Gravity wave clouds over the Gulf of Mexico, Texas, US. Gravity clouds are distinctive ripple-like clouds that usually form over the ocean. The ripples are caused by the movement of a high pressure area and its cold front. Dense air is pushed upwards into the less dense air above it, forming the crest of the wave. Gravity pulls the dense air back down, forming the trough. This image was taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA's Aqua satellite.