The invisible peril - freezing rain

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  • Ice from freezing rain covering a car in Skipton, North Yorkshire. Picture courtesy of @CarletonWeather via Twitter

    Two air temperature maps for 9am Thursday 9th February. The left hand one is temperatures at the surface; the right hand one is temperatures at around 900 metres above sea level. Note that temperatures are positive at the 900 metre level and negative lower down. Thus any snow falling from the clouds will melt as it moves into the warmer air aloft. However, it will then remain as rain even with temperatures below zero before hitting the surface. Here it will then coat anything with a temperature below zero with ice. This was happening across parts of the north. precipitation was not yet falling farther south. Image copyright MeteoGroup

    A stylized representation of a slice through the atmosphere during a freezing rain event. Image copyright MeteoGroup

  • The invisible peril - freezing rain
    09.02.2012 17:22

     

    Traffic accidents across northern parts of Britain early on Thursday were a manifestation of one of the most hazardous forms of precipitation – freezing rain.

    In
    Cumbria alone about 100 traffic accidents were likely to have been the result of the sheet ice that ensued after rain fell onto frigid road surfaces.

    A pedantic meteorologist might inform you that much of the freezing rain we see in the UK is not exactly that by the purest definition of the phrase, and might better be described as “rain, freezing”. But why parse semantics when the result is the same and equally treacherous?

    For the record, though, the strictest definition of freezing rain describes it as supercooled water droplets instantly freezing as they strike a surface to create what is called a glaze – a layer of ice that can thicken considerably as rain continues to fall, sometimes to several centimetres. This is what occurred on Thursday morning.

    Water is an odd substance sometimes, despite our familiarity with it. Rain can survive in a supercooled state as it passes through a frigid slice of the atmosphere just above the surface, colder than at cloud level. In other words, it can exist at temperatures well below zero degrees Celsius without freezing.

    It certainly cannot turn to snow, and the absence of freezing nuclei can preclude immediate freezing to, say, ice pellets, but the process is instantaneous when droplets come into contact with, for example, a street, a tree or a power line, encasing them in clear ice.

    If the freezing rain continues and ice thickens then additional perils may be tree branches or power lines collapsing under the weight.

    The subtly different other type of freezing rain, more common in the UK, sees run-of-the mill raindrops at temperatures above zero degrees falling onto surfaces that are below zero and then freezing.

    Either way, this glaze, or what we sometimes call “black ice” is the result. It can be more dangerous than snow in that it is less noticeable – a glance out of the window in the morning might reveal a rainy morning with wet roads, on the face of it less of a danger than snow until the first stretch of unexpected glaze is encountered.

    Roads are of course treated with grit and rock salt but it easily gets washed away as rain continues to fall, so treatment becomes a Herculean task.

    Thursday’s atmospheric set up was due to a wedge of deep cold air laying at the surface and a few hundred metres above while a warm front slowly approached from the northwest, pushing a layer of much milder and cloudy air ahead of it that rode over the stubborn frigidity. As cloud thickened in that warmer layer it started to produce rain that fell through the colder layer below, supercooling the water droplets that turned to ice on contact with sub-zero surfaces.

    One slice through the atmosphere revealed an air temperature of -4 degrees at an altitude of about 180 metres / 600 feet, and +3 degrees at 600 metres / 2000 feet.

    One of the most famous freezing rain events in the UK occurred on 27th January 1940. Rain fell persistently for a couple of days in some areas such as the Cotswolds, causing limbs and whole trees to fall. Birds froze to branches and perished, and some that got airborne dropped from the sky as the weight of ice brought them down.

    By: Stephen Davenport