text: is water vapour heavier/ lighter than dry air? substantiate your answer.?   sorry, water vapour is lighter than dry air.  imagine. take one cubic foot of dry air. on average, it consits of 78% nitrogen molecules and 21% oxygen molecules, and 1% of various other gases. now, each molecule of nitrogen has a molecular weight of 28. and each molecule of oxygen has a molecular weight of 32.  according to the laws of nature, if u add watr molecules now to that cubic foot of dry air, either some nitrogen molecules or oxygen molecules would have to leave.  but, the water molecules, which would replace nitrogen or oxygen, have a molecular weight of 18. this is lighter than both nitrogen and oxygen. that is, replacing nitrogen or oxygen wid water vapour decreases its weight per cubic foot. that meens its density decreases. read the source below for a more detailed explanation.