Dry air consists of oxygen O2 with a molecular weight of 32 and nitrogen N2 with a molecular weight of 28.
Water is obviously H20 meaning two hydrogens ( molecular weight of 2) and one oxygen molecular weight of 16. Therefore a water molecule 'weighs' 18.
Steam with a weight of 18 is lighter than something that is roughly 0.2x32 + 0.8x28 = 28.8. Air is actually a bit heavier however the math is simpler with 20% oxygen and 80% nitrogen.