Precipitation and evapotranspiration - Fundamentals of the Physical Environment - page 91

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 1,800 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 precip-
itation 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 5.3 ); 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.

Singapore
(10m)

Mean annual precipitation = 2282 mm

300

200

100

0

JFMAMJJASOND

Figure 5.6 Mean monthly precipitation at Singapore, in the
equatorial trough zone.

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 5.6 ),
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 ( Figure 5.7 ). Even within the same
pressure belt some seasonal pattern may be evident. In the

Las Vegas (Nevada)
(660m)

Mean annual precipitation = 100 mm

12

10
8
6
4
2

0

JFMAMJJASOND

Figure 5.7 Mean monthly precipitation at Las Vegas,
Nevada, in an arid zone.

Table 5.3 Rainfall at Yenbo, Saudi Arabia, 24° 8 e N, 38° 3 e 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