February 1, 2009 - A Rare
Severe Snow at London, England
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Figure 1. A double-decker bus traveling through the growing storm.
All buses were cancelled the next morning, leaving commuters stranded. Courtesy
BBC. |
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Figure 2. A woman tries to shovel out her car in a London suburb. Courtesy
BBC. |
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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. |
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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. |
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Figure 5. Surface mean sea level pressure. Courtesy
UKMET office. |
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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. |
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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. |
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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. |
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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.
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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. |
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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. |
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