The Saharan Air Layer (SAL) is an extremely hot, dry and sometimes dust-laden layer of the atmosphere that often overlies the cooler, more-humid surface air of the Atlantic Ocean. In the Sahara Desert region of North Africa, where it originates, it is the prevalent atmosphere, extending from the surface upwards several kilometers. As it drives, or is driven, over the ocean, it is lifted above the denser marine air. Sandstorm, strong dry wind blowing over the desert that raises and carries along clouds of sand or dust often so dense as to obscure the sun and reduce visibility almost to zero; also known as a duststorm. Such a wind is usually the result of convection currents created by intense heating of the ground. They usually come out of nowhere, blowing dust and sand into villages and the homes of nomadic tribes.