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The Harmattan is a dry and dusty northeasterly trade wind which blows from the Sahara Desert over the West African subcontinent into the Gulf of Guinea between the end of November and the middle of March (winter). The name comes from or is related to Ga haramata. It is cold in some places and hot in others, according to the circumstances.
The Harmattan blows during the dry season, which occurs during the lowest-sun months, when the subtropical ridge of high pressure stays over the central Sahara Desert and when the low-pressure Intertropical Convergence Zone (ITCZ) stays over the Gulf of Guinea. On its passage over the Sahara, it picks up fine dust and sand particles (between 0.5 and 10 microns).
The Harmattan season differs from winter, because it is characterized by cold, dry, dust-laden wind, and also wide ﬂuctuations in the ambient temperatures of the day and night. Temperatures can easily be as low as 9 °C (48 °F) all day, but sometimes in the afternoon the temperature can also soar to as high as 30 °C (86 °F), while the relative humidity drops under 10%.
The air is particularly dry and desiccating when the Harmattan blows over the region. 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. The wind can increase fire risk and cause severe crop damage. The interaction of the Harmattan with monsoon winds can cause tornadoes.
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. This effect is known as the Harmattan haze. It costs airlines millions of dollars in cancelled and diverted flights each year. When the haze is weak, the skies are clear. The dry air can break the trunks of trees growing in the region.
Humidity drops to as low as 15%, which can result in spontaneous nosebleeds for some people. Other health effects on humans may include conditions of the skin, eyes, and respiratory system, including aggravation of asthma.
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- Climate and Land Degradation (ISBN 3540724370, 2007): "At the southern fringe of the Sahara Desert, a special dry and hot wind, locally termed Harmattan, occurs."
- Geographical Review (1919): "Knox writes of this wind : The Harmattan is experienced as a wind which blows, especially in the months of December, January, and February, from the NE. and is a hot wind in some localities and a cold wind in others, according to circumstances."
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