Refer to figure.

FREEZING RAIN. The conditions needed for freezing rain are quite specific.
Generally, precipitation first falls from a cloud as snow. If it falls through warmer air before reaching the ground, it can melt and turn to rain droplets. If it then falls through cold air again before hitting the ground, the droplets can become supercooled (which means that they are still falling in liquid state, even though their temperature is below 0ºC).

• Ahead of the warm front where the freezing level in the cold air is lower than the freezing level in the warm air above it => As the rain falls from the warmer air (above 0°C) into the cold air (below 0°C) below it freezes => forming freezing rain.