Extratropical storm (winter storm)

Extratropical storms are different from tropical storms not only in terms of their areas of formation and their tracks but also, and above all, in terms of their intensity and geographical size. They form in the transition zone between subtropical and polar climate zones (roughly between latitudes 35°and 70° north and south of the equator).

When outbreaks of cold polar air meet up with subtropical warm air masses, extensive low-pressure vortices are generated. The storm intensity within these vortices increases in proportion to the temperature difference of the two air masses. It is highest in late autumn and winter, when the oceans are already cold but the polar air is still warm – hence the designation winter storm.

Maximum wind speeds are 140–200 km/h, although winter storms can also reach speeds far in excess of 250 km/h in exposed coastal locations and on higher mountains. Extratropical storms may have wind fields up to 2,000 km wide.

Ice storms and snowstorms (blizzards) are further types of extratropical storm. The damage caused by ice or snow load may – as in the case of the other extratropical storms, where high wind speeds are the main cause of damage – lead to losses amounting to tens of billions.

An ice storm lasting from 28 January to 4 February 1951 covered huge areas of the United States – from New England to Texas – with a layer of ice up to 10 cm thick. In terms of its geographical size, it was probably the largest ice storm of the 20th century.

Development of an extratropical low-pressure system (winter storm)

An air mass boundary forms between cold polar air in the north and warm subtropical air in the south. The heavier cold air starts moving southwards close to the surface. At the same time, the warm air advances northwards at higher levels, with the result that the pressure in the centre of the turbulence falls. The faster cold air catches up with the warm air, the two mix – leading to the formation of vortices.