Geoscience Reference
In-Depth Information
A further index of humidity is the dew-point
temperature. This is the temperature at which
saturation occurs if air is cooled at constant
pressure without addition or removal of vapor.
When the air temperature and dew-point are
equal the relative humidity is 100 percent, and it
is evident that relative humidity can also be
determined from
stations (AWS) commonly employ an electrical
method where a thin film of material changes
its capacitance in relation to relative humidity
The material is a thin metal film on a thin glass
substrate coated with an organic polymer that
forms the capacitor's dielectric.
2 Moisture transport
The atmosphere transports moisture horizon-
tally as well as vertically.
Figure 4.1
shows a net
transport from oceans to land areas. Moisture
must also be transported meridionally (south-
north) in order to maintain the required moisture
balance at a given latitude; i.e., evaporation -
precipitation = net horizontal transport of mois-
ture into an air column. Comparison of annual
average precipitation and evaporation totals for
latitude zones shows that in low and mid-latitudes
P
>
E
, whereas in the subtropics
P
<
E
(
Figure
4.4A
). These regional imbalances are maintained
by net moisture transport into (convergence) and
out of (divergence) the respective zones (D
Q
, where
divergence is positive).
e
s
at dew-point
-----------------
×
100
e
s
at air temperature
The relative humidity of a parcel of air will
change if either its temperature or its mixing ratio
is changed. In general, the relative humidity varies
inversely with temperature during the day,
tending to be lower in the early afternoon and
higher at night.
Atmospheric moisture can be measured by at
least six types of instrument. For routine measure-
ments the
wet-bulb thermometer
is installed in a
louvered instrument shelter (Stevenson screen).
The bulb of the standard thermometer is wrapped
in muslin, which is kept moist by a wick from a
reservoir of pure water. The evaporative cooling
of this wet bulb gives a reading that may be
used in conjunction with a simultaneous dry-bulb
temperature reading to calculate the dew-point
temperature. A similar portable device - the
aspirated
psychrometer
- uses a forced flow of air
at a fixed rate over the dry and wet bulbs. A
sophisticated instrument for determining the
dew-point, based on a different principle, is the
dew-point hygrometer
. This detects when conden-
sation first occurs on a cooled surface. Three
other types of instrument are used to determine
relative humidity. The
hygrograph
utilizes the
expansion/contraction of a bundle of human
hair, in response to humidity, to register relative
humidity continuously by a mechanical coupling
to a pen arm marking on a rotating drum. It has
an accuracy of ± 5-10 percent. For upper air
measurements, a
lithium chloride
element detects
changes in electrical resistance to vapor pres-
sure differences. Relative humidity changes are
accurate within ± 3 percent. Automatic weather
E
-
P
= D
Q
A prominent feature is the equatorward
transport into low latitudes and the poleward
transport in mid-latitudes (
Figure 4.4B
). Atmos-
pheric moisture is transported by the global
westerly wind systems of mid-latitudes towards
higher latitudes and by the easterly Trade Wind
systems towards the equatorial region (see Chapter
8). There is also significant exchange of moisture
between the hemispheres. During June to August
there is a moisture transport northward across
the equator of 18.8
10
8
kg s
-1
; during December
to February the southward transport is 13.6
×
×
10
8
kg s
-1
. The net annual south to north transport
is 3.2
10
8
kg s
-1
, giving an annual excess of net
precipitation in the Northern Hemisphere of
39mm. This is returned by terrestrial runoff into
the oceans.
It is important to stress that local evaporation
is, in general, not the major source of local
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