The climate is generally very dry and very hot; dust storms and sandstorms are frequent. Day and night temperatures vary greatly. From May to September, the hottest period, daytime temperatures reach 54° C (129° F ) in the interior and are among the highest recorded anywhere in the world. Temperatures are slightly lower along the coasts, but humidity reaches 90%, especially in the east, which is noted for heavy fogs. From October through April, the climate is more moderate, with evening temperatures between 16° and 21° C (61° and 70° F ). Average annual rainfall is 9 cm (3.5 in), with most rain falling from November to May. Between 25 and 50 cm (10 and 20 in) of rain falls in the mountainous 'Asir area, where there is a summer monsoon.