What is freezing rain?
Freezing rain is a type of precipitation that remains in liquid form, despite having a temperature below freezing. When it strikes a cold surface it freezes almost instantly. It actually start its life as snow, ice, sleet or hail, but passes through a layer of air that’s above 0 °C on the way down to the ground, melting into a liquid water droplet. If these droplets then fall through a zone of sub-zero air just above the ground, they become supercooled. When these supercooled droplets strike surfaces that are close to or below freezing, they freeze on impact forming a glaze of ice.
So just how is it possible to have liquid 'supercooled' water at a temperature below freezing?
Raindrops generally need some sort of microscopic particle to form around, such as a bit of dust or dirt and certainly the droplets can only freeze if this particle is present. However, it is possible that water droplets can exist several degrees below zero and remain in liquid form without such a particle. In this situation they are referred to as 'supercooled'.
Why is freezing rain such a hazard?
The icy glaze can collect on trees or power lines, the sheer weight then causing them to break and collapse resulting in disruption on a potentially large scale. The glaze is of course also extremely slippery and almost invisible, making driving and walking at best impossible and at worst lethal.
Fortunately it's a rare phenomenon here in the UK but worth listening out for it the weather forecasts and taking extra care when the risk is present.