LESSON 9

Moist Air is Lighter than Dry Air

For now we will define moist air as a volume of air with lots of water molecules and dry air as a volume of air that contains only a few water vapor molecules. To understand why moist air is lighter than dry air we have to define a few concepts.

1. A molecule is the smallest particle of a substance that still has the properties of that substance. Molecules are composed of atoms. A molecule of water has the properties of water and is composed of two hydrogen atoms and one oxygen atom.
2. The weight of an individual atom is represented by its atomic weight. The atomic weight of hydrogen (H) is 1, oxygen (O) is 16, nitrogen (N) 14 and carbon (C) has an atomic weight of 12. The weight of a molecule is determined by summing the atomic weights of its atoms. A water molecule (H₂O) has a molecular weight of 18 (1+1+16). Free nitrogen (N₂) has a molecular weight of 28 (14+14) and an oxygen molecule (O₂) a weight of 32.
3. A fixed volume of a gas at constant pressure and temperature has the same number of molecules. It does not matter what the gas is, the same number of molecules will exist in that volume. This is known as Avogadro's Law.

To make a given volume of air more moist, we need to add water vapor molecules to the volume. To add water molecules to the volume, we must remove other molecules to conserve the total number of molecules in the volume (Avogadro's Law). A fixed volume of air consists mostly of nitrogen and oxygen atoms, which weigh more than water vapor molecules. Thus to make a given volume of air more moist we replace heavy molecules with ones that are lighter. Thus moist air is lighter than dry air.

Return to Lesson 9 Page.

Return to Weather For Pilots Main Page

Local Madison Weather