Do Clouds Block UV?

Author: Staff Writer
Clouds do not significantly block UV.

This is a dangerous myth, and many people who have gone without sunscreen on a cloudy day regret having believed it. Clouds are better at blocking visible light than UV rays, and unless there is complete cloud cover (no shadows), some harmful UV radiation is still getting through. In fact, there is some evidence to suggest that UV rays may actually be more dangerous on partly cloudy days. This may seem absurd or counter-intuitive, but there are some good reasons having to do with the nature of UV and of clouds.

Ultraviolet Radiation

When you go outside during the day, you are actually exposed to two kinds of Ultraviolet light-UVA, linked to long-term skin damage, and the deadly UVB, which directly damage DNA and are thought to cause most skin cancers. The third kind, UVC, does not make it through our atmosphere. The amount of UV you are exposed to depends on many factors, including the time of year, time of day, elevation, and cloud cover; the EPA maintains a UV index with forecasts for your area. Under heavy cloud cover, you may be protected from some UV, but partly cloudy conditions are suspected to aggravate UV rays through reflection and diffusion. [1]

Cloud Enhancement and Ultraviolet Rays

Most clouds block some degree of UV rays.  The thicker and denser the clouds, the more UV rays are filtered. [2] But clouds can also enhance the intensity of the ultraviolet rays that reach ground level.  How is this possible? Because clouds are made up of tiny water droplets formed around dust, they are highly reflective; that is why they appear white.  Dense cumulus clouds reflect the UV radiation and allow it to bounce off their sides and get through cloud cover, while wispier clouds redirect the rays in all directions. High clouds refracting UV rays and lower clouds reflecting them may significantly increase the UV levels on the ground. The effect can be amplified by haze, fog, and especially snow. [3]

Recent studies have demonstrated that cloud enhancement of UV rays can be as high as 25%. [4] This is exacerbated by some people's failure to apply sunblock on cloudy days. Even on a completely overcast day, some UV radiation gets through, and on partly cloudy days, your risk for skin cancer and melanoma may actually increase.


Works Cited

What Is Ultraviolet Radiation?

[2] National Weather Service Climate Prediction Service
UV Index Information: Effects of Clouds, Elevation, and Surface Pollution

[3]American Scientist
Sunshine on a Cloudy Day

[4] Ultraviolet radiation at the Earth’s surface
Scientific Assessment of Ozone Depletion:
1998, Global Ozone Research and MonitoringProject–Report No. 44

Resource: Environmental Health and Safety: Oklahoma State University
Sun Safety: What Is Ultraviolet Radiation?

Resource: Scientific Assessment of Ozone Depletion: 2006
Surface Ultraviolet Radiation: Past, Present, and Future

purplearrowExpert Opinion

Quote:  "Investigators have known since 1964 that clouds can have paradoxical effects on incident UV radiation. In more than a dozen studies since then, every data set includes at least some examples of what is known as cloud enhancement of UV."

Source:Sunshine on a Cloudy Day    Scientific American.

purplearrowGlossary of Terms

UV: abbreviation for ultraviolet. Describes light that has a wavelength which is after the violet (= light purple) end of the range of colors that can be seen by humans. Light of this type causes the skin to become darker in the sun.
Cambridge Dictionary

UVA: radiation that is in the region of the ultraviolet spectrum which extends from about 320 to 400 nm in wavelength and that causes tanning and contributes to aging of the skin.
Merriam-Webster Dictionary

UVB: radiation that is in the region of the ultraviolet spectrum which extends from about 280 to 320 nm in wavelength and that is primarily responsible for sunburn, aging of the skin, and the development of skin cancer.
Merriam-Webster Dictionary

UVC:  radiation that is in the region of the ultraviolet spectrum which extends from about 200 to 280 nm in wavelength and that is more hazardous than UVB but is mostly absorbed by earth's upper atmosphere.
Merriam-Webster Dictionary

purple arrowCite this Article

"Do Clouds Block UV?." Sophisticated Edge. N.p., n.d. Web. . <>.  

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