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Transmission rayons UV nuages.

Misconception: UV rays do not penetrate through clouds.

Due to the inability to see UV rays, some people underestimate their harmfulness. For most, cloud cover reduces a person's exposure to UV rays, and where sun protection practices (sunscreen, protective clothing, etc.) would then depend on the forecasts of solar UV. They thus believe they can avoid skin photoaging and skin cancer in cloudy weather. But what is the reality?

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What are the effects of clouds on UV radiation?

Many people have heard the claim that UV rays are filtered by clouds and therefore do not reach the Earth's surface, believing they have a reducing effect. However, several studies report an increase in UV radiation levels at the surface. Thus, this idea is considered a myth, neglecting the application and/or reapplication of a sunscreen. On the contrary, a high amount of UV light penetrates the clouds. It is therefore important to take precautions, including reducing UV exposure and protecting your skin as if it were a sunny day.

Climatic variables are not reliable indicators of UV radiation. Indeed, people mistakenly associate two weather phenomena with UV radiation, namely perceived temperature and cloud cover.

Although the sun may not feel as hot during overcast weather due to a reduction in the transmission of infrared radiation, UV radiation can penetrate through the clouds. Indeed, they can block a certain amount of UV rays, but up to 50 to 90% of UV rays can still pass through the clouds during periods of overcast skies. Of course, this depends on the type of cloud in question, its quantity, thickness, and its relative position to the sun.

Light clouds do not significantly affect UV rays, thus offering little protection, and even increase UV levels due to a scattering effect. However, the thicker/denser the clouds are, the less UV rays can penetrate.

While clouds generally block visible light more effectively, it is UVA rays that are relatively more affected by cloud cover (45%). Moreover, cloudy days can be more dangerous for the skin. Compared to a completely clear sky, studies have shown that partially cloudy, or even overcast skies, have increased UV radiation by 25% at the surface (enhancement effect) and DNA damage by up to 40%. Scientists have proposed two hypotheses to explain this phenomenon.

  1. Clouds reflect UV rays, through the water particles or ice crystals they are composed of, which in turn bounce them back to the Earth's surface.

  2. Short-wavelength UV rays are dispersed more significantly by the oxygen and nitrogen molecules in the atmosphere (Rayleigh scattering).

This combination of refraction and reflection among different clouds results in a significantly higher level of UV at ground level.

Sources

  • ALBOLD A. & et al. The transmittance of a cloud varies depending on the wavelength in the UV-range. Geophysical Research Letters (1996).

  • GONZÁLEZ J. A. & al. Empirical Studies on the Effects of Clouds on UV Radiation: A Review. Reviews of Geophysics (2005).

  • DIGNA M. D. & co. Environmental indicators of ultraviolet radiation and personal sun protection in outdoor winter activities. Archives of Dermatology (2010).

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