The flow downstream of a body immersed in a stream, or the flow behind a body propagating through a fluid otherwise at rest. Wakes are narrow elongated regions aligned with the flow direction and filled with large and small eddies. The wake eddies of a bridge pier immersed in a river stream, or of a ship propelled through the water, are often visible on the surface. On windy days, similar wakes form downwind of towers, smoke stacks, or other structures, but such eddies in the air are not generally visible unless some smoke or dust is entrained in them. Wakes are sustained for very large distances downstream of a body. Ship wakes retain their turbulent character for miles behind a vessel and can be detected by special satellites hours after their generation. Similarly, condensation in the wake of aircraft sometimes looks like narrow braided clouds, traversing the sky.