Even if there is a mechanism to lift the air, clouds and precipitation may not occur if the low levels of the atmosphere do not contain sufficient moisture. The availability of moisture is revealed on a surface map through the dew point temperatures. If the dew point is close to the corresponding temperature, the air is nearly saturated, so precipitation is quite possible.

Consider the example below where a cold front was approaching the southeastern United States. The values of the temperatures and dew point temperatures at stations ahead of the front are close together, meaning the air is nearly saturated. Since the dew points are quite high, these factors indicate that there is sufficient moisture for precipitation to develop, and the radar echoes on the map reveal that precipitation occur.

Now consider a different example with a stationary front (depicted on the surface map below).

The values of the temperatures and dew points near the front are further apart, meaning the air is quite dry. Therefore, despite the lift provided by the convergence along the front, there is insufficient moisture for precipitation to develop.

frontal lifting

Terms for using data resources. CD-ROM available. Credits and Acknowledgments for WW2010. Department of Atmospheric Sciences (DAS) at the University of Illinois at Urbana-Champaign.

rain or snow