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. The clouds are composed of sulphuric acid and various other corrosive compounds, and the atmosphere contains little water.
There are several reasons to believe this. First, there is evidence that there was a lot of water on Venus at one time. This comes form Pioneer spacecraft observations which showed 150 times more DEUTERIUM (heavy isotope of hydrogen) per hydrogen atom than on Earth. This deuterium naturally forms from the dissociation of water (H20) into its respective components hydrogen and oxygen.
Because deuterium is a heavier isotope of hydrogen (having 1 neutron in the nucleus in addition to just the proton) it rises more slowly in the atmopshere. Thus the lighter isotope of hydrogen largely escaped but the heavier isotope remained. This provides evidence that there was more water on Venus. Enough to make a surface covering ocean about 25 meters deep.
The reason that the UV radiation could penetrate the atmosphere is because the oxygen was not able to form ozone but instead mainly formed oxides in the soil.
Now Venus is extremely dry and it is estimated that one could only form a surface covering ocean 0.3 m deep as compared with the Earth where one can form a surface covering ocean of 3000m deep.
Note that although the hot surface temperature of Venus is understood (see next section) the fact that there are high-velocity winds in the upper atmosphere but a relatively stagnant lower atmosphere with only very weak winds blowing at the surface is not well understood.