Sunday 8 December 2013

At least twelve dead as Cyclone Bodil hits northern Europe.

Twelve people are know to have died and several more are missing after Cyclone Bodil (also known as Sven and Xaver) hit nations in northern Europe from Thursday 5 to Saturday 7 December 2013. A lory (truck) driver was killed in Scotland when his vehicle was blown over by high winds near Edinburgh on Thursday 5 December, while a man was hit by a falling tree and killed in Nottinghamshire, England, on the same day. In Denmark a passenger in a truck was killed when the vehicle was overturned by high winds. Poland has reported five deaths relating to the storm, three of them killed when tree fell onto a car, and seven people have been reported killed, including an elderly woman found dead in a snowdrift and two Philippine sailors swept overboard from a Dutch container ship.

A lory blown over by high winds near Edinburgh, Scotland, an incident which led to the death of the driver. AP.

The storm led to extensive flooding along the northern coast of Wales and northwestern coast of England and west coast of Scotland, as well as on the east British coast as far south as Kent. Glasgow Central Railway Station was evacuated after part of a glass roof collapsed, and a number of major bridges were closed in Scotland, where winds of up to 228 km per hour were recorded. Around 10 000 people were evacuated from low-lying areas of Norfolk and Suffolk in eastern England, and a number of homes were destroyed in Hemsby on the Norfolk coast, after the collapse of a part of a cliff into the sea. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall, although it is likely that on this occasion the collapse was caused more directly by physical erosion due to waves battering the foot of the sandstone cliffs.

Houses undermined by coastal erosion at Hemsby on the Norfolk coast. Reuters.

In the European mainland flooding affected parts of Germany and the Netherlands, with the city of Hamburg being worst hit following a storm surge on the River Elbe. Winds of 150 km per hour were recorded in Germany, and bridges, roads, rail services, airports and public buildings closed as a precaution several countries, with many road and rail routes remaining out of action after the storm had passed due to large numbers of fallen trees. In addition many homes across northern Poland and southern Scandinavia have been left without electricity due to damaged power lines.

Flooding in Hamburg due to Cyclone Bodil. AFP.

Ocean storms form due to heating of air over the sea in tropical zones. As the air is heated the the air pressure drops and the air rises, causing new air to rush in from outside the forming storm zone. If this zone is sufficiently large, then it will be influenced by the Coriolis Effect, which loosely speaking means the winds closer to the equator will be faster than those further away, causing the storm to rotate, clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.

Storm surge at Dagebuell, on the German North Sea coast on 5 December 2013. AFP.

Whilst the high winds associated these storms is extremely dangerous, the real danger from such storms is often the flooding. Each millibar drop in air pressure can lead to a 1 cm rise in sea level, and large storms can be accompanied by storm surges several meters high. This tends to be accompanied by high levels of rainfall, caused by water picked up by the storm while still at sea, which can lead to flooding, swollen rivers and landslides; which occur when waterlogged soils on hill slopes lose their cohesion and slump downwards, over whatever happens to be in their path.

Flooding at Helsingborg in southern Sweden. AP.

Since the passage of Cyclone Xaver a surprising number of websites have appeared citing the storm as 'proof' that global warming is a hoax (such sites will appear after almost any weather event, but there are a larger than usual number on this occasion). While attempting to determine the precise relationship between long-term climatic events and individual storms is largely futile, it is worth noting that storms of this nature are always a result of warming over the sea, even though their outcome is something we perceive as 'wintry' weather. Extreme weather events are the direct outcome of additional energy within a weather system leading to increased large scale atmospheric turbulence. The occurrence of storms, cannot, therefore, be used as an inference that the weather is cooler than, or even the same as, usual, although the the time scale over which the heating persists cannot be determined from an individual storm.

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