What happened
At the end of February 2018 bitterly cold air spread west from Russia and Scandinavia across the British Isles. The newspapers called it the “Beast from the East”. Then, on 1 and 2 March, Storm Emma moved up from the south-west into the cold air, bringing heavy snow, especially to the south-west of England and to Ireland.
At this station 4 of the 9 days stayed below freezing all day. That’s remarkable so late in the winter: by the start of March the sun is as high at noon as it is in the middle of October, and usually warms even a cold day above zero. The coldest day, Wed 28 Feb, only reached −1.9°C, and the temperature fell to −6.8°C at Wed 28 Feb, 19:20.
Wind chill
Unlike the still cold of December 2010, this cold came with a wind. A wind strips away the thin layer of warmer air that clings to your skin, so you lose heat faster and it feels colder than the thermometer says. Weather services put a number on it, the “feels like” temperature or wind chill, from the temperature and the wind speed. Here, at Thu 1 Mar, 03:15, it felt like −12.1°C. More on temperature.
Where the cold came from
Most winters, westerly winds carry mild, damp air in from the Atlantic. In mid-February 2018 the air high above the Arctic warmed suddenly by tens of degrees, a “sudden stratospheric warming”. That upset the polar vortex, the ring of strong winds that usually keeps the coldest air bottled up around the pole, and over the following weeks the westerlies faltered. Cold air from the frozen continent flowed west across Europe instead.
When air masses meet
Storm Emma brought mild, moist air from the south into the cold. Where the two met, the moisture fell as snow. The cold air at the ground, dense and heavy, was slow to shift, and it took until Sun 4 Mar, 14:50 for the milder air to win out here, when the temperature reached 7.6°C. All the while the pressure kept falling as the lows moved in from the south-west, down to 986.5 hPa at Sun 4 Mar, 22:45. As in December 2010, the rain gauges counted the snow only as it melted: 6.2 mm over the 9 days. More on air pressure.
What to look for in the charts
- Temperature: the days below 0°, and how little it rose each afternoon, then the thaw at the end.
- Wind: a steady breeze through the coldest days, which is what made the wind chill.
- Pressure: falling through the week as the lows from the south-west moved in.
- Rain: almost none while it was frozen.