What is freezing rain and why does it happen?

8 02 2012

We take a look at what conditions lead to freezing rain.

English: Ice glaze on crabappleFreezing rain happens when rain droplets fall through air thats temperature is below zero and freezes on impact with the ground or another object to form clear ice – also known as glaze.  Conditions have to be ‘just right’ to get freezing rain and it doesn’t happen very often in the UK, making it a relatively rare phenomenon.

Generally, freezing rain starts its life as snow, ice, sleet or hail, but passes through a layer of warmer air on the way down. This causes it to melt and return to a liquid form. As it continues towards the ground, it then briefly passes through colder air again, causing the water droplets to become ‘supercooled’ – colder than 0 °C but still in liquid form. When these supercooled droplets fall on ground which is close to or below freezing, they can freeze on impact – quickly creating treacherous conditions.

An obvious question is how can water become ‘supercooled’ – as we know it usually turns into ice when it drops to 0 °C or below? This is because water needs small particles of dust or dirt – what’s called a nucleus – to freeze around. If there is no nucleus present, then water can remain in its liquid form at temperatures well below freezing – that is, until it comes into contact with something. It is common for water droplets in clouds to be in a supercooled state, but it is rarer for supercooled droplets to get to ground level. It’s also quite difficult to replicate the supercooling of water in a laboratory.

The effects of freezing rain can have large impacts. It can create a layer of ice on anything cold it comes into contact with – this means not only a potential for icy roads and pavements, but also a risk to things like aircraft and powerlines.

For more information on what to do in severe icy weather, visit our severe weather advice page. Keep up to date with your local weather forecast and warnings.


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