Stormier winter

During the winter months, on the other hand, there are signs of an additional increase in the windstorm hazard in the mid-latitudes. Rising temperatures there prevent with increasing frequency the extensive snow cover in lowlying areas that used to be common in the harsher winters of the past.

Often, a stable cold high-pressure system was able to build up above that snow cover. It acted as a barrier to extratropical low-pressure systems coming in from the sea and usually diverted them into higher latitudes before they could reach the densely populated coastal zones. With the winters becoming milder, the development of cold continental high-pressure systems has become more seldom and weaker, allowing low-pressure systems to penetrate further and more frequently into the continent.

That has led to a string of violent windstorm catastrophes in the last two decades, above all in western and central Europe (e.g. Daria, Vivian, and Wiebke in 1990 and Anatol, Lothar, and Martin in 1999). Besides the change in windstorm tracks (cf. Fig. 5), there has been an increase in windstorm activity over the oceans too. This may be generally associated with the warming of the oceans and the higher energy input into the atmosphere by way of evaporation.

As far as hurricanes, typhoons, and cyclones are concerned, the evidence continues to be contradictory. One group of scientists draws attention to the sea surface temperature of 26—27°C, which is known to be necessary for the birth of tropical cyclones. If the oceans warm up, this surface temperature will be reached in larger and larger expanses of sea and will extend over longer and longer seasons; this points to a possible increase in the number of cyclones.