The Harmattan is a hot, dry and dusty wind (continental trade wind) blowing over West Africa. This northeasterly wind blows from the Sahara Desert into the Gulf of Guinea between the end of November and the middle of March (winter). The name comes from or is related to an Akan cognate.
On its passage over the desert, it picks up fine dust and sand particles (between 0.5 and 10 micrometres). The air is particularly dry and desiccating when the Harmattan blows over the region. At morning, low temperatures can easily be as low as 15 °C (59 °F) or 20 °C (68 °F). At afternoon, high temperatures easily soar to more than 30 °C (86°F) and can reach as high as 40 °C (104 °F) sometimes, while the relative humidity drops under 10%.
The Harmattan blows during the dry season which occurs during the lowest-sun months, when a high pressure system of the subtropical ridge stays over the central Sahara Desert and when a low pressure system of the Intertropical Convergence Zone (ITCZ) stays over the Gulf of Guinea. The Harmattan brings desert-like weather conditions: it lowers the humidity, dissipates cloud cover, prevents rainfall formation and sometimes creates big clouds of dust or sand which can even result in violent duststorms or sandstorms but when the haze effect is weak, this dry wind creates beautiful sunny days with plenty of clear skies.
In some countries in West Africa, the heavy amount of dust in the air can severely limit visibility and block the sun for several days, comparable to a heavy fog. The dry air can break the trunks of trees growing in the region. The effect caused by the dust and sand stirred by these winds is known as the Harmattan haze, which costs airlines millions of dollars in cancelled and diverted flights each year, and risks public health by increasing meningitis cases.
The interaction of the Harmattan with monsoon winds can cause tornadoes. Humidity drops to as low as 15 percent, which can result in spontaneous nosebleeds for some people. The wind can cause severe crop damage.
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