Geoscience Reference
In-Depth Information
which saturation of air will occur following cooling
without the addition or removal of moisture. This is one
of the ways in which the amount of moisture which air
can hold may be assessed.
Relative humidity is the most popularly used term for
atmospheric moisture. It is the ratio of the amount of
moisture the air contains to the amount of moisture the
air could hold when saturated at that air temperature,
expressed as a percentage. Relative humidity may be
measured indirectly from wet-bulb and dry-bulb ther-
mometer readings, using humidity tables. Evaporation of
moisture from the wet bulb leads to cooling which is
inversely proportional to the relative humidity of the air.
If the air is saturated, there will be no evaporation, no
cooling and so no difference in temperature between
the dry and wet bulbs. Although frequently used,
relative humidity does have the disadvantage of being
temperature-dependent. For example, as air temperature
rises relative humidity will fall, because the air is able to
hold more moisture, even though the absolute moisture
content of the air has remained constant. An absolute
method of measuring moisture content is to determine
the vapour pressure , which is that part of the total
atmospheric pressure exerted by water vapour. Again it
can be obtained indirectly from the wet- and dry-bulb
thermometers, using tables and pressure readings. The
relationship between temperature and the moisture
content at saturation is indicated by the saturation vapour
pressure curve ( Figure 4.2 ). This shows the maximum
amount of moisture air can hold at any given temperature.
Thus as a rising air bubble cools, it may approach the
temperature at which condensation occurs. When the air
bubble reaches that temperature it becomes saturated and
net condensation takes place.
If condensation was the only thing that happened
on saturation, then, apart from the extra weight of the
droplets, the effect on the air bubble would be small.
There is, however, another major effect. As water changes
from its vapour state to a liquid it releases latent heat. This
heat acts to warm the air and thereby counteracts the
cooling resulting from expansion.
We can readily see the implications for our air bubble.
Instead of cooling at 9·8 C/1,000 m (its dry adiabatic
lapse rate), it cools more slowly as it rises. This new, lower
rate of cooling is known as the saturated adiabatic lapse
rate (SALR). Unlike the dry rate it is not a constant, for,
as we can imagine, it depends upon the amount of heat
released by condensation, and that, in turn, depends upon
the moisture content and hence the temperature of the air.
Warm air is able to hold a lot of moisture, and thus, on
cooling, it releases a lot of latent heat; cold air is able to
70
6
4
60
With respect
to water
Saturation vapour
pressure curve
with respect to
an ice surface
2
0
-50
50
-40
-30
-20
-10
0
Temperature (°C)
40
30
20
Saturation
point
Adding
moisture
Initial temperature and
vapour pressure
10
Cooling
0
0
10
20
30
40
Temperature (°C)
Figure 4.2 Saturation vapour pressure curve. Below 0°C
the curve is slightly different for an ice surface than for a
supercooled water droplet. If the initial air temperature and
vapour pressure contents are at point x with 20°C and 10hPa
respectively, the air can reach saturation, either by adding
moisture, or by cooling the air, or a combination of the two.
hold far less moisture, so the heat production during
condensation is much less. This is one reason why some
of the world's most severe storms are found in warm
climates.
Let us illustrate the effect of condensation by consider-
ing a specific example. Figure 4.3 shows the path curve for
the bubble. Its initial temperature as a result of surface
heating is at 21
C. As it is warmer than its environment,
it will cool at 9·8
C/1,000 m until saturation point is
reached. It is at this level that we first see the visible
evidence of our bubble - a small cloud will be seen
forming. Above condensation level the rate of cooling
 
 
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