The Saharan Air Layer (SAL) is a mass of extremely dry and arenaceous air that originates from the Sahara Desert of North Africa. The SAL is almost always constant around the globe, but it normally occurs during the northern hemisphere's summer. The Saharan Air Layer is located between ~5,000-20,000 ft (~1500-6000 m) above the ground. This means that it can spread quickly.
Because of the location of the Saharan Desert, the Saharan Air Layer only affects the northwest hemisphere.
Affect on tropical cyclones
The Saharan Air Layer normally produces a negative impact on tropical cyclonegenesis. The arenaceous air impedes hurricane development by reducing convection and creating a tradewind inversion. This causes the atmosphere to become stable. Because of the large scale of the air layer, it can affect entire hurricane seasons, regardless of other factors including reduced wind shear or warm sea temperatures. One example of a hurricane season affected by the Saharan Air Layer is the 2006 Atlantic hurricane season. It acted as a large field of dry air, causing the season to end early (October 3) and produce only 10 storms.