Which weighs more damp air or dry air?

Damp air is the lightweight. Dry air has more massive molecules like diatomic Oxygen and diatomic Nitrogen. In damp air these molecules are replaced by water molecules which have a smaller molecular weight. Water vapor weighs about 62 percent of an equal volume of dry air.

A good pilot knows that the high humidly will make a plane’s rate of climb longer and the landing speed higher. It’s a good time to have a nice long runway.