From AMS Glossary
(Also spelled harmatan, harmetan, hermitan.) A dry, dust-bearing wind from the northeast or east that blows in West Africa especially from late November until 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. It is associated with the high pressure area that lies over the northwest Sahara in winter and the adjoining part of the Atlantic in other seasons. In summer the cooler onshore southwest monsoon undercuts it, but the harmattan continues to blow at a height of about 2 km (3000–6000 ft) and sometimes deposits dust on ships at sea. This conflict of winds causes the so-called West African tornadoes.