Sleet

Definition: Generally transparent, globular, solid grains of ice which have formed from the freezing of raindrops or the refreezing of largely melted snowflakes when falling through a below-freezing layer of air near the earth's surface. (Glossary of Meteorology)

The red line is a temperature profile through a portion of the atmosphere. In this example, the freezing layer at the surface is large enough to freeze completely melted snowflakes (rain) or to refreeze partially melted snowflakes.

The Wednesday 6 am sounding on December 13th 2000 at Shreveport indicated a melting layer between 3,000 and 11,300 feet with approximately 4,000 feet where temperatures were between 45 to 50 degrees Fahrenheit. The shallow freezing layer was from the surface to 3,000 feet with the coolest temperature at 2,100 feet at around 23 degrees Fahrenheit. This temperature profile of the atmosphere (sounding) caused the snowflakes to completely melt and then not have time to refreeze into sleet before reaching the ground. Further north, across the Red River Valley of northeast Texas, southeast Oklahoma, and southwest Arkansas, the freezing layer at the surface was larger (more depth) so the liquid droplets had time to freeze into sleet.