As the seasons go by

Because of the altitude and the cold, the seasons in the mountains, in temperate countries, do not have the same duration as in the plains. Spring comes very late, sometimes only in June. Fortunately the sun warms the ground quickly and the summer is brief but beautiful.

From mid-August, the fall is felt on the slopes facing north: in the shadows all day, they can not warm up anymore.

Winter is the most important, since it imposes its law on the mountain and its inhabitants, for more than 6 months.

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The weather is changing fast

At altitude, the air is colder and drier, the sun and the light stronger, the rains more important, the winds more violent. The temperature differences between the shade and the sun, between day and night are enormous. The weather changes quickly.

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Why is it cold at altitude?

The higher you go, the colder the air and yet you get closer to the sun! The key to the mystery is atmospheric pressure. The higher you go, the more it goes down. At the summit of Mont-Blanc, it has dropped by 40% compared to sea level. This means that the air is more rare and retains less sunlight. The temperature drops by 0.6°C each time you climb 100m.

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The sun, uneven heat

Oddly, the sun can still burn very strong in the mountains: the surface of some rocks, a beautiful summer day, reaches the temperature of 80 ° C! But this heat is not manifest everywhere equally, because the slopes are not all oriented in the same way.

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The wind turns us into an ice cube

The stronger the wind, the more it cools us. It lowers the temperature. When it's -4°C on the slopes in winter and the wind is blowing at 40 km/h, it's like -20°C. That's why we always take windproof clothes on a hike.

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Storms

In the mountains, thunderstorms are more violent than anywhere else. The rocky peaks are constantly attracting lightning, and the valleys make the sound of thunder deafening. Mountain thunderstorms have particularities such that they pose significant risks because of their consequences, especially for mountaineers, hikers and paragliders. Thunderstorms in the mountains are also particular because of the proximity of clouds, the importance of electric fields, their speed of appearance, their particular danger. Sudden wind lift of a mass of air can radically change the initial conditions within them.

In addition to the common phenomena related to thunderstorms, the larger electric fields cause unexpected warning signs particularly related to the ionization of the air, the attraction of the fibers. French mountaineers then say that they heard "the bees": suddenly, on all surfaces, small noises are created. After this bee sound sensation, another observation is that the hair is straight up over the head, and sparks can form between the teeth when you open your mouth. These are the results of electrical phenomena, already described in other situations, and known as fire Saint-Elme or corona effect. Climbers, very little mobile, find themselves trapped in a thunderstorm. The metal ice axes of their climbing equipment can turn into lightning rods and attract lightning. The lightning strike of mountaineers represents a great danger in these situations. In case of thunderstorm, avoid all the sharp and isolated reliefs: summits, trees, haystacks, tent planted alone. The isolated peaks of the Alps are the most affected by lightning. Beware, the danger remains after the lightning has fallen. It continues to follow the shortest way down: it can be a ridge, a human body that makes the connection to the earth. That's why you should not take shelter under a tree. Attitude to adopt: in the thunderstorm, you must absolutely go down, flee the peaks and ridges. Do not look for shelter. Sit on the floor, with your head down, your legs together, your hands on your head (or your arms glued along your body) and form a ball.

FORMATION OF THUNDERSTORMS

Thunderstorms form when the air in a layer of the atmosphere is very unstable. A plot of land of air lifted on the base of this layer is then hotter than the environment and undergoes a push of Archimède upward. By rising, its temperature decreases by adiabatic relaxation and when the relative humidity of steam which it contains reaches saturation point, there is training(formation) of the convective cloud. To train(form) a thunderstorm this layer has to be very big and the temperature at the top of the cloud has to be under 20 °C.

Mountains can help trigger atmospheric convection in three ways:

• Direct uprising at the level of free convection in the unstable air by uprising with a synoptic wind which arrives with a perpendicular component at slopes. The air Is obliged to go back up the slope  and enters convection;

• thermal forcing in an unstable situation where there is a weak general circulation but a diurnal warming, the mountain breezes generate anabatic winds which also go back up the slope.

• dynamic forcing when the air is stable at low level, and the circulation is blocked by the relief, but that the air is unstable at higher altitude. In this case the air accumulating at the base of the mountain, the general circulation is forced above this blocking layer and can reach the level of the convective layer.

In a more general case, several effects may be present and if the synoptic wind is opposed to the anabatic wind, additional convergence occurs at the top of the mountain.

The uprising serves not only to destabilize the air and form the convective clouds, but it also enhances its intensity. Indeed, the raised air coming from the lower slopes being hotter and humid than the surrounding air, the heave index will be more negative which increases the vertical extension of the cloud. A simple difference in temperature of 2K is enough to make the violence of thunderstorms much worse.

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The snow

It can snow at any time, but in summer, this snow does not last more than a few days. It is particularly dangerous for cattle, which sometimes slide down the slope.

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Attention to avalanches

3 types :

• Powder avalanches are by far the most numerous and the most devastating with their blast effect. They occur in very cold weather, with fresh snow. Their violence increases rapidly as they advance, and they can hit the slopes at 400 km/h !!

• Plate avalanches are hard, packed snow, but based on another very slippery layer. Almost unpredictable, they are triggered after a small shock, after long periods of good weather very cold.

• The wet snow avalanches are heavy and powerful, crushing everything in their path. They are mostly seen in the spring, after a period of mild weather during which the snow has softened.

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Closer to the stars

At high altitude, the air is purer than in plain: there is less dust, less carbon dioxide, less water vapor ... A boon for astronomers: they can observe the stars with a lot more sharpness.

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