Geoscience Reference
In-Depth Information
Figure 4.3
Mean atmospheric water
vapour content in January and July
1970 to 1999, in mm of precipitable
water.
Source
: Climate Diagnostics Center,
CIRES-NOAA, Boulder, CO.
evaporative cooling of this wet bulb gives a reading
that can 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 condensation first occurs on a cooled surface. Two
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 to 10 per cent.
For upper air measurements, a
lithium chloride
element
detects changes in electrical resistance to vapour pres-
sure differences. Relative humidity changes are accurate
within ± 3 per cent.
2 Moisture transport
The atmosphere transports moisture horizontally as
well as vertically. Figure 4.1 shows a net transport
from oceans to land areas. Moisture must also be trans-
ported meridionally (south-north) in order to maintain
the required moisture balance at a given latitude
(i.e. evaporation - precipitation = net horizontal tran-
sport of moisture into an air column). Comparison of
annual average precipitation and evaporation totals
