It is possible get a tan when the sky is overcast. Clouds reduce the amount of tan-producing ultraviolet light, but up to 80 percent still comes through, according to the Skin Cancer Foundation. The type of cloud and whether or not the cloud cover is broken affect tanning.
Clouds are better at blocking visible light than UV, says American Scientist. When considering the tan factor or ultraviolet radiation present, the U.S. National Weather Service uses a factor of 89 percent for scattered clouds, 73 percent for broken clouds and 32 percent for overcast, compared to a clear-sky sunny day. Clouds may even enhance tanning under some circumstances. Experts advise fair-skinned individuals to wear sunscreen unless the overcast is so thick that there are no visible shadows.