Dry, dusty, north-easterly or easterly wind which occurs in W. Africa north of the equator. Its effect extends from just north of the equator in January, almost to the northern tropic in July. In W. Africa it is known as ‘the doctor’ because of its invigorating dryness compared with humid tropical air. The harmattan wind stream occasionally extends south of the equator during the northern winter as an upper air wind over the south-westerly monsoon.