Figure 1. A double-decker bus traveling through the growing storm. All buses were cancelled the next morning, leaving commuters stranded. Courtesy BBC.		Figure 2. A woman tries to shovel out her car in a London suburb. Courtesy BBC.

 

A significant snowstorm arrived in London, England late on the first day of February 2009. I first found out when a friend who lives there emailed that there were about 15 cm or 6 inches on the ground. I was quite surprised, because a snowfall of 15 cm or more is very rare in London and most of the rest of England due to their relatively mild climate, helped considerably by their proximity to the Gulf Stream. Although they average about 20 days with snow each winter, their average annual accumulation is less than 1 inch. Most of the first wave of snow came in only about 3-5 hours, a notably high rate in the U.K. In addition thunder and lightning were observed, making this indeed a rare event. Although many are saying that this is the most snow London has had since February 7-9, 1991, when snow was widespread over most of the U.K. and 15 cm fell at London, it appears that this storm was even more significant. Preliminary tallies so far have shown 20 cm (8 inches) in parts of London, making this their biggest snow since the severe winters of 1981-82 and 1984-85, when 15-30 cm or 6-12 inches fell. Regardless of the final numbers, this was a disaster for commuters, who woke up to find limited tube service, no bus service at all, and all airports basically closed.   This snowfall was caused by an atmospheric pattern that, although rare to this area, is very conducive to heavy snow over eastern England. It is basically a sea effect snow, caused by very cold air moving over a relatively warm sea surface. In this case unusually cold high pressure over Scandinavia and Russia brought very cold and moist air from over the North Sea into the English east coast on strong northeast winds. At the same time, low pressure in the upper levels over western mainland Europe brought in even colder air aloft. The large difference in the temperatures of the air over the relatively warm North Sea and the much colder air at upper levels caused the air to become very unstable; once unstable air begins to rise it will continue to do so, condensing into clouds and precipitation. What caused the air to rise? In cases of sea effect snow such as this, the warmer moist air that was originally moving over a relatively smooth sea surface encounters much rougher and higher surfaces as it moves over land. This creates frictional convergence, which causes the air to start rising. In this case the especially large temperature differences between the water and upper air caused such a rapid ascent that the resulting convection (clouds and precipitation) even produced thundersnow, a quite rare but spectacular winter phenomenon. Here is a short clip from the BBC showing a lightning strike at Kent, southeast of London.

 

The following is a short illustrated technical discussion of what caused this unusual weather event.

 

			At left in Figure 5 we can see the strong and quite cold 1034mb high pressure ridge established from Scandinavia eastward to Russia, with the ridge stacked up all the way to jet stream level (see the 500mb and 300mb analyses.) The clockwise circulation around it produced a strong northeasterly flow into London of very cold air that dropped surface temperatures to -2.4C (high 20s F) by evening on the 1st (see observations from UK weather.) Meanwhile, the air over the relatively warm (7C) North Sea was being overrun by very cold air higher up in the atmosphere: at 850mb (1500 meters) the air temperature was -12C or colder, brought in on easterly winds from the continent. This temperature difference of at least 19C was highly significant: differences of 13C or more are very conducive to producing ocean or sea effect snow. The net result can be seen in the cross-sectioned views of the atmosphere, the skewT diagrams in Figures 6a and b. Note how between 12Z (late night) and 00Z (afternoon hours) the height of the saturated layer rose rapidly from about 800mb (2200 meters) to 600mb (4200 meters), allowing convection to grow high enough to produce heavy snow showers and even occasional thundersnow.
	Figure 5. Surface mean sea level pressure. Courtesy UKMET office.

 

Figures 6a and b below show the skewT diagrams from Nottingham, U.K. at 12Z on February 1st, and at 00Z on the 2nd. Nottingham is about 160 km or 100 miles to the north-northwest of London. In the first sounding data at left we can see the strong inversion at 800mb capping the rapidly rising low level air. In the second at right we can see that the cap has been eroded to 600mb, allowing rapidly rising air, clouds and convection to about 4200 meters (13-14,000 feet) due to the CAA (cold air advection) from the east. Thus air parcels were rising rapidly and high enough to support strong convection, which led to high precipitation amounts and even some thundersnow.

 

	Figure 6a. SkewT (cross-section) analysis from Nottingham, UK at 12Z on 2009-February 01. Notice the saturated, moist layer, rises only to about 800mb or 2200 meters. Courtesy U of Wyoming.		Figure 6b. SkewT (cross-section) analysis from Nottingham, UK at 00Z on 2009-February 02. Notice that the saturated, moist layer has risen to 600mb or about 4200 meters. Courtesy U of Wyoming.

 

  Figure 7, an infrared satellite image taken at 1900, shows the lack of high clouds (brighter whites) over most of the U.K. This would lead us to believe that most of the convection was of a lower level type. However, a closer look from a polar orbiting satellite (2009-02-01 2132) reveals some areas of brighter white and banding of the clouds over eastern and southeastern England, where small pockets of strong convection produced localized areas of severe snow. This is confirmed by the radar image in Figure 8.

 

Figure 7. Infrared satellite image at 2009 Feb 01, 1900. Note the lack of bright white (higher clouds possibly indicating strong convection) over most of the the U.K. except for the southeastern areas. Courtesy MeteoFrance.   Figure 8. Radar image from 2009 Feb 01 at 2300. Note the moderate to heavy echo returns over London and southeast England, indicating heavy snow. Courtesy British Met Office.     Finally, Figures 9 and 10 show the meteograms for London from 03Z on February 1st through 23Z on February 2nd. Note the long period of almost continuous snow from February 1st at 18Z through February 2nd around 20Z, when the snow changed over to rain as warmer air moved in over the area.         Figure 9. Meteogram for London, England from 03Z on Feb 01 2009 to 03Z Feb 02. Note the change from snow showers at 12 and 15Z to heavy snow showers and north to northeast winds by the end of the period. Courtesy U of Wyoming.   Figure 10. Meteogram for London, England from 22Z on 2009 Feb 01 to 22Z on Feb 02. Note the periods of heavy snow and almost continuous snow until 20Z, when temps rose to 34F and the precipitation changed to rain. Courtesy U of Wyoming.

 

	Figure 11. A scene from a corner at South Ealing Road in London, as the snow picked up in intensity. Note the blowing snow to the left. Courtesy BBC/Sebastian Querac.		Figure 12. A deserted rail station at Epsom Downs, Surrey. Note the text at the bottom of the sign: "Please do not travel."  Courtesy BBC & Tony Hathaway.