Convection occurs because air near the ground is warmer than air at higher altitudes. On a clear night over flat terrain, air flow can be laminar near the ground. Usually, however, it is turbulent near the ground—producing, for example, random gusting in wind—but becomes laminar at higher altitudes. Turbulent convection nevertheless occurs in most clouds, leading to random billowing shapes. The "turbulence" that causes bumps in airplanes is often associated with clouds, though sometimes with larger-scale wave-like fluid motions such as the jet stream.