In most winter weather events, we see several different precipitation types ranging from snow, sleet, freezing rain, and plain rain. The temperature profile through the middle and lower levels of our atmosphere determine what precipitation type will fall.
When freezing rain occurs it can occur from two different processes. The first one involves a snowflake aloft that enters a warm layer. This warm layer melts the entire snowflake/ice crystal into a water droplet. The water droplet then falls to the ground, where the surface is at or below freezing (32F). The temperature could be warmer than 32F, but as long as the ground temp is at or below freezing the rain will freeze. This is displayed on a Skew-T/Log-P diagram with the temperature being displayed diagonal in Celsius. The solid black line is the temperature with the hatched line being the dew point:
The image taken from the National Weather Service in Louisville shows the temperature profile of a freezing rain event. The temperature could go back to below freezing after the snowflake melted in the cloud, but for ice crystallization to occur we typically need to see the droplet reside in temperatures at or below -10C. This leads us to our second way of freezing rain occurring…
The sounding shows temperatures below freezing throughout the column, but the precipitation type will still be freezing rain. The lack of saturation in the cloud above 700mb only allows for temperatures of -5C in the cloud. This is not cold enough for ice crystallization to occur resulting in water droplets remaining super cooled. As a result, the shallow layer of moisture ends up producing light freezing rain that freezes on the cold surface below freezing.
Sleet has a warm layer involved in the cloud, but it is typically shallow or not as warm. Instead of the snowflake completely melting, as mentioned in the freezing rain case, the snowflake partially melts. This keeps the ice nucleus intact allowing for refreezing once the temperature drops below freezing.
The whole column is below freezing with ample moisture at or below -10C. This allows for enough ice crystallization for the snow to fall to the ground.