Humid air is less dense than dry air because a molecule of water (m = 18) is less dense than a molecule of nitrogen(m = 28) and a molecule of oxygen (m = 32). About 78% of the molecules in dry air are nitrogen (N₂). Another 21% of the molecules in dry air are oxygen (O₂). The final 1% of dry air is a mixture of other gases. For any gas, at a given temperature and pressure, the number of molecules present is constant for a particular volume - see ideal gas law. So when water Louis J.

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