Most precipitation that reaches the ground actually begins as snow high in the atmosphere. These snow flakes develop somewhere above the freezing level where the air temperature is less than 32 F (the dashed blue line), and begin to fall toward the earth as snow. If ground temperature is above 32 F, the freezing level must be located somewhere above the ground. The falling snow passes through the freezing level into the warmer air, where it melts and changes to rain before reaching the ground.

When the air temperature at the ground is less than 32 F, the precipitation begins falling as snow from the clouds.

Since it is falling into cold air, the snow does not melt on the way down and reaches the ground as snow. This is why cold air is important for there to be snow.

Once in a while, a very thin layer of warm air is found near the surface and temperatures may be several degrees above freezing. However, since the layer of warm air is so shallow, the snow reaches the ground in tact before it has a chance to melt and become rain. This is how snow falls when the surface temperatures are above freezing.

Forecast Tip:
When forecasting precipitation type, if temperatures are expected to be above freezing, then rain is most likely. If temperatures are expected to be below freezing, then forecast for snow.

moisture

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.

Remote Sensing