Fact Check: Can Clouds Make UV Radiation Stronger?

Internet posts sometimes claim the sun's ultraviolet (UV) rays are stronger on cloudy days. The short answer: sometimes — but only under particular atmospheric and geometric conditions. In most cases clouds reduce the amount of UV that reaches the ground, though the degree of reduction depends on cloud type, thickness and structure.

How UV Radiation Reaches the Ground

Two main bands of solar ultraviolet radiation reach Earth: UV-A and UV-B. UV-A penetrates more deeply into the skin and contributes mainly to ageing and long-term damage, while UV-B is more energetic per photon, is the main cause of sunburn and contributes more strongly to skin cancer risk.

Clouds Usually Reduce UV, But Not Always Equally

Typical cloud cover tends to attenuate UV radiation. Government guidance and measurement studies note that the amount of attenuation varies: the U.S. Environmental Protection Agency estimates that broken clouds absorb roughly 27% of incoming UV, while overcast skies may absorb about 70%. Attenuation also varies by wavelength — UV-B can sometimes penetrate certain clouds more effectively than UV-A.

When Clouds Can Enhance UV: The "Enhancement Effect"

Under some specific conditions, clouds can temporarily increase the UV that reaches the surface. Peer-reviewed reviews and field studies describe a real phenomenon often called the "enhancement effect". This happens when sunlight is scattered, reflected or focused by particular cloud geometries — for example, bright cloud edges, gaps between clouds, or layered clouds that redirect more radiation toward the surface through multiple scattering.

"A significant number of cases" in the literature show cloud-related increases in surface UV when sunlight and cloud geometry align.

Field work has documented such events in several locations. For example, a study in northeastern Brazil reported enhancement episodes lasting up to about 13 minutes and described them as "not uncommon" locally, though such conditions are generally short-lived and geographically variable.

Why This Matters

Enhancement events are relatively rare compared with the general tendency of clouds to reduce UV, but they matter for public health because cloudy skies can create a misleading sense of safety. People often skip sunscreen when it looks overcast, yet measurable UV can still reach the skin — and in some rare situations it can be temporarily amplified.

Practical Advice

- Check the UV index before spending time outdoors and use sun protection (sunscreen, hats, protective clothing) when the index is moderate or higher.
- Remember that cloud cover alone is not a reliable visual cue for UV risk — thin clouds, broken skies, or bright cloud edges can still allow significant UV through.
- Use broad-spectrum sunscreen that protects against both UV-A and UV-B and reapply per product guidance, especially after swimming or heavy sweating.

Sources and Further Reading

Key references include reviews and measurements in the peer-reviewed literature (e.g., Reviews of Geophysics; Geophysical Research Letters), measurement compilations such as Parisi et al.'s Scattered and Filtered Solar UV Measurements, field studies reporting local enhancement events, and public resources from NOAA, NASA and the U.S. EPA on UV and the UV index.