Dear Tom,
When water evaporates, it goes into the air. How much heavier is moist air than absolutely dry air?
Alan Majon

Dear Alan,
The visible and gaseous form of water is water vapor, and water vapor occurs in highly variable amounts in the atmosphere. It is always present in air, though it is present in very small amounts in very dry air.

Pure water is composed of two hydrogen atoms and one oxygen atom, H2O. It has a molecular weight of 18 grams per mole. One mole of a gas has a volume of 22.4 liters at standard conditions of temperature and air pressure (32 degrees Fahrenheit and one atmosphere of pressure, 1 bar).  

Absolutely dry air is air that contains no water vapor and it is mostly a mixture of molecular nitrogen (N2) and molecular oxygen (O2). The molecular weight of dry air is 28.97 grams per mole.

Completely dry air, weighing 28.97 grams per mole, will always weigh more than an equal volume of moist air (which consists of dry air mixed with water vapor that weighs 18 grams per mole). That is to say, pure dry air is heavier than any mixture of dry air and water vapor.

When water evaporates, the air into which it evaporates becomes lighter. Humid air might feel heavier than dry air, but in reality it is lighter.