The atmosphere of Venus is composed of about 96% carbon dioxide, with most of the remainder being nitrogen. The atmosphere appears to be relatively clear until the cloud deck starts about 50 km above the surface. As noted in the previous section, the clouds are composed of sulphuric acid and various other corrosive compounds, and the atmosphere contains little water.
The pressure of the atmosphere is about 90 times that of the Earth at the surface, and the surface temperatures on Venus are around 500 degrees Celsius, exceeding that of Mercury and hot enough to melt soft metals. Calculations indicate that for the temperatures to be so high there must be a mechanism in the Venusian atmosphere that traps solar radiation very effectively. As we will see in the next section, these rather remarkable properties of the atmosphere are thought to be a consequence of a runaway greenhouse effect.
There are high-velocity winds in the upper atmosphere, but the atmosphere below the cloud deck appears to be relatively stagnant, with only very weak winds blowing at the surface. Convection driven by differential solar heating should give rise to winds of only a few meters per second, so the high velocity upper level winds, and the contrasting stagnation of the lower atmosphere, are not well understood.