Alps
Climate

The Alps are a classic example of what happens when a temperate area at lower altitude gives way to higher-elevation terrain. Elevations around the world that have cold climates similar to those of the polar regions have been called Alpine. A rise from sea level into the upper regions of the atmosphere causes the temperature to decrease (see adiabatic lapse rate). The effect of mountain chains on prevailing winds is to carry warm air belonging to the lower region into an upper zone, where it expands in volume at the cost of a proportionate loss of temperature, often accompanied by precipitation in the form of snow or rain. The height of the Alps is sufficient to divide the weather patterns in Europe into a wet north and dry south because moisture is sucked from the air as it flows over the high peaks.

The severe weather in the Alps has been studied since the 18th century; particularly the weather patterns such as the seasonal foehn wind. Numerous weather stations were placed in the mountains early in the early 20th century, providing continuous data for climatologists. Some of the valleys are quite arid such as the Aosta Valley in Italy, the Maurienne in France, the Valais in Switzerland, and northern Tyrol.

The areas that are not arid and receive high precipitation experience periodic flooding from rapid snowmelt and runoff. The mean precipitation in the Alps ranges from a low of 2,600 mm (100 in) per year to 3,600 mm (140 in) per year, with the higher levels occurring at high altitudes. At altitudes between 1,000 and 3,000 m (3,300 and 9,800 ft), snowfall begins in November and accumulates through to April or May when the melt begins. Snow lines vary from 2,400 to 3,000 m (7,900 to 9,800 ft), above which the snow is permanent and the temperatures hover around the freezing point even during July and August. High-water levels in streams and rivers peak in June and July when the snow is still melting at the higher altitudes.

The Alps are split into five climatic zones, each with different vegetation. The climate, plant life, and animal life vary among the different sections or zones of the mountains. The lowest zone is the colline zone, which exists between 500 and 1,000 m (1,600 and 3,300 ft), depending on the location. The montane zone extends from 800 to 1,700 m (2,600 to 5,600 ft), followed by the sub-Alpine zone from 1,600 to 2,400 m (5,200 to 7,900 ft). The Alpine zone, extending from tree line to the snow line, is followed by the glacial zone, which covers the glaciated areas of the mountain. Climatic conditions show variances within the same zones; for example, weather conditions at the head of a mountain valley, extending directly from the peaks, are colder and more severe than those at the mouth of a valley which tend to be less severe and receive less snowfall.

Various models of climate change have been projected into the 22nd century for the Alps, with an expectation that a trend toward increased temperatures will have an effect on snowfall, snowpack, glaciation, and river runoff. Significant changes, of both natural and anthropogenic origins, have already been diagnosed from observations, including a 5.6% reduction per decade in snow cover duration over the last 50 years, which also highlights climate change adaptation needs due to impacts on the climate and regional socio-economic activities.
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