Ernst Rauch

Windstorm – The most significant natural hazard worldwide

Winter Storms Daria, Lothar, and Kyrill in Europe, Typhoon Mireille in Japan, Hurricane Andrew and Hurricane Katrina in the United States: just a few of the major windstorm catastrophes of recent years that have devastated whole areas, destroyed forests and coastal resorts, and cost billions of euros. The frequency and dimension of the losses have had a major impact on the insurance industry around the globe.

Windstorm is the most important natural hazard of recent decades, in terms of the frequency of loss events, the total expanse of the areas affected, and, above all, the scale of the damage caused. The insurance industry has consequently had to carry higher and higher losses due to windstorm, the natural hazard responsible for about 79% of the US$ 370bn (2007 values) which the insurance industry had to pay for major natural disasters between 1950 and 2007.

What do we know about the wind?

Meteorological observations of windstorm events have been documented for centuries – for almost as long, in fact, as written history. On the other hand, instrumental measurements of wind fields have only existed for a relatively brief one hundred years. Moreover, since wind fields are very sensitive to the coarseness of a region – topography, vegetation, built environment – it is very seldom possible to compare them with each other over relatively long observation periods. This is one of the reasons why there are few areas with indicative wind statistics and windstorm hazard zoning to date. What is more, the windstorm hazard in mountainous areas may be subject to extreme smallscale changes due to topographical features like river valleys. However, routine meteorological monitoring networks are usually too large-meshed to pick up local changes in wind fields or confined windstorm phenomena like tornadoes and thundersqualls.

Far ahead of the rest. Historically, windstorms have been the most important natural hazard for the insurance industry – even more than earthquakes, volcanic eruptions, or floods. The loss frequency, the scale of the damage caused, and not least the high windstorm insurance penetration are all responsible for this.

As fast as the wind

Observations of the wind present another problem, too: the wind‘s speed increases with its height above the ground – usually following power law. However, it also reacts strongly to the coarseness of the earth‘s surface. In short, the smoother the surface, the less the wind current is decelerated. For this reason, wind speeds are on average much higher over the sea than over a surface covered with vegetation or an urban area.

Since the height at which the wind is measured plays such a decisive role, a standard reference height of 10 m above the ground has been agreed on for the purposes of comparison by the World Meteorological Organization.

Since the height at which the wind is measured plays such a decisive role, a standard reference height of 10 m above the ground has been agreed on for the purposes of comparison by the World Meteorological Organization.

Turbulent risk assessment

For an accurate assessment of the windstorm risk, however, the insurance industry needs even more information. One of the essential parameters for the extent of damage is the duration of wind stress. Many losses are only caused by a multitude of "wind attacks" or load changes, which cause material fatigue and finally failure.

Besides speed and duration, the direction of the wind is also decisive. Severe changes in direction can influence the extent of loss considerably, if trees with their root system and buildings with their specific load design cannot cope with them.

The wind is turbulent. The wind speeds of short gusts are much higher than the average, with the gust factor – the ratio of gust speed to mean wind speed – usually being between 1.2 and 1.5. In very rough terrain, however, values exceeding 2 may also be reached. The Beaufort Wind Scale defines windstorm strength as the ten-minute mean value.

Last but not least, the turbulent nature of the wind leads to its kinetic energy fluctuating very strongly, too. Known as the energy spectrum of the wind, this property has a decisive impact on the extent of damage to trees and resonating structures, particularly bridges, towers, or chimney stacks.

Windstorms – from tropical to wintry

In meteorological terms, windstorms can be essentially divided into four classes: tropical cyclones, extratropical storms (winter storms), regional storms (including monsoon storms), and local windstorms (tornadoes, thunderstorms/hailstorms). The world map of windstorms present the typical tracks and origins of the various windstorm types.