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Storms
The making of a winter storm
When Winter Storm Xynthia swept over Europe in 2010, it left behind insured losses in the order of €2bn. The year before, it was Klaus that raged across Europe. The balance: insured losses also amounting to some €2bn. In 1999, Lothar outstripped both these events, costing insurers approximately €6bn. Kyrill (2007) inflicted insured losses of over €2bn in Germany alone. The damage caused by Joachim, the first big winter storm in 2011, was mild in comparison. However, a recent study by the GDV shows that such storms will increase in the future as a result of climate change.
07.02.2012
Winter storms form over the North Atlantic, Europe's "weather kitchen". This is where cold air from the north meets warm, humid subtropical air from the south, leading to the formation of extensive low-pressure vortices. Storm intensity inside these vortices increases in proportion to the temperature difference between the two air masses. This difference is at its highest in late autumn and winter – hence the term "winter storm". Maximum wind speeds lie in the range of 140 to 200 km/h; in exposed coastal areas and higher mountains they can also attain speeds well in excess of 250 km/h. The wind fields of extratropical storms can be as much as 2,000 km wide.
Differences compared with tropical storms
Extratropical storms differ from tropical storms not only in respect to their areas of origin and paths but above all in their intensity and geographic extent. They develop in the transitional area between subtropical and polar climate zones (approximately latitudes 35° to 70° north and south of the equator). Ice and snowstorms (blizzards) are a further variety of subtropical storm. As with other extratropical storms in which high wind speeds are the main cause of damage, the potential losses due to ice or snow pressure can be in the order of single- or double-digit billion euros. A blizzard that lasted in 1951 from 28 January to 4 February affected enormous areas of the USA – from the states of New England to Texas – with a 10 cm thick layer of ice. In relation to its geographic extent, this was presumably the largest blizzard of the 20th century.
Formation of an extratropical low-pressure storm front (winter storm)
© Munich Re
Fig. 1: Cold, dry air flowing from north to south collides with warm, humid subtropical air flowing from the equator to the north. This leads to the formation of a weather front. Fig. 2: The cold, heavier air wedges its way below the warm air and flows over the ground towards the south; the warm air rides up over the cold air and flows to the north. Air pressure falls as a result of the lift processes. At the same time, the warm, humid air in the higher altitudes progressively cools down, the water vapour condensates, clouds form. Huge amounts of energy are released and drive circulation further; the wind becomes stronger. Fig. 3: The cold front draws closer and closer to the warm front. The warm, humid air continues to rise to ever higher altitudes where it cools down and forms clouds. Fig. 4: The cold air has caught up with the warm air; at this stage the air pressure at the centre of the vortex is at its lowest.
© Munich Re
Winter Storm Joachim sweeps across Europe
From 15 to 17 December 2011, Winter Storm Joachim swept across parts of central Europe. Winds reached hurricane force over France, Switzerland, southern Germany and other areas.
