The weather's in line!

  • Radar image of line convection stretching from N Ireland to London.

    Overlay of 1hr rainfall totals at 14:00. Note 10mm at Hawarden, NE Wales and >16mm in Stormont Castle, N Ireland.

    Approaching squall line over Clapham Junction, London, during a line convection event. Photo by Brendan Jones.

    Wednesday's squall line developing "kinks", sometimes hinting at tornadic development. Arrows represent surface wind direction.

    A squall line roars towards the photographer near Rugby in 2007. Photo by Brendan Jones.

  • The weather's in line!
    16.08.2012 11:09


    Heat and humidity were pile-driven away by “A waterfall from the sky” on Wednesday afternoon.

    Trouble was brewing during Tuesday and Wednesday this week as a huge area of low pressure, more at home in the turbulence of autumn, exploded into life off the coast of Ireland. Wind gusts in excess of 50-60mph battered south-western areas whilst a huge arc of rain raced eastwards through places which were bathing in 25 degree sunshine just minutes earlier. Embedded within this area of rain was a spectacular example of a phenomenon known as line-convection.

    Wednesday’s rain was caused by a cold front. These are, essentially, where masses of cold air move in and push warmer air away. Some cold fronts can be relative weak features whilst other cold fronts can be far more vigorous with torrential rain, hail, damaging winds and lightning. The latter are known as ‘ana cold fronts’ and one such event took place on Wednesday.

    To understand the processes involved within an ana cold front, imagine dragging a sharp knife along the surface of soft butter. The knife represents the cold, dense wedge of air behind a cold front, whilst the surface of the butter represents the warmer and more buoyant air ahead of it. Digging the knife through the surface causes the butter to peel vertically upwards above the knife. The same happens when cold air forces its way underneath a slab of warmer air; the warm air is forced to rise vertically upwards, sometimes at great speed. As the air rises, it quickly cools to form cloud and rain, and the faster this rising motion, the more vigorous the resultant storm cloud.

    Radar imagery, used for the detection of precipitation location and intensity, can pick out the digging motion of this cold air very nicely, showing up the location of line convection (sometimes known as a squall line). Wednesday’s radar imagery displays this feature beautifully as a solid, narrow line of intense rain the length of the country. Some areas saw between 10 and 15mm of rain in just 5 or 10 minutes, resulting in flooding. Thunderstorm activity was also noted, especially in the Midlands.

    Occasionally, ‘kinks’ develop in these lines of intense rain, where surges of cold air move forward at varying paces. Within these kinks, very strong winds can occur with tornadic activity not uncommon. No tornadoes were reported on Wednesday but the large rainfall totals, local flooding and multiple lightning flashes made for quite a spectacular event.


    By: Brendan Jones