Geoscience Reference
In-Depth Information
2.2.3 Precipitation
Precipitation provides the external mass
flux to drainage basins. The level of precipitation
is connected to atmospheric humidity, temperature and circulation, and orography, since
the saturation water vapour pressure is sensitive to temperature (Fig. 2.7 ). Therefore
precipitation is usually lower in colder climate (see Table 2.8 ). The main exceptions are in
the tropics, where descending air masses produce very small amount of precipitation, and
at mountains, where orographic effects may force air masses upward and result in heavy
precipitation. The highest precipitation levels over ice-covered lakes are observed in mid-
latitude mountain areas, e.g. in European Alps, Rocky Mountains in North America, and
Japanese Alps, where the annual precipitation is 2,000
fl
3,000 mm. Toward the polar
regions, the precipitation level decreases, and as an extreme case, there is no precipitation
in Antarctic dry valleys.
Precipitation data are available from most weather stations. It is given in terms of
accumulation rate of an equivalent liquid water layer, normally in millimetres per time.
Precipitation gauges are used for the measurements. There are large dif
-
culties to deter-
mine the solid precipitation, especially during snowstorms due to turbulent transport of
snow
akes, and the recorded snow accumulation data are often biased down. Instead of
simple cylindrical gauges, in cold climate regions precipitation gauges are equipped with
speci
fl
c structures to manage the snowfall aerodynamics properly (Fig. 2.9 ).
In cloud altitudes the air temperature is typically below the freezing point due to
adiabatic cooling. Because the saturation water vapour pressure is lower at ice surface than
at water surface (Fig. 2.7 ), deposition of water vapour into ice crystals is more likely than
condensation into water droplets. If the atmosphere is cold down to the Earth
is surface,
precipitation lands in the solid phase. Hailstones fall fast and can reach the ground even in
warm conditions. The shape of the falling ice or snow crystals depends on the temperature
and the degree of oversaturation in clouds (Fig. 2.10 ). If the phase of precipitation is
unknown, a practical approach is to take it solid when the air temperature is below a
speci
'
ed level, e.g. below
1
°
C.
Table 2.8 Quality of ice season in different cold climate zones
P(I) = 0 0 < P(I) < 1 P(I) = 1
P(O) = 0 X X Perennial
0 < P(O) < 1 X Perennial/seasonal/ephemeral Perennial/seasonal
P(O) = 1 Ice free Ephemeral Seasonal
P(I) and P(O) are the probabilities of the annual occurrences of ice and open lake, respectively; X
stands for
' not possible '
 
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