Freezing rain

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  • Freezing rain
    06.12.2010 15:50

    The tens of centimetres of snow that choked much of the UK were followed on Saturday in the south by an equally, if not more, dangerous phenomenon.

    Rain arrived and the temperature lifted sufficiently for there to be a rather rapid thaw across southern England, and this in itself might cause problems with flooding at other times and in other places, especially if accompanied by heavy and persistent rain.

    In this case that wasn’t the main issue, although water did gush from guttering and rush down the streets, and as the snow and ice at least partially melted away many homeowners were left pondering burst pipes.

    The real danger came in the first few hours of the transition from snow to sleet and rain on Friday night and early Saturday, when it fell onto sub-zero surfaces and froze on impact, causing what is popularly known as “black ice” but more properly termed “glaze”.

    This was by no means the worst freezing rain event, and in most areas the surface temperature rose sufficiently for any ice to melt as the rain continued. A far more hazardous episode occurred last winter, on 13 January, in parts of Yorkshire and the north Midlands, when a whole day of freezing rain and drizzle persistently fell, building up an almost impenetrable layer of ice, impossible to drive or even to walk on. Pedestrians were reduced to crawling through the streets on hands and knees for fear of falling.

    North America has some of the worst freezing rain imaginable, and glaze has been known to grow several centimetres thick. Salting the road has little or no effect, especially if snow subsequently falls to cover it.

    Freezing rain can occur when the air temperature has risen above freezing but surfaces remain below for a while, especially at times when snow is lying, or there is already some ice. This is what happened last Friday night.

    More properly, however, and this is what causes the very dangerous events, cloud releases snow which turns to rain as it falls through a warmer layer of air a couple of thousand metres above our heads.

    This relative warmth is overlaying a colder wedge of air entrenched closer to the surface, perhaps in the bottom kilometre of the atmosphere. This often occurs when a warm front is approaching and cold air becomes trapped near the surface while warmer air encroaches above it.

    The raindrops become “supercooled” as they fall into the lower, colder layer – they do not freeze, lacking a nucleus around which to do so. Neither can they turn back to snow, so they continue to fall to the ground while their temperature decreases and falls below zero degrees Celsius. If the air is very frigid, however, the rain can be transformed into ice pellets, which are relatively less  dangerous.

    On impact they immediately freeze – not only onto roads and pavements but also power lines and branches, which can collapse under the weight of ice, or onto aircraft wings, a highly dangerous state of affairs because it changes the shape of the aerofoil. This is why airlines invest in sophisticated and robust de-icing equipment, and why even so flights have to be cancelled.

    The most extreme outbreaks of freezing rain are called “ice storms”, and one of these notoriously struck eastern Canada and the north-eastern USA in January 1998. In fact, it comprised five successive ice storms, leaving millions of people without power for several days or even weeks, causing at least 30 fatalities and virtually shutting down the cities of Ottawa and Montreal, where ice grew to thicknesses between 6 and 10 centimetres.

     

    By: Stephen Davenport