Snow: For snow to occur the air temperature has to be at or below 32 degrees (cold air) from the surface up to the clouds.
Sleet: For sleet to occur there has to be a thin layer of air above 32 degrees (warm air) between two thicker layers with cold air. Sleet starts as snow when it leaves the cloud. As it falls it moves through a warm air layer and partially melts. The partially melted snowflake then falls through another cold layer and it refreezes causing sleet.
Rain: During the winter months rain starts out as snow. For rain to occur the snowflake falls through a thick layer of warm air all the way to the surface. Because the air is above 32 degrees all the way to the ground the droplet never refreezes and remain as rain.
Freezing rain: For freezing rain to occur there has to be a thin layer of cold air at the surface. The layer is so thin that the rain doesn’t have time to freeze while falling but when it hits the ground it freezes into ice.
So, it all comes down to how thick the layers of cold and warm air are between the cloud and the ground. The thicker the cold layers are the better the chance for frozen precipitation like sleet and snow to happen. Then thicker the warm layers are the better chance for rain or freezing rain to happen.