Water existing as a gas is called water vapor. When referring to the amount of moisture in the air, we are actually referring to the amount of water vapor. If the air is described as "moist", that means the air contains large amounts of water vapor. Common sources of moisture for the United States are the warm moist air masses that flow northward from the Gulf of Mexico and western Atlantic Ocean.

As cyclones move eastward from the Rocky Mountains, southerly winds ahead of these storm systems transport the warm moist air northward. Moisture is a necessary ingredient for the production of clouds and precipitation.