1. A dry, dusty wind from the northeast or east that blows from the Sahara across western Africa; especially, from late November until about mid-March.

It originates in the Sahara as a desert wind and extends southward to about 5°N in January and 18°N in July. In the humid lands along the Gulf of Guinea, its dryness is considered to be refreshing.

The harmattan is extremely dry and often reaches hurricane force which transports large amounts of dust and sand to the Atlantic Ocean.

It is associated with the high pressure area that lies over the northwest Sahara in the winter and the adjoining part of the Atlantic during other seasons.

During the summer, the cooler on-shore southwest monsoon undercuts it, but the harmattan continues to blow at a height of about two kilometers (3000–6000 feet) and sometimes deposits dust on ships that are at sea and this conflict of winds causes the so-called "West African tornadoes".

2. Also defined as a wind of western Africa, a north-east trade wind between 0 ° and about 20 ° latitude North, appearing primarily in the dry season and in winter (November till possibly March), and followed by the southwest monsoon (rainy season).