The harmattan is a dry, dusty northeasterly wind that blows from the Sahara across West Africa toward the Gulf of Guinea between roughly late November and mid-March. It is the dry-season half of the region's wind cycle: for the rest of the year the prevailing flow is the southwesterly monsoon bringing moist Atlantic air inland, and the harmattan is what replaces it when the Intertropical Convergence Zone migrates south. The wind carries enormous quantities of fine Saharan dust, which fills the air as a persistent haze that can cut visibility below a kilometre, ground aircraft for days and turn the sun into a pale disc at midday. It drops relative humidity dramatically — often to 15 per cent or lower in places that spend the rest of the year above 80 — producing cracked skin, splitting timber and severe fire risk. It also brings a striking diurnal range, with cool nights that can approach 9 °C in the Sahel and hot dusty afternoons, and its dust has consequences that reach as far as the Caribbean and the Amazon basin.
From the Sahara to the Gulf of Guinea
West Africa's climate is governed by the seasonal migration of the Intertropical Convergence Zone, the belt where the northeast and southeast trade winds meet. It follows the sun north in boreal summer, bringing the rains as far as the southern Sahara, and retreats south in winter.
When it retreats, the northeasterly trade flow off the Sahara takes over, and that is the harmattan. The wind's character changes along its path. Over the Sahara and the northern Sahel it is at its driest, dustiest and most extreme. As it moves south toward the coast it gradually mixes with residual moist air, so Kano and Niamey experience a far more severe harmattan than Lagos or Abidjan, where it arrives weakened and intermittent, sometimes as no more than a few hazy weeks.
The seasonal timing shifts with latitude too. The northern Sahel enters harmattan conditions in October and stays in them into April; the Guinea coast may only see clear harmattan episodes in December and January. The transition at either end is often abrupt — a shift of wind direction over a day or two, and the air changes completely.
The Haze That Grounds Aircraft
The dust is the harmattan's most conspicuous feature and its most economically disruptive one. Suspended particles reduce horizontal visibility routinely to 2 to 5 km and, in severe episodes, to a few hundred metres, with the haze extending several kilometres up rather than sitting as a surface layer.
Aviation bears the brunt. Airports across Nigeria, Ghana, Niger, Mali, Burkina Faso and Chad suffer repeated closures and diversions each season, and multi-day disruptions at Kano, Abuja, Accra and Bamako are a normal feature of December and January. The haze is particularly awkward because it is not a discrete storm that passes: it is a persistent condition that can hold visibility just below landing minima for days.
Road transport suffers similarly, and solar generation across the Sahel loses substantial output during heavy dust periods — an increasingly material problem as the region builds out solar capacity.
The Word, and the First European Account
The name comes from a West African language rather than a European one — most accounts trace it to a Twi or Fante term along the lines of haramata — and it entered English through traders on the Gold Coast in the late seventeenth century. That makes it one of the relatively few wind names in international meteorological use that originated with the people who actually live under the wind, rather than with visiting sailors.
The first detailed scientific account in Europe was published in the Philosophical Transactions of the Royal Society in 1781, drawing on observations made on the Guinea coast. It described the wind's extraordinary drying power with some precision — the splitting of furniture and panelling, the withering of vegetation, the peeling of skin — and included the striking and now thoroughly obsolete claim that the harmattan was beneficial to health, curing or relieving fevers and certain infections. Given what is now understood about the meningitis belt, that conclusion has aged about as badly as an eighteenth-century medical judgement can.
What the account got right was the physical description, and it remains a usable one. Modern instrumentation has since put numbers to it: particulate concentrations during heavy harmattan episodes routinely exceed several hundred micrograms per cubic metre and have been measured above 1,000 in the Sahel, against a World Health Organization daily guideline an order of magnitude lower. The dust is also fine, with a large share in the size range that penetrates deep into the lungs, which is why the health effects are not merely a matter of discomfort.
What It Does to People, Crops and Cattle
The wind has a double reputation. In some coastal areas it is welcomed as the doctor, because the drop in humidity is a genuine relief after months of oppressive damp, and the cooler nights are the most comfortable of the year. Inland the assessment is harsher.
- Health: dry air and suspended particulates irritate airways, and respiratory presentations rise sharply during the season. Cracked lips, split skin and nosebleeds are near universal.
- Agriculture: the wind desiccates crops and shortens the growing season at its margins. Cocoa in Ghana and Ivory Coast is particularly sensitive, and a long or severe harmattan is a recognised driver of poor pod development and of movements in the world cocoa price.
- Livestock: pasture dries out and water sources shrink, and the season drives much of the seasonal southward movement of pastoralist herds.
- Fire: humidity in single figures combined with steady wind makes bush fire risk extreme, and the harmattan season is the burning season across the savanna belt.
- Materials: timber shrinks and splits, and instrument makers and furniture builders in the region work around the season deliberately.
The Meningitis Belt
One consequence deserves separate treatment because of its scale. A band of sub-Saharan Africa running roughly from Senegal east to Ethiopia, identified in the mid-twentieth century and known as the meningitis belt, experiences recurring epidemics of meningococcal meningitis — and those epidemics track the harmattan season with striking precision.
Epidemics begin as the dry dusty season sets in, peak in the hottest, driest part of it, and stop abruptly with the first rains, often within days. The correspondence is strong enough that seasonal forecasting of the wind has been examined as a public health tool.
The mechanism is generally understood as damage to the mucosal lining of the nose and throat by dry air and abrasive dust, which lowers the barrier to invasive infection by bacteria already carried harmlessly by a substantial fraction of the population, compounded by crowding indoors and by co-occurring respiratory infections. Vaccination campaigns since 2010 have dramatically reduced the dominant serogroup A epidemics, but the seasonal pattern in the remaining disease still follows the wind.
The season also has a distinctive social rhythm. Because the harmattan brings the only reliably cool nights of the year across much of the region, it coincides with the period of heaviest travel, festivals, weddings and returns home, and the West African calendar is organised around it as much as around the rains. The same months bring the harvest of the main cereal crops and the beginning of the cocoa main crop further south. The wind is not simply an inconvenience that people endure; it defines the half of the year in which most of the region's public life happens.
Dust That Crosses an Ocean
The harmattan is one arm of the largest dust transport system on the planet, and its reach extends far beyond Africa.
The single most productive dust source on Earth is the Bodele Depression in northern Chad — the dried floor of what was once a vastly larger Lake Chad — where a gap between two highland masses channels the wind across ancient diatomite deposits and lifts them. A disproportionate share of all Saharan dust originates in that one relatively small basin.
Dust lifted by the harmattan and related systems rises into the Saharan Air Layer, a hot dry dusty mass that travels west across the Atlantic. It reaches the Caribbean and the southeastern United States each summer, where it produces hazy skies and elevated particulate readings. It also suppresses Atlantic hurricanes, since dry air and the wind shear associated with the layer both inhibit tropical cyclone development — an unusual case of a desert wind moderating a marine hazard thousands of kilometres away.
And it fertilises the Amazon. Satellite estimates put the phosphorus delivered annually from Saharan dust to the Amazon basin at a scale comparable to what the rainforest loses to runoff each year, with the Bodele the dominant source. The Amazon rainforest is, in a real and measurable sense, partly sustained by the dried bed of a Chadian lake, delivered by the same wind that grounds flights in Kano. The Mediterranean version of the same story — Saharan dust falling on Europe and on Alpine snow — is covered in the guide to the sirocco.