Saharan Air Layer

The Saharan Air Layer is a hot, dry, dust-filled air mass that forms over the Sahara Desert each year from about June to August. The air layers typically develop over the Northern African desert every few days and move west over the Atlantic Ocean. The dry air combined with dust particles in the atmosphere can hamper the development of tropical storm systems and suppress the formation of hurricanes. The air masses are often more than 2 miles (3.2 kilometers) thick and sometimes travel as far west as the Gulf of Mexico. In summer 2020, the largest dust-filled Saharan Air Layer ever recorded traveled more than 5,000 miles (8,047 kilometers) across the Atlantic. At its peak, the air mass was about the size of the lower forty-eight United States and affected a wide area of the southeastern states.

Background

The Sahara in Northern Africa is the largest hot desert on Earth, covering more than 3.5 million square miles (9 million square kilometers). The Sahara accounts for about 10 percent of the continent’s total area, stretching from Egypt and Sudan on the Red Sea to Mauritania and Morocco in western Africa. The desert’s geography consists of sand dunes, plateaus, salt flats, dry valleys, and windswept sand and gravel fields. On average, parts of the Sahara receive fewer than 4 inches (10.2 centimeters) of rain per year, while the driest areas receive less than an inch (2.5 centimeters). The average temperature is about 86 degrees Fahrenheit (30 Celsius), although summer highs can top 120 Fahrenheit (49 Celsius).

To the south of the Sahara is a narrow transition zone of semi-arid plains known as the Sahel. The Sahel extends for about 3,330 miles (5,310 kilometers) from Sudan in the east to Senegal in the west. It acts as a geographical buffer between the hot, dry desert to the north and the slightly cooler, humid savannas and rainforests to the south. The region has a yearly average of about 4 to 8 inches (10.2 to 20.3 centimeters) of rain, which falls primarily from June through August.

Overview

The temperature difference between the hot Saharan air and the relatively cooler humid air to the south creates a fast-moving, high-altitude stream of air called the African Easterly Jet. This jet stream fluctuates from north to south, forming waves of air that travel westward over the continent, eventually reaching the Atlantic Ocean. If these waves can pick up enough moisture from the humid regions south of the Sahara and generate atmospheric lift, they can form clusters of thunderstorms that move off the coast and over the ocean. During the late summer months, the waters of the Atlantic are much warmer, providing fuel for the thunderstorms to potentially develop into tropical storms or hurricanes.

Beginning in January and continuing into summer, the African Easterly Jet moves north over the Sahara. There, the jet stream can produce large masses of hot, dry, dust-filled air known as the Saharan Air Layer. The Saharan Air Layer typically forms in late spring and reaches its peak period from mid-June to mid-August. During this time, air layer outbreaks usually develop every three to five days and move west over the continent and into the Atlantic. The dust clouds can range from 2 to 2.5 miles (3.2 to 4 kilometers) thick with a base that can start as low as 1 mile (1.6 kilometers) above the surface. As the African Easterly Jet begins to dip south in mid-August and September, Saharan Air Layer outbreaks quickly become less frequent.

Normally, the dust clouds generated are thin enough that they spread out and disperse as they move across the ocean. However, denser air layers often reach Florida, Texas, and parts of Central America. This dust can cause a thick layer of haze and produce more colorful sunrises and sunsets as the sunlight interacts with the dust particles in the air. It can also pose a problem for people with asthma and who suffer from allergies.

The Saharan Air Layer also has a significant impact on the formation of tropical cyclones in the Atlantic Ocean. The air masses contain about half the moisture of typical topical air masses that move out from the African coast. Because tropical storms or hurricanes thrive in warm, humid air, the air layer can weaken tropical systems that do form. The dust particles within the air also absorb heat energy from the sun, stabilizing the atmosphere and negating another factor in storm formation. The strong winds from the African Easterly Jet travel at a speed of about 25 to 55 miles per hour (40 to 88.5 kilometers per hour), which can create wind shear in a developing storm. Wind shear is a sudden change in wind speed and direction over a short distance. In tropical systems, wind shear can weaken the processes that cause storms to strengthen. Normally, the Atlantic hurricane season intensifies after mid-August when the Saharan Air Layer has subsided, with storms becoming stronger and more frequent.

Meteorologists have been studying the phenomenon since the mid-1960s, when the University of Miami set up a scientific research station on the island of Barbados to collect samples of dust particles. Today, the National Oceanic and Atmospheric Administration (NOAA) uses satellite data and ground-based sensors to track and forecast the Saharan Air Layer. NOAA has also been keeping a record of its observations since the early twenty-first century.

In late June 2020, NOAA meteorologists observed the most dust-filled Saharan Air Layer since they began recording the phenomenon. They estimated the air mass carried 60 to 70 percent more dust than normal and grew to cover an area larger than the forty-eight contiguous United States. The large cloud traveled more than 5,000 miles (8,047 kilometers) across the ocean and eventually covered an area from North Carolina across the Gulf of Mexico to Texas. It affected air quality across the southeastern United States and resulted in the highest concentration of dust particles on the island of Puerto Rico in fifteen years.

Bibliography

Beitler, Jane. “Saharan Dust Versus Atlantic Hurricanes.” National Aeronautics and Space Administration, 28 July 2020, earthdata.nasa.gov/learn/sensing-our-planet/saharan-dust-versus-atlantic-hurricanes. Accessed 2 Sept. 2020.

Belles, Jonathan. “Where African Cyclones Come From.” The Weather Channel, 24 Apr. 2019, weather.com/storms/hurricane/news/2019-04-24-african-tropical-cyclone-hurricane-origins. Accessed 2 Sept. 2020.

Fiallo, Josh. “Saharan Dust Was Florida’s Summer Hurricane Protector. Now It’s Going Away.” Tampa Bay Times, 10 Aug. 2020, www.tampabay.com/hurricane/2020/08/10/saharan-dust-was-floridas-summer-hurricane-protector-now-its-going-away/. Accessed 2 Sept. 2020.

Gorham, Dave. “The Sahara Desert Plays a Major Role in Atlantic Hurricane Development.” StormGeo, 2020, www.stormgeo.com/weather/articles/sahara-desert-atlantic-hurricane-development/. Accessed 2 Sept. 2020.

Harvey, Chelsea. “Saharan Dust Plume Slams U.S., Kicking Up Climate Questions.” Scientific American, 26 June 2020, www.scientificamerican.com/article/saharan-dust-plume-slams-u-s-kicking-up-climate-questions/. Accessed 2 Sept. 2020.

Horstmeyer, Steven L. “El Niño, La Niña, and the Southern Oscillation.” The Weather Almanac, 12th ed. Wiley, 2011, pp. 297–316.

“The Saharan Air Layer.” National Oceanic and Atmospheric Administration, 2020, www.aoml.noaa.gov/saharan-air-layer/. Accessed 2 Sept. 2020.

Sillin, Jack. “What Are Tropical Waves, How Do They Form, And Why Are They Important?” Weather.us, 24 June 2019, blog.weather.us/what-are-tropical-waves-how-do-they-form-and-why-are-they-important/. Accessed 2 Sept. 2020.

Zhang, Zhenxi, and Wen Zhou. "Impact of Saharan Dust on Landfalling North Atlantic Tropical Cyclones over North America in September." Atmospheric and Oceanic Science Letters, vol. 16, no. 1, Jan. 2023, doi.org/10.1016/j.aosl.2022.100276. Accessed 18 Nov. 2024.