However, a typical 1 kilometer thick cloud contains only enough water for a millimeter of rainfall, whereas the amount of moisture in the atmosphere is usually 10-50 times greater. Most water is transported in the form of water vapor, which is actually the third most abundant gas in the atmosphere. Water vapor may be invisible to us, but not to satellites, which are capable of collecting data about the moisture content of the atmosphere. From this data, visualizations like this water vapor image are generated, providing a visual picture of moisture transport in the atmosphere. Bright areas indicate higher amounts of moisture and are often associated with clouds.
Dark areas indicate less moisture, or relatively drier air.
However, moist air does not always contain clouds . Credits and Acknowledgments for WW2010.