During the last days of February and first days of March 2001 Britain was affected by some cold and wintry weather, including a good deal of snow in parts of Scotland and northern England. Scotland recorded some very cold nights during this time too and the minumum temperature of -20.6C recorded at Altnaharra (Highlands, northern Scotland) on the morning of Saturday March 3rd was the coldest minimum temperature registered in March in Britain since 1965 when, conicidentally, on the morning of the 3rd Corwen (Denbighshire, northern Wales) recorded -21.7C.
By way of comparison the coldest March nights on record were the nights of March 12th 1958, when Grantown on Spey (Aberdeenshire, north-eastern Scotland) registered a minimum temperature of -22.2C, and March 14th 1958 when Logie Coldstone (also in Aberdeenshire) dropped down to -22.8C. The coldest nights ever recorded in Britain were 11th February 1895, 10 January 1982 and 30th December 1995. On these 3 nights the mercury sank to -27.2C, on the first two occasions at Braemar (close to Balmoral in Aberdeenshire) and in 1995 in Altnaharra.
So why did it get so cold then? First of all it's no secret that clear nights are usually colder than cloudy nights. Why? Because at night heat is radiated (ie given out) from the ground and moves away from the earth. There's no incoming warmth from the sun to replace this outgoing heat so the air near the ground gets cold. Cloud cover will act like a blanket and prevent at least some of this heat escaping, so that explains why clear nights are colder than cloudy nights.
Secondly, windy nights are milder than still nights. Why? Well, on a still night the lack of movement of air (which is all that wind is) means that air near the ground stays in the same place all night and simply spends all night becoming colder and colder. On a windy night the air near the ground is always moving so can't cool off as fast. It's like bit like soup being heated at the side of a pan but in reverse; if you are heating soup in a pan and don't stir it the soup in the side of the pan will stay in the same and simply heat up really quickly. If, on the other hand, you stir the soup every time a bit of soup gets a bit hot the stirring will bring another bit of soup to the side of the pan to get heated up. In other words stirring the soup affects the rate at which soup at the side of a pan heats up - soup at the side of a pan will heat up quicker if it isn't stirred. The reverse happens on a still night - no stirring (or movement of air, ie wind), means the air near the ground gets colder quicker than if the air was being moved (ie it was windy).
Also, places that are in valleys are colder on clear, still nights than places not in valley bottoms. Why? Well, because cold air is denser than warm air it will sink down valley sides into valley bottoms during cold, clear nights, making conditions there even colder than they otherwise would be. This process is called Katabatic Drainage.
OK, so that's why clear, still nights will be colder than windy, cloudy nights and why places at the bottom of valleys will be colder than places not in valley bottoms. Now, when there's snow on the ground the air near the ground will be colder than if the snow wasn't there, especially if the snow is fresh, deep and powdery. Why? Because normally when the air near the ground is cooling off at night some of this coldness will sink away into the ground. Snow, however, acts a bit like a blanket over the ground and stop the coldness sinking into the ground so the air near the ground winds up being colder than it would be if snow wasn't there.
So, a clear, still night with fresh, deep and powdery snow on the ground will produce the coldest nights, and on such nights the coldest places will be in valley bottoms. Clear, still nights occur when high pressure is overhead so the ideal conditions for a really, really cold night are when heavy snow falls and then after the snow has fallen skies clear and winds ease, or in other words when a weather system produces snow over a region for a time and then high pressure moves over and stays over the region in question for at least a night.
This is exactly what happened over Scotland on the night of the 2nd/3rd March 2001. The chart to the left shows the weather situation on the night in question. The front over the Baltic Sea and northern Germany brought some snow to Scotland and northern England at the end of February and then as the front moved away south-east an area of high pressure moved over Britain. This area of high pressure brought clear skies and light winds to Scotland. Bearing in mind that snow was already on the ground we can see that the optimum conditions for a really, really cold night over Scotland were then in place for the night of the 2nd/3rd March (as they had also been on the nights before when temperatures dropped close to -15C in some parts of Scotland) and in this respect mother nature didn't disappoint and delivered Britain's coldest March night since 1965. You also won't be surprised to learn that Altnaharra (and the other places mentioned in the first two paragraphs!) are in valley bottoms!
This late February/early March wintry weather wasn't just restricted to Scotland though. Snow affected much of northern England too and in some parts of south-western England saw snow lying on the ground for the first time since December 1997. The Scilly Isles had their first lying snow since Febuary 1993 (Roger Brugge's British Weather Diary). Further afield southern France and northern Italy also had some snow during this time (50 cms snow fell in some places, and in southern France it was some of the worst wintry weather since the 1980's) whilst minimum temperatures in parts of Norway and Sweden fell below -30C. Portugal and Spain were milder but extremely unsettled. Here some places received some tremendous amounts of rain - including falls of 50mm rain or in just less than a day in a few spots (Cordoba, southern Spain for example).
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Dan Suri, 4 March 2001