Clouds over Venus. False-colour image made at ultraviolet wavelengths of the upper cloud layers over Venus, from data gathered by the Pioneer Venus Orbiter. The general structure of these clouds, composed mainly of sulphuric acid, has remained the same over many years. There is a dense (pale) band of cloud near each pole, and the cloud banks at lower latitudes form bow-shaped waves. At the top of the clouds (about 80km altitude), wind speeds may reach 360 km/h. This is probably a result of momentum transfer from the very dense lower atmosphere under the influence of strong Hadley cell circulation.