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Precipitation (April – September)

from 978-3-14-100790-9 from page 45 fig. 4
Diercke Karte Precipitation (April – September)
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Precipitation (April – September)
The distribution of precipitation in Europe is influenced largely by topography. Both elevation and lee and windward effects play important roles in this context.
Most precipitation in Europe is released from wandering low-pressure vortices. These low-pressure systems are formed in a dynamic process below the high-altitude belt of the westerly jet stream (polar front jet stream), which begins to meander at a certain temperature gradient between the Equator and the North Pole. As a result, cold-air troughs shift southward, while warm-air troughs move toward the North. A low-lying low-pressure system forms beneath the front edge of a trough below the westerly jet stream under the influence of upward suction forces. This system steers warm tropical air in a counter-clockwise pattern toward the North along its eastern edge and cold polar air toward the South along its back edge, creating typical warm and cold fronts.
The masses of air in the advancing warm front glide over the lower-lying masses of cooler, and thus heavier air. Stratus clouds form along the inclined edge of the front, which may extend over several hundred kilometres, under the influence of condensation. These clouds typically release precipitation in the form of steady rain. At the edge of the cold front, polar air masses collide with the warmer and thus lighter topical air. The lighter air is lifted, creating a convective (cumulus) cloud formation, from which showers are released. These low-pressure systems are coupled with the overlying westerly winds and thus wander inland from the Atlantic.
Because the lifetime of these "wandering cyclones" rarely exceeds a period of several days, the precipitation associated with them decreases gradually as they move across Europe from west to east. The result is a corresponding precipitation gradient in the continental interior. This effect is compounded by an annual pattern of increasing precipitation toward the East, which reaches its peak during the summer months. This phenomenon is known as hygric continentality. Increased precipitation in the summer is the result of a substantial rise in the temperature of mainland masses in the continental interior. This produces vertical movements of air and convective cloud formations with corresponding precipitation.
n addition to the predominant east-to-west gradient in the distribution of precipitation in Europe, precipitation quantities also increase markedly at higher elevations. This is the result of accumulation effects combined with higher wind speeds in the mountainous regions, which accelerate the influx of moist air masses. The north-to-south alignment of mountain chains intensifies this effect, as westerly winds prevail at higher elevations. This is particularly evident at the western slope of the Scandinavian Mountains (see the climatic diagram "Mountains"). Precipitation decreases significantly on the leeward sides of such mountain ranges under the influence of foehn wind effects, as is the case in the Spanish highlands.
The high-altitude westerly wind currents, beneath which low-pressure systems form and are integrated into their west-to-east circulation pattern, shift over the course of the year. The tracks along which the "wandering cyclones" are transported to Europe change accordingly. During the summer, a wedge of the subtropical high over the Azores steers low-pressure systems northward across Europe, thereby keeping them away from the Mediterranean region. In the winter, the westerly jet stream shifts southward and steers Atlantic lows into the Mediterranean region as well. Autonomous low-pressure systems also form above the relatively warm Mediterranean region itself, as often occurs over the Gulf of Lion and the Gulf of Genoa. This explains why precipitation along the entire Mediterranean coast is heaviest during the winter months.
By virtue of their intrinsic circulation, high- and low-pressure systems control the influx of different masses of air and the temperature and moisture conditions associated with them toward Europe. Westerly winds bring moist masses of air from the Atlantic, causing frequent precipitation. Cool to cold masses flow inland from the North, while southerly and south-westerly air currents promote the influx of moist, warm air masses. Easterly winds in Europe usually bring dry weather, which is associated in the summer with warm temperatures from the continental interior and in winter with cold Siberian air.
P. Frankenberg, A. Siegmund, D. Volz; Ü: J. Southard


Keywords: climate Europe precipitation weather station

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