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The Harmattan is a cold-dry and dusty trade wind, blowing over the West African subregion. 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.
Temperatures can easily be as low as 9 °C (48 °F). However, sometimes, in the afternoon, the temperature can soar to as high as 30 °C (86°F), while the relative humidity drops under 10%. The Harmattan season differs from winter, because it is characterized by cold–dry, dust-laden wind, and also wide ﬂunctuations in the ambient temperatures of the day and night. On its passage over the Sahara, 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.
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 which can result in duststorms or sandstorms. However, when the haze effect is weak, this dry wind creates visually stunning sceneries with beautiful clear skies.
In some countries in the 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 stirred by these winds is known as the Harmattan haze, which costs airlines millions of dollars in cancelled and diverted flights each year,
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 also cause severe crop damage.
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