Group News

Stormy weather

Storm front "Burglind" brings the first storm of the new year to Germany. Fallen trees and heavy rain massively obstruct traffic. How do gales that can even reach hurricane force develop at all – with sometimes major impacts on humans and the environment?

Why is stormy weather more common in the autumn?
What we perceive as wind is actually the equalisation of pressure within the atmosphere between air masses of different temperatures. The difference in temperature – and hence also in pressure – in temperate latitudes is particularly marked in autumn: whereas the Mediterranean still retains some warmth after the end of summer, temperatures in Northern and Central Europe fall quite rapidly. As a result, a massive air mass boundary – the so-called Polar Front – forms right across Central Europe, just where the warm Mediterranean air meets the continental cold air. The pressure difference along this air mass boundary is released in the form of strong winds – the greater the temperature difference between north and south, the more violent the storms.

What is the definition of a storm?
Wind strength is normally expressed according to the Beaufort scale (named after Francis Beaufort). The determinative criterion for measuring wind strength on the Beaufort scale is wind speed: the scale ranges from zero (calm) to twelve (hurricane with wind speeds in excess of 117 kilometres per hour). A storm, or more accurately a "strong gale", is designated by Beaufort number 9, with wind speeds of between 75 and 88 kilometres per hour.

How is an extreme low-pressure system formed?
The origin of an autumn or winter storm is a so-called baroclinic wave in the atmosphere at the boundary between polar and temperate air masses. Through the formation of the wave at the Polar Front the temperate warm air at the leading edge of the wave slides above the cold air and thus expands the warm front at ground level. On the trailing edge the heavier cold polar air mass in turn expels the warm air, which is forced to rise. This leads to the formation of the cold front at ground level. Both processes cause clouds to form, lead to the onset of precipitation and result in the creation of low-pressure areas. As they are drawn into the newly formed depression both air currents are deflected to the right owing to the Coriolis effect in the northern hemisphere caused by the Earth's rotation, as a consequence of which a vortex forms around the centre of the low and rotates in an anticlockwise direction. Thus, cold air is deflected westwards past the low-pressure zone towards the south, while warm air flows on the eastern side of the depression towards the north.

Normally, the cold front associated with the low-pressure area advances more rapidly, with the result that at some point in time it catches up with the warm front due to the rotational movement of the low. The mixing of the cold and warm front (known as an occluded front) leads to turbulence in the air masses – and in such cases the storm frequently proves to be especially severe. The typical autumn and winter storms are major low-pressure vortices. Summer storms, on the other hand, are usually geographically localised and often accompanied by thunder and hail.

How is it possible to protect against storm damage?
Thanks to numerical weather models, meteorologists are able to predict weather events relatively well. Similarly, information systems for risk analysis and loss prevention, such as the ARGOS system (Accumulation Risk Geospatial Online System) used by HDI Global SE, help to identify natural perils in good time. Generally speaking, the storm damage caused by falling branches and uprooted trees tends to be greater, the earlier in autumn the storm occurs. This is because deciduous trees that have still to shed their leaves offer a particularly large surface area for strong winds to attack. Despite this even with state-of-the-art technology it is impossible to predict whether storm damage will occur and how extensive the ensuing losses will be. With this in mind, therefore, Frank Manekeller at HDI insurance urges private individuals in particular to take a close look at their insurance coverage for home, motor vehicle and personal liability in order to ensure that the next storm does not leave a financial burden in its aftermath as well.