reference
In-Depth Information
formation of clouds and the production of
rainfall.
The average storage of water vapor in the
atmosphere (
Table 4.1
), termed the precipitable
water content (about 25mm), is sufficient for only
about 10 days, supply of rainfall over the earth as
a whole. However, intense (horizontal) influx
of moisture into the air over a given region makes
possible short-term rainfall totals greatly in
excess of 30mm. The phenomenal record total
of 1870mm fell on the island of Réunion, off
Madagascar, during 24 hours in March 1952, and
much greater intensities have been observed over
shorter periods (see E.2a, this chapter).
400
500
700
Tucson
850
Miami
950
Surface
0
2
4
6 8
Mixing ratio (g/kg)
10
12
14
16
Figure 4.2
The vertical variation of atmospheric
vapor content (g/kg) at Tucson, AZ, and Miami, FL,
at 12 UTC on 27 March 2002.
B
HUMIDITY
latitudes. It is also apparent that the seasonal effect
is most marked in the lowest 3000m (i.e., below
700mb). Air temperature sets an upper limit
to water vapor pressure - the saturation value
(i.e., 100 percent relative humidity); consequently
we may expect the distribution of mean vapor
content to reflect this control. In January,
minimum values of 1-2mm (equivalent depth of
water) occur in northern continental interiors and
high latitudes, with secondary minima of 5-10mm
in tropical desert areas, where there is subsiding
air (
Figure 4.3
). Maximum vapor contents of
50-60mm are over southern Asia during the
summer monsoon and over equatorial latitudes of
Africa and South America.
Another important measure is
relative
humidity
(
r
), which expresses the actual moisture
content of a sample of air as a percentage of that
contained in the same volume of saturated air at
the same temperature. The relative humidity is
defined with reference to the mixing ratio, but it
can be determined approximately in several ways:
x
1
Moisture content
Atmospheric moisture comprises water vapor,
and water droplets and ice crystals in clouds.
Moisture content is determined by local
evaporation, air temperature and the horizontal
atmospheric transport of moisture. Cloud water,
on average, amounts to only 4 percent of atmos-
pheric moisture. The moisture content of the
atmosphere can be expressed in several ways, apart
from the vapor pressure (p. 31), depending on
which aspect the user wishes to emphasize.
The total mass of water in a given volume of air
(i.e., the density of the water vapor) is one such
measure. This is termed the
absolute humidity
(r
w
)
and is measured in grams per cubic metre (g m
-3
).
Volumetric measurements are seldom used in
meteorology and more convenient is the
mass
mixing ratio
(
x
). This is the mass of water vapor
in grams per kilogram of dry air. For most
practical purposes, the
specific humidity
(
q
) is
identical, being the mass of vapor per kilogram of
air, including its moisture.
More than 50 percent of the atmospheric
moisture content is located below 850mb
(approximately 1450m) and more than 90 percent
below 500mb (5575m).
Figure 4.2
illustrates
typical vertical distributions in spring in mid-
q
e
r
= --
×
100 < --
×
100 < --
×
100
x
s
q
s
e
s
where the subscript
s
refers to the respective
saturation values at the same temperature;
e
denotes vapor pressure.
