How does a winter storm occur?
Demaged forest Reust
A typical winter storm may develop when differences in air pressure are considerable between North Pole and the equator. This often occurs in fall and winter when the temperature differences are large as well. In this period almost no energy input exists in the polar region which could heat the polar air masses. In mid-latitudes, however, the air masses are still heated by the influx of tropical air.
The low-pressure system
A large-scale depression (low pressure system) of 3000 km diameter may occur during arctic outflow winds from the pole to the mid-latitudes together with a strong jet-stream. The centre has a very low air pressure. The wind speed at the edge of the low is very high, maximum wind speed can reach 200 km/h. The wind speed is altered by the shape of the earth surface (channelization and friction effects). Therefore, the wind speed in mountainous areas are generally higher than in the plateau. The climax of the gale-force winds is exceeded when the faster moving cold front has met the warm front in the centre of the storm (occlusion).
› Hurricane Lothar, 26 December 1999
What damage may occur?
The suction force of a winter storm can carry away individual tiles or whole roofs. For persons outside of houses it is particularly dangerous to be hit by falling branches or by other objects in the air.
The gales cause objects to swing. For instance, trees are up-rooted and tilted. The wind can also erode top soil by wearing away the upper most soil particles, thus depriving soil nutrients and reducing fertility.
Severe side effects
Winter storms may have particular severe side effects: The storm surge is pushed and carries water from the sea towards the coast inundating coastal areas. In addition, the strong rainfall, often accompanying a winter storm, can cause flooding and trigger landslides. In the high mountains the strong winds transfer snow to the lee-ward slopes. There the probability for avalanches increases considerably.