Most of Egypt is a dry subtropical area, but the southern part of Upper Egypt is tropical. Northern winds temper the climate along the Mediterranean, but the interior areas are very hot. The temperature sinks quickly after sunset because of the high radiation rate under cloudless skies. Annual rainfall averages 2.5 cm (1 in) south of Cairo and 20 cm (8 in) on the Mediterranean coast, but sudden storms sometimes cause devastating flash floods. Hot, dry sandstorms, known as khamsins, come off the Western Desert in the spring. In Cairo, average temperatures range from 14° C (57° F ) in January to 28° C (82° F ) in July. Relative humidity varies from 68% in February to over 70% in August and 77% in December.