In the UK, many of our snow events are right on the knife-edge between rain and snow. Many of us will have experienced days where we’ve had cold, heavy rain gradually turn into snow. However, you may not realise that the melting of snowflakes into raindrops way above our heads can cause snow to fall at lower levels as time progresses.
But how does this occur? One process by which a rainy day can turn progressively more wintry is known as evaporative cooling. This process relies on the principle that when a substance changes state – in this case from solid to liquid – it either releases or absorbs heat from its surroundings.
When snowflakes melt into raindrops, they require energy from their surroundings to do so in the form of heat – much like when you hold an ice cube in your hand, it cools your hand as it melts.
As these snowflakes melt into raindrops as they fall through the atmosphere they cool the surrounding air, eventually enough to drag the freezing level down and bring a greater risk of snow surviving to the surface.
This process relies on their air being still, however. Too much wind can stir up the atmosphere and mix up the cooled air with less cold air nearby.
