Venus essentially has zero humidity because of its high surface temperature of 480 degree Celsius and its atmosphere of 97 percent carbon dioxide. These conditions result to the runaway greenhouse effect, wherein water vapor that rises in the atmosphere, to the so-called "cold trap" region, quickly condenses back to liquid.
The water may fall back to Venus' surface or it may be destroyed by ultraviolet light. Ultraviolet light dissociates water into hydrogen gas, which escapes to the atmosphere, and one oxygen atom, which combines with carbon to form carbon monoxide or carbon monoxide to form carbon dioxide. This partly explains why the carbon dioxide composition of Venus' atmosphere is so high.