A:The answer is humid air is less dense. Q:But why humid air is less dense? A:Because water molecules have less mass than the average air molecules do. But It will happen if and only if the pressure is unchanged. Given that dry and humid air are both under the same pressure then we have smaller density for humid air. But when a place is saturate with water vapour the pressure increases (i.e for the vapour pressure of the water). I am thinking of a closed container half filled up with water. At first the air over the water surface is dry but as time passes water molecules from the surface escapes and eventually the air becomes saturated. So the density of the air should increase when air is saturated by water vapour (for the volume is unchanged and there is more molecules). This is the part which I can't understand. Please help me.