From Glossary of Meteorology
(Also called barih; also spelled shemaal, shimal, shumal.) The northwest wind in the lower valley of the Tigris and Euphrates and the Persian Gulf.
It may set in suddenly at any time, and generally lasts from one to five days, dying down at night and freshening again by day; however, in June and early July it continues almost without cessation (the "great" or "forty-day" shamal). Although the wind rarely exceeds 13 m s-1 (30 mph), it is very hot, dry, and dusty. The sky is cloudless but the haze is often so thick as to obscure the land, making navigation dangerous.