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 has remained the same over many years. There are two dense bands of cloud near each pole, and the cloud banks at lower latitudes form bow- shaped waves. These high clouds are mainly sulphuric acid droplets. At the top of the clouds (about 80km altitude), wind speeds may reach 360 km/h. This is thought to be a result of momentum transfer from the very dense lower atmosphere under the influence of strong Hadley cell circulation.