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The Alpine Climate
The Weather in the Mountains |
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Bordering both Switzerland and Italy, the French Alps are one of the principal mountain ranges in France; the others most notably being the Vosges in Alsace and Lorraine, the Pyrenees in the extreme south, the Massif Central and the Jura. The weather and climate of the French Alps and Jura tend to be very similar to that found in the Swiss Alps. These high mountain areas have their own distinctive climates, with heavier precipitation and generally colder temperatures than the rest of the country throughout the year. At elevations of 1500 m and above, almost all precipitation in winter falls as snow that will usually remain on the ground from November until as late as May or June, particularly in areas above 2000 m.
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Local Climate in the Alps
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Cloud inversion (Photo credit: Richard Lundberg) |
The Alps have a profound effect on our weather and climate as they act as a natural barrier on which large scale weather systems can be deflected, modified or even created anew. Mild, moist air reaches the mountains from the west; bitterly cold artic air comes down from the north; dry continental air - hot in summer and cold in winter – comes in from the east, and Mediterranean air arrives from the south; so it’s hardly surprising that the area has unpredicatable and rapidly changing weather patterns.
In summer, the climate is generally hot and sunny in the lower valleys and pleasantly cool higher up. The valleys accumulate the heat because the air remains in contact with the sheltered, sun drenched ground for longer and thus increases the air temperature. However, in winter, conditions are often reversed and these valleys become much chillier as the cold, dense air sinks down the mountainside to form a pool in the valley itself. This is why many alpine villages are built part way up the mountain rather than on the valley floors, so they benefit from pleasant summer breezes and make the most of winter sunshine.
Bands of high pressure often dominate the weather in winter and can cause temperature inversions, where cold air is trapped beneath warmer air. On these occasions it is common for fog to form in the pools of cold valley air which can sometimes persist for weeks on end and is very depressing! This is a phenomenon that is often seen when out skiing where thick cloud has sunk down into the valley whilst the pistes are bathed in sunshine and are wonderfully warm. |
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The Altitude Effect
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Variation in altitude also plays an important part in influencing the weather and climate of the Alps. Air temperature generally decreases by an average of 6.5°C per 1000m of height gain, and as air is only heated by its contact with a ground surface, strong winds have usually swept the air away in the high mountains before it has had time to warm up. This means that it can feel relatively warm, even in sub-zero temperatures, if the sky is clear and the sun is shining, while a higher temperature can feel relatively cool if accompanied by a biting cold wind. The cold wind can also bring the added problems of wind burn and dehydration in the dry air.
Of course the thinner or less dense air found at altitude not only makes breathing more difficult, but it also affords less protection against UV radiation than at a lower altitude. Therefore the risk of sun burn is far greater (even on cloudy days) without adequate sun protection and the risk is multiplied further in snow covered areas as the sun's rays are reflected back off the snow. |
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Lenticular Clouds
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These wonderfully unique clouds form as a result of high winds that are forced upwards over an obstacle such as a hill or mountain. They are cumulus type clouds and are commonly associated with the strong winds of the Fohn. The wind literally sculpts the cloud to create a smooth lens-shaped form that almost looks like a flying saucer. You can occasionally get multiple layers of lenticular clouds that form on the top of each wave. However, unlike other cloud types, lenticulars appear to be relatively stationery even though they form in high wind conditions. The air actually blows through the cloud, constantly renewing the water droplets within, which then evaporate as the air drops down the lee side of the mountain and thus gives the impression that the cloud is stationary. Lenticular cloud is very common on the top of Mont Blanc and is known as “L’âne de Mont Blanc”.
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Mountain Winds
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Mountains often produce their own local winds, and wind currents can be shaped as they are forced to rise or are funnelled through valleys which greatly increase their strength. As an air mass descends a mountain slope it is compressed and becomes a warm dry wind. The effect is most noticeable in spring when these winds can melt snow very quickly. They are called Föhn winds in the Alps and Chinook ("snow-eaters") in North America. As well as causing unstable snow conditions and sometimes avalanches; they can also affect the mood of a person and general sense of wellbeing.
Other winds are truly local and are caused by a daily pattern of air-flow up and down valleys.
Katabatic winds are created by air flowing down an incline. In the Alps, they most commonly occur at night when cool air sinks down mountain slopes and valleys. These night time wind can be very cold if there are glaciers in the valley. A Katabatic wind can also be warm, for example, the Föhn wind. Anabatic winds or “valley breezes” occur when sun-warmed air rises and gently flows up mountain slopes and valleys. |
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The Föhn Wind
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A more widespread type of wind known as the Föhn can affect large areas of the Alps under certain meteorological conditions. It is a warm, dry wind that blows down the lee (sheltered side) of mountains, bringing air of very low relative humidity. The Föhn forms when a low pressure area pushes air up the windward (unsheltered) side of the mountain. As the air rises it cools and condenses into clouds when dewpoint is reached. This may lead to snow or rain on windward side of the mountain range. As the air mass descends the other side of the mountain it is warmed by compression at around 10C per 1000m. At altitudes above 2000m, this can mean that air is around 7C warmer in the valley on the lee side of the mountain range. With the onset of a Föhn wind temperature may rise as much as 15°C/27°F–20°C/36°F within an hour and such conditions may last for two or three days.
Föhn winds can be very strong, rolling back colder air masses and causing snow to be loaded onto lee slopes, forming hard snow slabs - a potential avalanche hazard. They are most noticeable in late winter and spring as they can lead to substantial thawing on lee slopes, hence the North American Indian name of Chinook or “snow eater”. Although it can blow in valleys on the southern side of the Alps, it is more severe on the northern side and blows particularly where valleys run from south to north. |
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Wind Chill
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Anyone who has ever been stuck on a chairlift on a cold winter’s day knows that you feel even colder when the wind blows. We call the cooling sensation caused by the combined effect of temperature and wind - the 'wind chill'. The wind chill factor used by weather forecasters is designed to indicate the dangers posed by different combinations of wind and temperature on the bodies of humans and animals.
On a calm day, our bodies insulate us from the outside temperature by warming up a thin layer of air close to our skin, known as the boundary layer. When the wind blows, it takes this protective layer away - exposing our skin to the outside air. It takes energy for our bodies to warm up a new layer, and if each one keeps getting blown away, our skin temperature will drop, and we'll feel colder. Wind also makes us feel colder by evaporating any moisture on our skin - a process that draws more heat away from our bodies.
Studies show that when our skin is wet, it loses heat much faster than when it is dry. Interestingly humans don't sense the temperature of the air directly. When we feel that it's cold, we're actually sensing the temperature of our skin. |
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