The extreme daytime heating in the Sahara Desert, especially during the summer, causes instability in the lowest level of the atmosphere. Dust-laden air rises and begins moving westward. As the air travels—a trip that often takes several days—it continues heating. When this Saharan Air Layer moves off the African coast and over the Atlantic Ocean, it is undercut by a cooler, wetter layer of air. Air normally cools with altitude, but the Saharan Air Layer passing over cooler air currents causes a temperature inversion, which suppresses mixing. As a result, Saharan dust often travels across the Atlantic, sometimes remaining visible throughout the trip.