There must be a layer of air near the ground whose temperature is below freezing, where water turns to ice. Above this layer of freezing air must be a layer of warmer air. As snow falls through the warm air, it melts or partially melts into raindrops. It forms sleet before it reaches the ground.
Sleet is pellets of ice that form when snow falls into a warm layer and melts into rain. The rain then falls into a freezing layer of air that is deep enough to refreeze the raindrops into pellets. Sometimes the snow does not completely melt and the partially melted snowflakes refreeze into snow pellets.:
