Winter storms

A risk to entire continents

Winter storms and blizzards affect North America and Europe the most

Major winter storms can cause as much damage as a hurricane. Whilst the wind speeds of these winter storms do not reach those of cyclones, the storm fronts can be over 1,000 km long and sweep across half a continent.

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.

In February 2021, winter storm Uri brought icy temperatures as far as the southern USA. Millions of people were left without electricity. It was the costliest winter storm to date.

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

The large number of claims and rather moderate average individual loss amount mean that prevention plays a key role. At the same time, it is critical that insurers know precisely what factors influence the frequency and intensity of winter storms. Whilst natural climate oscillation has influenced certain years, and storms balance each other out in the long run, climate change still needs to be carefully analysed as a risk of change. So far, scientific studies have assumed that the number of winter storms has not changed significantly, however the proportion of severe storms will increase.

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.