Claim:
The sun’s UV rays are stronger on cloudy days.
Rating:
What’s True
UV rays can be stronger on cloudy days. However …
What’s False
… this only occurs in specific, somewhat rare conditions. Otherwise, clouds generally tend to dampen UV rays.
Since at least 2020, internet users have debated the strength of ultraviolet (UV) rays — radiation from the sun outside of the visible spectrum — on cloudy days. Some users, like in this Reddit post (archived), warn others to wear sunscreen even on cloudy days. Others, like in this TikTok (archived), don’t buy into the idea that UV rays can penetrate clouds at all. A small amount of posts, like this one (archived) on Instagram, go as far as to claim that UV rays are actually intensified by cloud cover.
So which is it?
In short, while studies have found that clouds can magnify UV rays, it is a rare occurrence and one that requires specific conditions to be met. Typically, UV rays are dampened by clouds, though the amount of rays that pass through varies greatly depending on cloud formations. As a general practice, wearing sunscreen, even on cloudy days, helps protect from the harmful rays that slip through cloud cover — more on this below.
What are UV rays?
Two types of UV rays originate from the sun and penetrate Earth’s atmosphere: UV-A and UV-B. UV-A rays cause the most skin aging and wrinkles while UV-B rays cause sunburns and contribute the most to skin cancer.
Do UV rays penetrate clouds?
For the most part, typical cloud cover can dampen UV rays. However, this dampening is not always as significant as people might think. According to “Scattered and Filtered Solar UV Measurements,” a book about the UV environment and the effects human UV exposure, “normal UV precautions” (Page 21) should be maintained even on cloudy days to prevent health hazards.Â
Clouds are constantly morphing and can form in a variety of shapes and sizes, making it difficult to know for sure how much UV is getting through. According to the U.S. Envrionmenal Protection Agency (EPA), broken clouds absorb only about 27% of UV rays while overcast skies absorb nearly 70%. UV-B rays typically penetrate clouds more effectively than UV-A rays can.
Counterintuitively, some formations of clouds can actually increase UV. The peer-reviewed journal Reviews of Geophysics published an article, “Empirical studies of cloud effects on UV radiation: A review,” explaining that there were a “significant number of cases” where clouds actually increased UV radiation. When rays from the sun hit certain formations of clouds the right way, UV radiation actually increased. The paper referred to the phenomenon as the “enhancement effect.”
Cloudy days can be dangerous as they dissuade the use of sunscreen, causing people to absorb more UV than they should. It is advisable to wear sunscreen during high-UV days, even if there appears to be heavy cloud cover.
What does the enhancement effect look like?
A peer-reviewed journal, the International Journal of Photoenergy, published a paper, “Enhancement of UV Radiation by Cloud Effect in NE of Brazil,” which discusses a specific occurrence of the enhancement effect in Brazil. The study included images showing an example of a cloud formation that caused the phenomenon, though it did not clarify why that specific configuration of clouds would result in its occurrence.
The enhancement effect also increases light in the visible spectrum, which explains the intensity of the sun in the image:
(From “Enhancement of UV Radiation by Cloud Effect in NE of Brazil” with authors’ permission)
The paper found that the enhancement effect could reach up to “13 minutes” in duration and that its occurrence was “not uncommon” in the studied area.
For further reading, Snopes investigated a video allegedly showing a rare “fallstreak hole” cloud formation.
