Augmentation du rayonnement
Les rayons dont il faut se protéger sont les UltraViolets (UV). Il y a trois sortes d’UV: les UVC, UVB et UVA par ordre décroissant de nocivité. Les UVC sont arrêtés par l’atmosphère et ne nous atteignent pratiquement pas. Par contre toute exposition prolongée aux UVB et UVA va provoquer des brûlures de la peau et des yeux.
Plus on s’élève, plus la couche de protection atmosphérique diminue et le rayonnement UVB augmente.
L’intensité du rayonnement augmente de 4% tous les 300 mètres.
En outre, plus le soleil est bas sur l’horizon, plus la traversée atmosphérique est longue et moins intense est le rayonnement qui parvient jusqu’au sol. Il y a donc un maximum d’UVB entre 11 h et 14 h. Les rayons ne tombent pas tout droit sur la terre. Ils sont diffusés par l’air, les particules de vapeur d’eau et de poussière.
Si les alto-cumulus de moyenne altitude absorbent la majeure partie des UV, les cirrus de haute altitude qui donnent un ciel gris très lumineux transmettent presque autant d’UV qu’un ciel clair. La réflexion du sol dépend de sa nature, elle peut être importante (jusqu’à 90% sur la neige).
Increased radiation
The rays we need to protect ourselves from are ultraviolet rays (UV). There are three types of UV: UVC, UVB and UVA, in decreasing order of harmfulness. UVC is blocked by the atmosphere and hardly reaches us at all. However, prolonged exposure to UVB and UVA will cause burns to the skin and eyes.
The higher you go, the more the layer of atmospheric protection diminishes and UVB radiation increases.
The intensity of the radiation
increases by 4% every 300 metres.
What’s more, the lower the sun is on the horizon, the longer it takes to cross the atmosphere and the less intense the radiation that reaches the ground. There is therefore a maximum of UVB between 11am and 2pm. The rays do not fall straight to earth. They are scattered by the air, water vapour and dust particles.
While mid-altitude alto-cumulus clouds absorb most of the UV, high-altitude cirrus clouds, which give a very bright grey sky, transmit almost as much UV as a clear sky. Reflection from the ground depends on its nature, and can be significant (up to 90% on snow).
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