Venus as a planet resembles the Earth, but has a much hotter and denser atmosphere due to an extreme case of the greenhouse effect, caused by compositional differences and the thick cloud cover. Studies of the lower atmosphere are inhibited by the cloud opacity, which makes remote measurements at most frequencies short of the radio range quite difficult. Progress in understanding of the composition and thermal structure below the clouds has been made by the Pioneer and Venera entry probes of the 1970s, and more recently with results from the Galileo fly-by in 1990. The latter exploited the newly discovered near-infrared ‘windows’ to achieve measurements of carbon monoxide and water vapour abundances in the deep atmosphere, and provided the first detailed view of the global cloud structure. The morphology and spatial variations seen in the main mass of clouds are remarkable, and suggest a powerful and diverse meteorology dominated by convection. Carbon monoxide is significantly more abundant at high northern latitudes than at low latitudes in either hemisphere.
