Question

Sound travels faster in rarer medium but Why density of air decreases with increasing humidity or moisture and is said that density of humid air is 5\8 th times density of normal air? Should not density of air increase with more water vapour ?

Answer 1

Water molecules (H2O, molecular weight 18) actually weigh less than the major constituents of atmoshphere which are oxygen or nitrogen molecules (which are O2, molecular weight around 32, and N2, molecular weight 28). Air is primarily nitrogen and oxygen. Each water molecule in the air displaces one molecule of "air". Consequently, the more humid the air, actually the less dense the air becomes .