During spells of wintry weather, often when the wind is blowing from the east, you may hear weather forecasters glibly talk of rain turning to snow as a frontal system approaches the UK from the southwest. This is not just sloppy, it is seriously misleading, because rain never actually turns to snow; it is a physical impossibility. However snow does turn to rain, all the time. In fact, much of the rain that falls in the UK starts off as snow, even in high summer, because the temperature at cloud-level is frequently below the freezing-point. As the snow falls through the air below the cloud the ambient temperature becomes progressively higher, so the snowflakes turn to raindrops.
So what happens when we perceive rain turning to snow? It is simply the result of the air beneath the cloud getting colder for one reason or another. Thus, instead of the snow turning to rain as it falls, it simply remains as snow all the way down from cloud to ground. Why does the air beneath the cloud get colder? There are three main mechanisms: advection (colder air flowing in from elsewhere), evaporative cooling (raindrops evaporating as they fall through a layer of dry air, and cooling it as they absorb the energy needed to change from liquid to vapour), and snowmelt cooling.
The best way to explain what happens in the latter case is to look at an example: the best recent occurrence was on Wednesday 14 January 2004 when a shallow depression tracked eastward across Wales and the Midlands. Ahead of the area of rain and snow, temperatures stood at 5 or 6°C, so why did the rain - turn so readily to snow? The clue was the absence of much wind. Crucial also was the heaviness of the rain. In the UK, nearly all the rain that falls starts off as snow at cloud-level. This is because the temperature normally decreases with height above the Earth's surface. When the air is saturated, according to the laws of physics the temperature drops by 0.5°C for every 100 metres above the ground. Thus on that Wednesday morning the temperature was around zero about 1000 metres above places like Coventry and Northampton an d at that altitude the falling snowflakes would begin to melt.
However, a large amount of energy is needed to melt snow, and high above England on a dull January day the only source of energy is the air itself. This energy is known as 'latent heat' specifically, the latent heat of fusion. Thus the melting snowflakes absorbed heat from the surrounding air, causing the temperature to drop, in turn allowing the falling snow to penetrate lower and lower until it eventually reached the ground. The heavier the precipitation the more energy is absorbed and the more rapid the temperature will drop, but this can only happen when there is no wind to mix the cooled air with "uncooled" air arriving from elsewhere.
This mechanism produced even bigger surprise snowfalls in central and southern England in late-January 2001 and early-January 1994, so forecasters should always be on the lookout for it.