In the winter, forecasting what kind of precipitation will fall from the sky can be very tricky. It all depends on temperature, but not just the temperature at the surface. It also deals with the temperature profile in the upper levels of the atmosphere.
Rain happens all year round, but did you know that all precipitation starts out as snow and ice in the upper levels of the atmosphere? In the far upper levels of the atmosphere, temperatures are way too cold, in turn all precipitation is snow. As the snow fall through the layers of the atmosphere temperatures stay above the freezing point allowing the snow to melt and fall as rain at surface.
Freezing rain: Precipitation continues to stay below freezing in the upper levels and falls as snow and ice. There is a large area of warm air above the surface that will allow the snow to melt. Different than regular rain, there is a small area of cold air near the surface. This allows the water droplets to cool just enough that when it falls at the surface it freezes on contact.
Freezing rain can be deadly, especially if it lasts over a long period of time. Enough build-up of ice can cause tree limbs to fall and the roads to be very slick and almost impassable. If the ice becomes heavy enough on power lines, it can cause them to fall and break. This then could lead to massive power outages.
Sleet: For sleet to form, there must be a smaller area of warm air aloft, this will allow the snow to partially melt. A larger area of cold air above the surface will allow the partially melted snowflake to re-freeze into a solid. These are also known as ice pellets.
Snow: It’s simply just ice crystals which then form into flakes. The entire atmosphere from the upper levels to the surface need to be below freezing. This then allows the snowflakes from the upper levels to make it down to the surface.
One form of precipitation you may not have heard of is graupel, also known as snow pellets. Graupel forms when snowflakes are covered with a layer of ice. Unlike sleet, graupel is soft to touch and can fall apart in your hand because it is mainly made of snow instead of ice.