Environmental Engineering Reference
In-Depth Information
Even at Bahrain (26° N), on the Arabian Gulf, night-time temperatures are cool in winter,
though frost is very rare. Precipitation is very low. Rain falls on an average of ten days
per year, with a mean annual total of 75 mm. Most of it falls in winter and spring, when
temperate-latitude depressions extend their effects far south and do give occasional rain.
Farther south, at Atbara (17° N) (Figure 27·3), the short summer rainy season is
Figure 27.1
Dryland areas of the world.
Figure 27.2
Mean surface pressure (hPa) over the Sahara in
summer.
the result of the northward movement of the equatorial trough in July. Thus we find
different rainfall regimes on opposite sides of the subtropical anticyclones: cool-season
rains on the northern limb and hot-season rains on the southern limb. Where amounts are
similar, the cool-season rain is more effective, as evaporation will be less at that time of
year.
Desert rainfall is notoriously unreliable. Several years without rain may be followed
by heavy showers giving tens of millimetres. It is this variability that makes the average
rainfall figures for desert areas almost meaningless. Annual rainfall totals at Al Wejd on
