The Harmattan is a seasonal trade wind that sweeps across West Africa, defining the dry season. It is characterized as a cool, dry, and dusty wind, representing a major shift from the usual tropical, humid weather. This climatic event affects nations from the Gulf of Guinea coast northward deep into the Sahara’s transitional zones.
Meteorological Origins and Timing
The Harmattan is a northeasterly trade wind strongest between late November and mid-March, though its exact timing varies. The wind’s movement is driven by the seasonal shift of the Intertropical Convergence Zone (ITCZ), which is a belt of low pressure near the equator where the northeast and southeast trade winds meet. During the Northern Hemisphere winter, the ITCZ migrates southward toward the Gulf of Guinea, allowing the dry air mass from the north to dominate.
A high-pressure ridge, a subtropical anticyclone, settles over the central Sahara Desert during this period. The air mass from this high-pressure system begins its journey, blowing southwestward across the vast desert landscape toward the low-pressure zone near the coast. This pressure gradient is the engine that propels the wind, giving it speed and consistency.
The wind flow is part of a larger atmospheric circulation pattern, where the air mass is strengthened by a low-pressure center situated over the North Coast of the Gulf of Guinea. The intensity of the Harmattan is directly related to the strength of this pressure difference between the high over the Sahara and the low over the Atlantic. As the wind moves over the desert, it picks up fine dust and sand particles.
Defining Characteristics of the Air Mass
The most palpable characteristic of the Harmattan air mass is its extreme dryness, often causing the relative humidity to plummet significantly. In areas most affected, the relative humidity can drop below 15%, sometimes even under 5%, leading to desiccating conditions. This low moisture content is so pronounced that the wind is sometimes referred to as the “doctor wind” for its invigorating contrast to the typically humid tropical air.
As the wind traverses the Sahara, it lifts immense quantities of fine particulate matter, with particle sizes typically ranging between 0.5 and 10 micrometers. This dust load creates a phenomenon known as the Harmattan Haze, which severely limits atmospheric visibility. The haze can occasionally be so thick that it blocks the sun for several days, and the dust particles can be transported thousands of kilometers, even reaching the Caribbean and South America.
The temperature profile during the season is marked by significant diurnal fluctuations, meaning a large difference between day and night temperatures. The lack of moisture in the air and the clear skies allow heat to escape rapidly after sunset, causing night and early morning temperatures to drop considerably, sometimes as low as 9 degrees Celsius. Conversely, daytime temperatures can climb substantially, occasionally soaring to 30 degrees Celsius or higher, particularly in the later stages of the season.
Practical Impacts on Environment and Health
The extreme dryness and high dust concentration of the Harmattan affect both the environment and human health across West Africa. The fine dust particles, which fall into the PM10 air pollutant group, can severely irritate the respiratory system, worsening conditions like asthma and bronchitis, and are linked to an increased risk of respiratory infections. The parched air also dries out the nasal passages, making the area more susceptible to infection and sometimes causing spontaneous nosebleeds.
Beyond respiratory issues, the low humidity impacts the skin and mucous membranes, leading to cracked lips, dry skin, and eye irritation. Environmentally, the Harmattan rapidly dries out vegetation, increasing the risk of fire outbreaks across the region. The season also causes significant surface water evaporation and can accelerate soil erosion due to the strong, dry winds.
The reduced visibility from the Harmattan Haze creates substantial challenges for transportation, particularly for air travel, resulting in numerous canceled or diverted flights annually. Despite the challenges, the Harmattan creates conditions unfavorable for mosquitoes, reducing their population. The cooler temperatures at night also offer a welcome reprieve from the intense heat and humidity typical of other seasons in the region.