Alpine Storms

Mountain thunderstorms pose significant risks because of their consequences, especially for mountaineers, hikers and paragliders. Despite monitoring weather conditions to prevent accidents, disasters occurring during such phenomena often remain unpredictable and are the subject of novels or other stories.

Formation

Thunderstorms are formed when air in a layer of the atmosphere is unstable. A parcel of air raised at the base of this layer is then warmer than the environment and rises. In rising, its temperature decreases by adiabatic expansion, and when the relative humidity of water vapor reaches saturation, this forms a convective cloud. To form a storm, this layer must be very large and the temperature at the top of the cloud must be below −20C.

Mountains can help trigger atmospheric convection in three ways:.

* ''direct lifting'' at the level of free convection in unstable air, lifting with a synoptic wind that arrives w/ a component perpendicular to the slopes. The air is forced up the slope and into convection;
* ''thermal forcing'' in an unstable situation where there is weak general circulation and diurnal warming. The mountain breezes then generate anabatic winds which also rise.
* ''dynamic forcing'' when air is stable lower down, and circulation is blocked by the mountain, but the air is unstable at higher altitude. In this case general circulation is forced above this blocking layer and can reach 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. It is also possible to create a convergence zone behind the mountain when the synoptic wind can divide and go around it to meet again (ex. Puget Sound Convergence Zone ) that will promote convection.

The uprising serves to not only destabilize the air and form convective clouds, but also to enhance its intensity. As the rising air from the bottom of the slopes is hotter and humid than the surrounding air, the ''lifted index'' will be more negative, increasing the vertical extension of the cloud . A simple temperature difference of 2C is enough to greatly aggravate the violence of thunderstorms.

Special phenomena

Mountain storms have particularities because of the proximity of clouds, the importance of electric fields, their rapidity of appearance, their particular danger. Sudden wind lift of a mass of air can radically change the initial conditions within them.

Immediate consequences

Observable phenomena give an idea of the peculiarities of mountain storms .

Mountaineers are the most exposed to these phenomena, in ridges and pass passes.

In addition to the common phenomena related to the storm, larger electric fields cause unexpected warning signs related to the ionization of the air. French mountaineers say they hear "bees": suddenly, on all surfaces, are small noisy discharges. After this bee-sound, another observation is that the hair stands straight up over the head, and sparks can form between the teeth when one opens one's mouth. These are the results of electrical phenomena, already described in other situations, and known by the name of fire of St. Elmo or corona effect also called crown effect.

Climbers, not very mobile, can find themselves trapped in a storm. The metal ice axes of climbing equipment can attract lightning, a great danger in these situations.

Indirect consequences

The force of thunder, lightning, lightning, rain, hail, bursts and whirlwinds can cause other dangerous situations. Some natural phenomena are increased tenfold by altitude,. in particular:

* torrential floods down slopes at rapid speeds
* landslides,
* avalanches.

Prevention

Weather forecasting is an essential tool for preventing accidents related to mountain storms but does not prevent them. Climbers must know how to respond to a storm in the mountains.

In the United States, the National Weather Service provides forecasts for every point in its territory, including the mountainous areas. In other countries, similar services and guidance are provided by the Meteorological Service of Canada, by the UK Met Office, and by Météo France in France. France also maintains a Safe Mountain Foundation and Keraunos, an observatory for violent storms.

Stories of mountain storms in literature

Novel

*''Premier de cordée'' (''First on the Rope'') by Roger Frison-Roche touched on the spectacle of mountain climbing and the experience of high mountain guides in these extreme situations. It was adapted to the cinema in a 1944 film by Louis Daquin with Maurice Baquet

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