Every year, from December to April, the Harmattan sweeps across Africa. This cool, dry, and dust-laden wind gets its name from haramata, a word in the Ghanaian language Fanti.
The wind originates from high-pressure systems over the Sahara, south of Algeria and Libya, and north of Mauritania, Mali, Niger, and Chad. It significantly alters the climatic and environmental conditions of the regions it crosses.
The Harmattan’s effects go beyond reduced visibility and dryness. It impacts human health, agriculture, and ecosystems. The phenomenon plays a significant role in weather and climate hazards across West Africa, influencing dry and wet periods.
During the boreal (northern hemisphere) winter, from October to March, large-scale components of atmospheric circulation shift southward. In West Africa, this shift causes the inter-tropical convergence zone where moist air from the south meets dry air from the north to migrate from the Sahel towards the Gulf of Guinea. These airflows are well known: the monsoon in the south and the Harmattan in the north. From December to late March, nearly all of West Africa falls under the influence of the Harmattan.
The formation of the Harmattan is driven by a significant difference in atmospheric pressure. To the north, around 30°N, a high-pressure belt, such as the Azores High, pushes down dry air from high altitudes. To the south, near the equator and the sun’s azimuth, low-pressure zones create frequent storms. Large-scale, steady winds develop between these contrasting regions, flowing close to the surface. These winds exist throughout the year across the tropical belt. Over the ocean, these winds are known as the trade winds, which sailors have long relied on for transatlantic navigation from east to west.
The air masses carried by the Harmattan come from high altitudes and dry out as they rise. As these masses move southward toward the equatorial low-pressure zones, they pass only through hot and dry regions like the Sahara and Sahel, remaining dry. Areas affected by the Harmattan experience significant dryness, unlike the rainy season when the monsoon wind from the south prevails.
The air masses are affected by a strong pattern of cool nights and extremely hot days, particularly in March and April. At night, the cooler surface temperatures create a stable atmospheric layer, limiting vertical air movement. This stability reduces the vertical layer where the Harmattan blows, intensifying winds, especially in the early morning. The stronger nighttime winds, known as the low-level jet, occur over dry, erosion-prone soils, particularly in regions like the Bodélé Depression in the Chadian Sahara. The combination of powerful winds and erodible soils makes this area the primary source of desert dust in West Africa. It is recognized as the world’s most productive region for generating fine dust particles.
The Harmattan, a dry, cool wind filled with dust, microbes, and parasites, has significant health impacts. It is linked to numerous conditions affecting the skin and mucous membranes of people and animals. By degrading air quality, it contributes to respiratory infections such as bronchitis, pneumonia, and asthma. The Harmattan also facilitates the spread of diseases such as influenza and the proliferation of Mycobacterium tuberculosis, responsible for tuberculosis, as well as meningococci, which cause meningitis. This disease is particularly common in Sahelian countries.
Studies have shown that a 15% increase in fine dust particle concentrations in recent years has been linked to a 24% increase in infant mortality. The Harmattan also affects local ecosystems, causing significant surface water evaporation and accelerating soil erosion. Its dust-laden winds create visibility issues for aviation and reduce the efficiency of solar farms by coating photovoltaic panels with dust and haze.
Extensive research and measurement campaigns have been carried out to better understand dust plumes, including their formation, transport, and impacts. These efforts have contributed to improving data on dust plumes, especially through advanced monitoring programs. At the end of the Harmattan season, around April, air quality declines and high daytime temperatures worsen respiratory problems, particularly during heat waves. This makes accurate predictions crucial to inform and protect vulnerable populations.
Scientists and meteorological services in Sahelian countries are developing user-friendly indicators and simplified early warning systems. People can take some steps to protect themselves, such as covering the mouth and nose with a scarf, using saline solutions to moisten the airways, and getting vaccinated when possible.
Strengthening impact observation networks across West Africa and enhancing air quality monitoring, especially in urban areas where local pollution exacerbates the dangers of dust and Harmattan heat, is crucial for public health and environmental stability.
