Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 23. joulukuuta 2010 klo 17:25 (GMT)
It will be a white Christmas over most of the UK this year, an unusual occurrence in a nation where heavy snows typically occur just a few times per year. High temperatures in London over the past week have averaged about 6°C (11°F) below the average high of 7°C (44°F), and will remain below average through Christmas. Winter has hit Western Europe hard for over a month, with heavy snows significantly disrupting flights all across the continent. For November, the UK saw its heaviest and most widespread snows and coldest temperatures since 1993, and deepest snows since 1965. On Monday, December 20, the temperature plummeted to -18°C (0°F) at Castlederg in Northern Ireland in the UK. According to weather records researcher Maximiliano Herrera, who maintains a comprehensive set of world extreme temperature records on his web site, this beats the previous coldest temperature in Northern Ireland, -17.5°C at Magherally in January 1979. `
Figure 1. Temperatures for Dec. 20, 2010, were wildly different from average over much of the Northern Hemisphere. The British Isles shivered under temperatures 6 - 10°C (11 - 18°F) below average, and Northern Ireland set the record for its coldest temperature ever measured, -18°C (0°F) at Castlederg. At the same time, most of the Arctic had record-breaking temperatures up to 20°C (36°F) above average. Image credit: NOAA/ESRL.
Reasons for the cold weather in Europe
The exceptionally cold weather in Europe is due to a very unusual shift in the atmospheric circulation over the Northern Hemisphere in recent weeks that has allowed cold air to spill out of the Arctic into the mid-latitudes, and warm air to surge northwards into the Arctic. I discussed this Hot Arctic-Cold Continents pattern last week. The Arctic is usually dominated by low pressure and counter-clockwise circulating winds, and this "Arctic Vortex" has broken down and reversed its flow direction to clockwise, as high pressure is now in place over the Pole. Natural variability in the weather can cause this pattern, though significant loss of Arctic sea ice, such as occurred this fall, can also be a contributing factor. The extreme conditions in the Arctic this December has led to a rather remarkable event--sea ice decreased this week, during a period when we normally see some of the fastest rates of ice formation in the Arctic. Arctic sea ice extent is now at its lowest extent ever recorded for this time of year, due to the combined effects of unusual wind patterns and temperatures in excess of 10°C (18°F) above average over most of the Arctic.
Figure 2. The average surface pressure pattern in the Northern Hemisphere on December 20 between 1968 - 1996 (left panel) featured two familiar semi-permanent low pressure systems, the Icelandic Low and the Aleutian Low. Low pressure and counter-clockwise winds dominated the Arctic. But on December 20, 2010 (right panel), surface high pressure dominated the Arctic, and the normal flow of winds reversed from counter-clockwise to clockwise. The Icelandic Low and the Aleutian Low had shifted far to the south. This is an extremely unusual weather pattern for the Northern Hemisphere.
Major atmospheric pattern shift coming
The unseasonably cold weather is due to break in the UK next week, as the atmosphere undergoes a major shift in its circulation. The high pressure region over the Arctic is forecast to break down and be replaced by the usual low pressure region that typically dominates in winter. By New Year's, the Aleutian Low and Icelandic Low should re-establish themselves in their usual locations as the atmosphere reconfigures itself into a more typical pattern, and Europe should have near average to warmer than average temperatures for the first week of 2011.
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