DOWNWARD PENETRATION OF SNOW

Falling snow modifies the temperature structure of the atmospheric boundary layer as both melting & evaporation takes place. Even if snow does not initially penetrate to the surface (after having fallen out of the parent cloud), if the Wet Bulb Freezing Level (q.v.) is low enough, the intensity of the precipitation is more than just 'light' and the mean wind strength in the melting layer is not too strong, then the snow level can descend considerably below initial conditions. The depth of this 'downward penetration of snow' as it is called, increases as the intensity of rain increases, and/or the wind speed decreases. It will be immediately apparent that the prospect for error in snow forecasting due to these variables in 'marginal' situations will be large!