The winter season can produce various types of precipitation – rain, freezing rain, sleet, or snow. The form we see at the surface depends on the temperature profile of the lower atmosphere.
All precipitation starts out as snow up in the clouds. But, as it falls toward the Earth, it can pass through one or more layers of air with different temperatures. When the snow passes through a thick layer of warm air – above 32°F – it melts into rain. If the warm air layer extends all the way to the ground, rain will fall at the surface. However, if there is a thin layer of cold air – below 32°F – near the ground, the rain becomes supercooled and freezes upon impact with anything that has a temperature at or below 32°F. This is known as freezing rain. It is one of the most dangerous types of winter precipitation, as it forms a glaze of ice on almost everything it encounters, including roads, tree branches, and power lines.
Sleet is a frozen type precipitation that takes the form of ice-pellets. Passing through a thick layer of sub-freezing air near the surface, liquid raindrops are given enough time to re-freeze before reaching the ground. Sleet often bounces when it hits a surface, but does not stick to anything. It can, however, accumulate.
Snow is another type of frozen precipitation. It takes the shape of six-sided ice crystals, often called flakes. Snow will fall at the surface when the air temperature is below freezing all the way from the cloud-level down to the ground. In order for the snow to stick and accumulate, surface temperatures must also be at or below freezing.
When two or more of these precipitation types fall during a single storm, it is called a wintry mix.