It sounds crazy, and many people just ignore the claim entirely believing it to be a myth. It’s no secret that one can certainly get sunburned on cloudy days, but is there any science backing up the claim that UV rays are actually stronger on cloudy days?
What are UV rays?
UV rays (or ultraviolet light) is electromagnetic radiation that has a wavelength shorter than that of the visible light we see everyday. It is similar to that of x-rays, gamma rays and microwaves in that it is invisible to our eyes. It’s a type of light we cannot see.
The most common source of UV light is the sun, electrical arc emissions and is produced by specialized light sources like black lights. UV light can also cause chemical reactions, which is what causes some substances to glow or fluoresce.
The naturally occurring UV light produced by the sun come in two different flavors; UV-A which is more common, and UV-B which is more potent. While each has a role in skin cancer and eye damage, UV-B is the type responsible for vitamin D production, and is directly responsible for those horrible sunburns.
So, are UV rays stronger on cloudy days?
UV intensity can vary widely with time of day, time of the year, and a locations latitude. (The equator at noon receives much more UV exposure than noon in Norway, for example). Clouds do usually block UV rays, particularly the more nefarious UV-B. On a really overcast day, they can stop 70 to 90% of the UV-B from reaching the surface.
Under skies that are only partly cloudy, something interesting happens. A phenomenon called the ‘broken-cloud effect‘ can occur, which causes higher UV levels – higher than a completely clear sky would allow. A survey conducted at six U.S. cites in 1994 found cumulus clouds could raise surface UV-B by 25%, and in 2004 Australian researchers reported that the UV-B rays associated with DNA damage were up to 40% stronger under partly cloudy skies.
How does this happen?
Scientists don’t have a solid answer, but there seems to be two main causes. The first is UV rays bouncing off, and being reflected off the sides of dense clouds, and the second is UV rays being redirected as they pass through very fine wisp-like clouds. An American Scientist article suggested last year that a combination of these thin refracting clouds high up and thick reflecting clouds down low could result in a significant UV boost at the surface level.
Another contributing factor is simple haze. Natural haze and haze caused by pollution can have a redistributing effect on UV radiation. While they can stop UV-B from reaching the ground directly, they also scatter it, effectively turning the entire sky into a source of radiation. Standing out in the open on a hazy day, you will likely will be exposed to less UV-B than if the sky were clear, but in the shade of haze means you’ll be exposed to a lot more – a risk to those living in the smoggier parts of the world.
People are also less likely to take precautions against sun when it’s cloudy which adds to that risk. This leaves them open to any UV-amplifying mechanisms taking place.
Broken-Cloud Enhancement of Solar Radiation Absorption (pdf)
Process for Determining Solar Irradiances (pdf)
Hockberger, P. E. (2002). “A history of ultraviolet photobiology for humans, animals and microorganisms“. Photobiol. 76 (6): 561–579.