Environmental Engineering Reference
In-Depth Information
If we monitored a storm, we would normally find that precipitation intensity - that is,
the amount of rainfall per unit time - varied considerably. Heavy bursts of rain are
normally seen to alternate with relatively quiet periods. All types of rainfall show these
variations; there is rarely such a thing as steady rain. In fact, it is only when the source of
precipitation is held stationary that we get anything like steady rainfall. One of the most
common situations in which this occurs is where moist air is forced to rise over a
mountain barrier. If the moist air is blowing from the sea at a constant speed, the air will
be fairly uniform and the conversion of vapour to water droplets will proceed at a
constant rate. Rainfall then is often prolonged and steady.
The short-term variability of rainfall differs greatly from one area to another. It tends
to be greatest in the tropics; at Djakarta (Indonesia), for example, the annual rainfall of
1800 mm falls in only 360 hours on average. By contrast, the average rainfall in London
is only 600 mm, yet it falls in about 500 hours. Variability in precipitation is often most
important, however, in the more arid parts of the world, for there even quite small storms
may be a rare event (Table 5.4); channels that have been dry for months or even years
may fill with water, and the baked clay (adobe) used to make houses may crumble and be
washed away. Within a matter of hours the rainfall may have ceased and the water almost
vanished; within weeks the vegetation will have died down again.
SEASONAL VARIABILITY
In many climates there is a predictable and consistent cycle of rainfall during the course
of the year related to the latitudinal migration of the wind and pressure systems.
Precipitation areas associated with areas of convergence and uplift tend to shift polewards
in summer and equatorwards in winter. Some areas, like the British Isles, remain within
the same pressure system throughout the year and so seasonal variations are subdued.
This is also true in the equatorial trough zone, where rainfall can occur at any time
throughout the year (Figure 5.4), and in deserts, where rainfall is almost negligible. The
brief, rare storms which do occur can come at any time, so monthly rainfall, averaged
over the long term, shows little variation
Table 5.4 Rainfall at Yenbo, Saudi Arabia, 24 ° 8′
N, 38 ° 3′ E, 1967-90
Incidence
No.
%
Days with no rain
8634
98·5
Days with rain
78
0·9
Days with a trace of rain
35
0·4
Missing data
19
0·2
Amount (mm)
Under 1
1·1-2·0
2·1-3·0
3·1-5·0
5·1-8·0
8·1-15
Over 15
No. of days in 24 years
27
15
5
14
6
5
6
% of all days
0·31
0·17
0·06
0·16
0·07
0·06
0·07
% of rain days
35
19
6
18
8
6
8
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