The Saharan Air Layer (SAL), as its name suggests, is a very dry deep layer of air (typically found between 5,000 and 20,000 feet above the ground) originating from the Sahara Desert of Africa. Deep easterly winds in the low and middle levels of the atmosphere transport this SAL across the tropical and portions of the subtropical Atlantic during the summer, through the Caribbean, and even into the Gulf of Mexico and surrounding land areas, as well as the southeastern U.S. Episodes of the SAL passing across the local area of Puerto Rico and the U.S. Virgin Islands frequently occur between tropical waves during the summer months. Minerals and dust from the Sahara suspended within the SAL gradually precipitate out of the atmosphere as the SAL moves across the Atlantic. It is this suspended dust that produces the hazy skies during SAL events.
Recent research into the effects of SAL on hurricanes, tropical cyclones, and tropical waves, shows that this dry layer of air can wrap into the circulation of developing tropical cyclones and act to slow down or retard intensification. Also, showers and thunderstorms developing within and along the edge of the SAL have been shown to produce squall lines and arcing outflow boundaries as the dry SAL enhances evaporatively driven downdrafts. A classic case of this occurred across the local area on the afternoon of June 20th, 2008, as a tropical wave moved through the local area with strong low level winds, and a broad zone of SAL stretched across and to the north of the tropical wave.
A line of thunderstorms developed across the Anegada Passage and the Caribbean waters south and southeast of St. Croix. As these thunderstorms grew and marched northwestward across the Virgin Islands and adjacent waters, they developed into a squall line with widespread wind gusts of 30 to 40 knots reported throughout the area, and then continue to expand west and northwestward as they moved deeper into the SAL.