Freezing rain is one of the rarest and most dangerous weather phenomena in the UK, creating instant ice that can cause black ice, power outages, and treacherous travel conditions. But how does freezing rain form, and why doesn’t it freeze until it hits the ground?
In this video, we break down the science behind freezing rain, explain how it differs from snow and sleet, and revisit the icy chaos caused by the Beast from the East in 2018.
In this video, we break down the science behind freezing rain, explain how it differs from snow and sleet, and revisit the icy chaos caused by the Beast from the East in 2018.
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00:00What is the most hazardous type of weather
00:04that falls from the clouds?
00:06Snow, drizzle, hail?
00:09Certainly one that's right up there is freezing rain.
00:14Now, thankfully, it's pretty rare in the UK,
00:16but when true freezing rain does fall,
00:19it's very, very dangerous,
00:22coating everything instantly in a layer of clear ice,
00:25causing power failures
00:27and making simple journeys on foot, bike, or by car
00:31truly treacherous.
00:33So just what is this mystery slip-inducing threat?
00:38Well, it's basically very cold rain,
00:41but the actual temperature of the air
00:44that the rain is falling through, well, that is the key.
00:48Now, most rain that we see at the surface in the UK
00:53actually starts off life as snow up in the cloud,
00:57but it melts on the way down
00:59because generally temperatures rise
01:02as you get closer to the surface of the Earth.
01:06If the temperature of the air
01:08between the cloud and the ground
01:10stays below freezing all the way,
01:14so all of this is at or below freezing,
01:19then the snow doesn't melt,
01:21and we get snow on the surface,
01:23creating beautiful white landscapes.
01:26If just the very bottom part of the air is above freezing,
01:29then the snow will start to melt.
01:32So if we have this zone below freezing
01:37and then a narrow layer towards the surface
01:41where temperatures are above freezing,
01:43then the snow will start to melt,
01:45and that's what basically makes sleet,
01:47a mixture of rain and melting snow.
01:51But sometimes conditions can be just right
01:55where the bottom part of the atmosphere
01:57is actually colder than the bits above.
02:01So what do I mean?
02:02There's a narrow layer of the atmosphere
02:06just above the surface that's below freezing.
02:09Then you have this zone
02:11where temperatures are above freezing, above that,
02:15and then you're back below freezing up at cloud level.
02:21So what you're gonna have now
02:22is that snow forms in the cloud, starts to fall.
02:26It melts on the way down through this warmer zone,
02:31and then right at the end of its journey,
02:35the rain, because it's now rain,
02:37goes through this bottom part of the atmosphere
02:40that is below freezing.
02:43And here's the key.
02:44The rain doesn't instantly freeze here.
02:48It travels through this layer still as a liquid.
02:52As it falls through this final layer
02:56that is below freezing, it becomes supercooled,
03:00basically liquid that is below zero Celsius.
03:04If this supercooled water, supercooled rain,
03:08is falling fast enough
03:10and the layer of the cold air is shallow enough,
03:15it remains in this state, a liquid,
03:17even though the temperature is below freezing.
03:20And then it hits something, a tree, a road,
03:23a power line, a pavement.
03:25All of these things on the ground are cold.
03:29They're in the air that is below freezing.
03:32So the freezing rain hits a surface
03:35and freezes instantly.
03:38It can create beautiful patterns
03:40and make for spectacular photos,
03:42but it also makes things very, very slippery.
03:46And on roads, you now have black ice,
03:49glaze of ice that you just can't see.
03:53It's also heavy.
03:54So the weight of the ice can bring down branches
03:56or power lines, creating power cuts.
04:00When the Beast from the East hit in 2018,
04:03it was the first time I'd properly experienced
04:06freezing rain, and it fell on top
04:09of several centimeters of snow.
04:11It really was quite a sight right here at Met Office HQ.
04:16Just because we forecast it,
04:17doesn't make us immune from it.
04:19Heavy snow fell here at Met Office headquarters
04:22on Thursday afternoon.
04:23It's pretty rare to see this much snow
04:25in this part of the world.
04:27It's also very rare to see this.
04:31A thick layer of ice has formed on top of the snow,
04:35and that's caused by freezing rain,
04:37which fell through much of the night.
04:38Freezing rain.
04:39That was a lot of fun to play in.
04:42Not so much fun if you had to actually get anywhere.
04:45Thankfully, as I said, freezing rain is quite rare
04:48in the UK.
04:49It happens much more frequently in North America,
04:51where during the winter months,
04:54ice storms can turn major highways into ice rinks.
05:00Now, the best conditions for getting freezing rain
05:03in the UK occur when a warm front approaches.
05:06Might sound strange, because as you might imagine,
05:09warm fronts actually bring warm air with them.
05:12But warm fronts also slope forwards through the sky.
05:18So if it's traveling in this direction,
05:20the warm air is tucked in here,
05:25behind the warm front.
05:26But you can imagine that the bottom part
05:30could still be in air that is cold
05:34and potentially below freezing.
05:37And in that scenario,
05:38well, you've got that setup,
05:40that perfect profile of the atmosphere
05:42for making freezing rain.
05:45But even then, things are very finely balanced.
05:47We get lots of warm fronts across the UK in winter,
05:50but hardly any of them produce proper freezing rain.
05:53A more common cause of ice in the UK
05:56is rain falling onto frozen surfaces,
05:59but that is not the same.
06:01The ice isn't as thick or instantaneous
06:05in the way that it forms as it does with freezing rain.
06:08It's the fact that the rain in freezing rain is super cooled
06:12that makes it so dangerous.
06:16Thanks so much for watching this video about freezing rain.
06:18When freezing rain does occur in the UK,
06:21there's a pretty good chance
06:22we'll be issuing a weather warning for it.
06:24But who decides when we issue a weather warning
06:28and what do the different colors mean?
06:30Well, you can watch this video
06:31to find out all about the national severe weather warnings
06:36Alex Burkill talks you through
06:37the different types of warnings
06:39and what those colors mean.