We are ending our coldest March in 51 years with some truely bitter nights and a push for not only the coldest night of March (-13C) but the coldest night in all of winter for the UK. It’s amazing to see this depth of cold so late in the year and of course we ‘spring forward’ to British summertime at 2am tomorrow morning, don’t forget!

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After about of week of stinging easterly winds and hefty, frequent snow showers, a cold high has now slide over Scotland and much of the UK, putting somewhat of a lid on the atmosphere, particularly over the North although snow showers are still affecting the East and Southeast coast of England where winds are fresher. These are bringing upwards of an inch of fresh snow to some spots.

The high has presented sunnier skies and so with lighter winds and plenty of strong late March sunshine, conditions aren’t that bad out there, feel the warmth of that sunshine when your out of the breeze? At this late stage of the year, the sun really does erode low level cold air and with very cold air aloft and only 1,000ft above the surface, this cold which gets lifted into the mid levels durting the day as the sun heats the lower atmosphere, quickly returns back to the surface after sunset.

Maximum Radiational Cooling

With a clear, calm and dry air within a cold atmosphere (-5 to -10C temperatures at 5,000ft), even without snow cover, towns and cities across much of Great Britain are making it to between -3 and -5C, -6C or colder in rural areas but where the snow lies and particularly in shallow snow covered valleys, the temperature is plummeting to -10C. The snow covered village of Braemar, tucked within the Grampian Mountains has been king of cold these past few nights with the mercury falling -10.3C Friday morning, -11C this morning and it will almost certainly make it back below -10C tonight and perhaps even through the first few nights of April as conditions remain near perfect.

The perfect recipe for maximum radiational cooling is a valley location with gently sloping hillsides in which cold air can drain down the slopes into the valley bottom and collect (valleys with slopes too steep and the air mixes as it drains towards the valley floor), clear calm skies and dry air which allows warmth to radiate back to space.

The Importance Of Snow Cover

By day, bare ground absorbs solar radiational which gets released into the atmosphere by night and even beneath clear, calm skies, the warmth from the ground is released into the lower atmosphere, keeping a limit on potential cooling. The high reflectively of snow cover simplu ‘bounces’ the majority of incoming solar radiational straight back up to space during the day and so there is minimal warming of the lower atmosphere and so it’s far colder where snow lies.

The effect of snow cover by night is two fold, it not only sets a ‘colder base temperature’ for cooling at night but the snow cover radiates any daytime warmth even quicker back to space and this refrigerated atmosphere allows bitterly cold air aloft to sink down to the surface without any limit, this presents maximum cooling potential throughout the night.

So, in the sheltered Grampian and Highlands Glens where snow remains on the ground, conditions into early next week look highly favourable for producing more sub -10C nights. The coldest UK Easter Sunday on record was set in 1986 with a low of -9.8C, Easter 2013 is almost sure to dawn below that 1986 benchmark and we may see a near close to -12C tonight.

Interestingly, temperatures all the way down to -17C have been recorded as late as March 29th, so while it’s cold, it’s been colder this late in the year. However, I believe the April record is -13C, while this will be tough to reach, we may at least have a good go at this early next week. April begins on Monday!

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