Typically, it gets colder as you go higher in the atmosphere, but that's not always the case. Sometimes, cold, relatively heavy air gets trapped near the ground, with a warmer layer just above. This topsy-turvy layering can create forms of precipitation other than rain or snow.

Most precipitation actually starts as snow in cold clouds many thousands of feet up. If these flakes fall into a warmer layer, they can partially or totally melt into raindrops. Then, when these drops fall into a cold layer near the ground, they sometimes refreeze in the air, forming pellets of ice that we call sleet.

If the drops don't freeze until they reach the surface, you get a coating of ice - the infamous freezing rain. The lesson here for forecasters: it's not enough to just know the temperature at the surface in order to predict what form precipitation will take.