If the freezing temperatures, snow flurries and traffic chaos weren’t bad enough, Storm Emma is set to add freezing rain into the mix.
As the name suggests, rain-like water droplets in the air freeze as they move through the atmosphere and layers of air with different temperatures. Here we explain what causes freezing rain and how it differs to snow.
What is freezing rain?
At its most simple, freezing rain is rain that starts its life as snow or sleet and warms slightly as it travels down to the Earth’s surface. If snow or sleet touches a layer of air above 0°C (32°F) any time before it reaches the surface, the slight increase in termperature melts the ice into an extremely cold, or a so-called “supercooled” water droplet. This droplet then freezes again, almost immediately, once it lands on a cold surface.
Water typically freezes at 0°C and only does so around an ice nucleus, but when it falls as rain the process is slightly different. Raindrops have a similar nucleus, such as particles of dust and dirt, that the water drop forms around. It is possible for water droplets to go below 0°C while staying as a liquid (so without freezing) without a nucleus in certain conditions – like those we’re seeing in the UK – and this is when they become supercooled.
When supercooled, or freezing rain, accumulates on trees or powerlines, the weight can cause them to collapse or break leading to power cuts, damage to homes and cars and road blockages. It can also be the cause of black ice on the roads.
Freezing rain vs snow
Even though, at one point in its lifecycle, snow, sleet and freezing rain can effectively be the same thing, the main differences between freezing rain and snow are:
Snow is made up of ice crystals, is white and “fluffy” and is defined by the Met Office as “solid precipitation which occurs in a variety of minute ice crystals at temperatures well below 0°C but as larger snowflakes at temperatures near 0°C”.
It forms when ice crystals in clouds merge to a point where they way so much, gravity causes them to fall to the ground as snowflakes. The temperatures need to be low, below 2°C and there needs to be enough moisture in the atmosphere via ice crystals to form snowflakes. It doesn’t, however, need to be below zero to snow, which is why it often feels warmer before snow falls.
Sleet forms when the layer of sub-freezing air is fairly “deep”, between 3,000ft and 4,000ft. This depth means the water droplets have time to freeze into ice which then fall to the ground as sleet. Sleet and hail are not the same thing; hail is larger and is linked with thunderstorms.
Freezing rain and ice storms are common in parts of the US but because temperatures don’t drop in the same way across the UK as they do in the US, freezing rain is much more rare over here.
Freezing rain is typically the weather threat that creates the most car accidents, injuries, and deaths in winter storms. Many people can drive in the rain and snow, but when the roads become icy, it is almost impossible to drive. Severe ice storms can shut down large cities, result in thousands of power outages, and the most violent ones can also become billion dollar disasters (rare).
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