Temperatures are below freezing in the lowest portion of troposphere while there is a layer just above
freezing above the below freezing layer. Precipitation falls as snow aloft, then partial melts as it
falls through the layer that is just above freezing, then refreezes in the subfreezing layer before reaching the
Sleet commonly occurs with a wintry mix of snow, freezing rain and/or rain
Sleet may change over to snow through
evaporative cooling or
cold air advection. Rain may change
to sleet through cooling processes in the lower troposphere.
Forecast sleet using the thickness values: http://www.theweatherprediction.com/winterwx/thicknesscriteria/.