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 layer of air and partially melts. The partially melted snowflake then falls through another cold layer and it refreezes causing sleet.
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.
Graupel: Otherwise known as snow pellets, is snow that falls but then supercooled water droplets form around it. This makes it look like mini Styrofoam balls. They usually have a white hue to them.
Supercooled water is water below our freezing mark of 32 degrees. Graupel is often confused for hail but they are made differently in our atmosphere.
For Graupel to form, you need cloud temperatures to be below the freezing mark of 32 degrees and surface temperatures to be below 45 degrees.
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.
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. The thicker the warm layers are the better chance for rain or freezing rain to happen.