Water vapor is water in its gaseous form, and is one of the most important components of the earth’s atmosphere. It not only serves as the raw materials for clouds and rain, it is a major player in regulating the earth’s temperature. Water vapor is actually the most prevalent greenhouse gas. The amount of water vapor present varies widely over both area and time, due to the many sources of evaporation and condensation. Since water vapor has such a big impact on cloud and storm formation, its presence is characterized in many different ways, including satellite imagery. Two of the most common ways to measure the amount in a particular area are dew point and relative humidity.
Mary Knapp, Weather Data Library