Freezing rain is a type of liquid precipitation that falls as a supercooled water droplet until it strikes a cold surface, at which point it freezes almost instantly.
We all know that water freezes at 0 °C but in the same way that raindrops need a nucleus (microscopic particles of dust or dirt) to form around, water droplets can only freeze around an ice nucleus. However, it is possible that water droplets can exist several degrees below zero and remain in liquid form without a nucleus. This is when they are supercooled.
Supercooled water droplets occur in clouds a lot of the time, especially in winter, and they form a very important role in the way that rain forms.
Freezing rain tends to 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.
It’s common across parts of the USA, for example, for weather systems to produce a lot of freezing rain. These are called ice storms, and if enough glaze collects on trees or power lines, the weight of the ice can cause them to break and collapse resulting in disruption on a large scale. The glaze is of course also slippery, which makes driving and walking almost impossible.
The conditions needed for freezing rain to occur are quite specific and we don’t see them very often, making this phenomenon quite rare in the UK. We are more likely to see rain falling onto already frozen surfaces, or wet surfaces that freeze as temperatures drop below 0 °C overnight.