What happened
December 2010 was the UK’s coldest December for more than a century. Arctic air spread south over the country, and on 18 December heavy snow fell across the south-east, closing Heathrow airport for days.
At this station 7 of the 13 days stayed below freezing from start to finish, and 12 had an air frost. The coldest day, Sun 19 Dec, only got up to −2.6°C. The temperature fell to −11.2°C at Mon 20 Dec, 04:00, the coldest the station has recorded.
Ice days and air frosts
An air frost is when the air, measured at about head height, falls below freezing: most winter nights with a frost on the car are air frosts. An ice day is rarer: the temperature stays below freezing all day, so nothing thaws even in the afternoon. In the mild south-east of England a run of them is unusual. More on temperature.
Why still, clear nights get so cold
Everything gives off heat to the sky as invisible infrared. Clouds send much of it back; a clear sky doesn’t, so on a clear night the ground cools quickly and chills the air just above it. A wind would mix in warmer air from higher up, but on a still night the cold air just settles and gets colder. Snow makes it colder still: it reflects the little winter sunshine, and it lies like a blanket between the air and the warmer ground beneath. Through the night of 19 to 20 December, as it fell to −11.2°C, the wind here was calm in 98% of the readings, and never gusted above 4 mph.
Where the cold came from
Our winter air usually comes in off the Atlantic, mild and damp. In December 2010 high pressure sat far to the north-west, near Greenland, and blocked that flow, so the wind came from the north and east instead: out of the Arctic, and across a frozen northern Europe. The station found a likely cold front at Thu 16 Dec, 13:30, as the colder air arrived. The pressure here wasn’t especially high; it even fell to 988.3 hPa at Sat 18 Dec, 14:30 as a low passed to the south, bringing the snow. How cold it gets depends on where the air has come from, not on the pressure overhead. More on air pressure.
Snow and the rain gauge
A rain gauge catches snow in its funnel, but only counts it as it melts. So the 9.6 mm recorded over these 13 days is far less than really fell: much of the snow only showed up in the gauges when the thaw came at the end of the month. The wettest day of the spell was Mon 27 Dec. More on rain.
What to look for in the charts
- Temperature: days on end below 0°, with the deepest drops on the still, clear nights, and the dew point close by.
- Pressure: the dip as the low passed to the south with the snow.
- Wind: how little there was, especially on the coldest nights.
- Rain: almost nothing while it was frozen, until the thaw.