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 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.

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