The rainfall is very unequally distributed: in the western part, which comes near to the limits of the rainless region of Asia, it is very scanty, and scarcely averages more than 5 in.; in the south-west the fall is more copious, sometimes exceeding 100 in.
North of the apex of the delta and the boundary between the deltaic and inland tracts, the rainfall gradually lessens as far as Minbu, where what was formerly called the rainless zone commences and extends as far as Katha.
The climate is extremely dry, but this is compensated for by the heavy mists which sweep up from the plains during the rainless months and exercise a most beneficial effect in the coffee-growing districts.
Evaporation is naturally greatest in the enclosed seas of the nearly rainless subtropical zone such as the Mediterranean and Red Sea.
In countries nearly rainless, such as Egypt or Sind, there can be no cultivation without irrigation.