Winter Precipitation
 
Forecasting precipitation in the cold season is notoriously difficult, and is often the bane of the weather forecaster. When the temperature hovers close to freezing, it is possible for several different precipitation types to fall, sometimes within in very narrow area. These can range from rain, to sleet and then snow, but it is possible for freezing rain and graupel to also fall: these varying types of precipitation all produce different and sometimes significant impacts and so forecasting accurately exactly when and where they occur is very important. The type of precipitation depends on the temperature profile of the atmosphere, and whether it is above or below freezing at the surface. Rain can rapidly change to sleet and snow as you cross the freezing line at the surface, for example, but this is the change in precipitation that must be forecast most accurately, even if it is hardest to do so.
 
Atmospheric Profile for Rain
 
 
 
 
Typical vertical profile during a rain event in a mid-latitude location in the winter. Most of the atmosphere remains below freezing except close to the surface. 
Credit: NWS JetStream
Typically, rain that falls in the winter starts off its life in frozen form: as snow and ice crystals. This is because at high altitude, the atmosphere remains below freezing. Sometimes, the temperature close to the surface may be above freezing (as is typical for much of the UK during the winter), meaning that the snow that initially falls, melts and reaches the surface as rain. Since most of the atmosphere is usually below freezing, only the bottom-most layer needs to be cooled for snow to reach the surface. 
 
Atmospheric Profile for Snow
Typical vertical profile during a snow event in a mid-latitude location in the winter. The whole of the atmosphere must remain below freezing.
Credit: NWS JetStream
When the whole vertical profile of the atmosphere is below freezing, then the snow and ice crystals that fall from the cloud layer will remain solid as they reach the surface. Therefore, we see snow falling. What makes forecasting where rain will turn to snow so difficult is that this transition depends on a change in temperature of just one or two degrees at the surface, from above to below freezing - something that is not easy to forecast accurately. It is also possible for snow to fall when the surface temperature is a few degrees above freezing, provided that the upper atmosphere is cold enough, and the snowflakes are large and fall fast, reducing the time that they have to melt. Sleet will fall when the temperature close to the surface is above freezing, and the snowflakes are in the process of melting as they reach the surface.
 
Atmospheric Profile for Freezing Rain
Typical vertical profile during a freezing rain event in a mid-latitude location in the winter. There must be a temperature inversion, with the temperature close to the surface below 0°C.
Credit: NWS JetStream
Under specific atmospheric conditions, freezing rain can occur. A temperature inversion in the atmosphere is required: that is a layer of the atmosphere where the temperature increases with height, rather than decreases. This temperature inversion must have a region with temperatures above 0°C, so that any snow that falls from clouds above will melt. The new raindrops fall through the inversion and into a below-freezing layer close to the surface. The droplets do not have time to re-freeze; instead, they become supercooled (they remain liquid, but are below 0°C). This means that they instantly freeze onto anything they touch at the surface - assuming that is below freezing too, forming a glaze of ice over everything. The resulting ice storms can be devastating for everything at the surface, bringing down trees, power lines and making roads and footpaths impassable. 
 
Atmospheric Profile for Graupel
Typical vertical profile during a graupel event in a mid-latitude location in the winter. There must be a temperature inversion, with the temperature close to the surface well below 0°C.
Credit: NWS JetStream
A slight modification of the profile for freezing rain can lead to the formation of graupel (ice pellets). An overall colder atmosphere with a temperature inversion, but a smaller layer above freezing and a much deeper, colder layer close to the surface is required. As before, snow falling from the clouds will melt in the temperature inversion. As the rain continues to fall into the much colder layer, it has time to fully re-freeze into ice pellets before reaching the ground. Temperatures at the surface are typically below -4°C, but this depends on the depth of the cold layer. Both graupel and freezing rain are relatively rare in the UK.