sleet, precipitation of small, partially melted grains of ice. As raindrops fall from clouds, they pass through layers of air at different temperatures. If they pass through a layer with a temperature below the freezing point, they turn into sleet. Snowflakes that have melted by passing through a warm layer will turn into sleet if they then pass through a freezing layer. Sleet often falls together with snow and rain, and may deposit an icy coating on exposed surfaces. Sleet occurs only during the winter, while hail, a different form of icy precipitation, may fall at any time of the year.