Satellite image of cloud gravity waves off the west coast of Africa. These distinctive criss-cross patterns of clouds are formed by the rise and fall of colliding air columns. Dry air coming off the Namib desert moves out under the balmy, moist air over the Atlantic ocean and forces it upwards. As the humid air rises to a higher altitude, the moisture condenses into droplets, forming clouds. Gravity rolls these newly formed clouds into a wave-like shape. When moist air goes up, it cools, and then gravity pushes it down again. As it plummets toward the earth, the moist air is pushed up again by the dry air. Repeated again and again, this process creates gravity waves. Clouds occur at the upward wave motions, while they evaporate at the downward motions. During the course of the day, the clouds move out to sea and stretch out, as the dry air flowing off the land pushes the moist ocean air westward.