A risk to entire continents
Winter storms and blizzards affect North America and Europe the most
The most expensive extratropical storm in history remains winter storm Lothar, which tore through central Europe in 1999 - It cost the insurance industry €8.6bn
Losses from winter storms chiefly come from damage to buildings, vehicles and infrastructure. The storms’ wind speeds often reach over 160 km/h and can bring heavy snowfall or freezing rain. A prime example of this are blizzards in North America. Business interruption losses are also increasing, for example when storm damage or large amounts of snow and ice cripple infrastructure.
The worst losses in recent decades were caused by winter storms in Europe – three catastrophic events in central Europe head the list of the world’s costliest winter storms and blizzards since 1980. Time and time again, large-scale weather patterns that persist over weeks and months bring a series of winter storms, such as in 1990 and 1999.
What makes winter storms different
Winter storms differ from tropical storms not only in terms of their areas of formation and tracks, but above all in their intensity and geographical extent. They form in the transition zone between subtropical and polar climate zones. When outbreaks of cold polar air meet subtropical warm air masses, extensive low-pressure vortices are generated. The storm intensity peaks in late autumn and winter, when oceans are still warm but the polar air is already cold – hence the designation winter storm. Maximum wind speeds are 140–200km/h, although speeds far in excess of 250 km/h are possible in exposed coastal locations and on higher mountains. The wind fields can be as much as 2,000km wide.
Ice and snowstorms (blizzards) are other types of extratropical storm. As with other extratropical storms, which mainly inflict damage through high wind speeds, losses from blizzards can reach tens of billions of euros and are caused by ice or snow pressure. Severe blizzards on the north-east coast of the US and Canada, where they are referred to as “nor’easters”, can be particularly costly.
The making of a 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, meaning that the pressure in the middle falls. The faster cold air catches up with the warm air, the two mix – leading to the formation of vortices.
Stormy weather ahead: the influence of climate change
Damage caused by storms
Challenges for risk assessment and risk management
Winter storms often come with secondary perils that can vary widely from region to region. The risk spectrum ranges from a period of warm air bringing double-digit Celsius temperatures and acute snowmelt, heavy rainfall, freezing rain or snowfall to drifting and compacting ice in rivers or coastal areas.
Extreme frost can also follow a winter storm. Business interruption, such as the closure of an airport due to extreme weather, can significantly impact businesses and substantially increase overall losses.