Getting a sunburn when it is cloudy is possible. This common misconception often leads people to skip sun protection, increasing their risk of skin damage even when the sun is not visible. While clouds effectively block the infrared radiation that causes warmth, the rays that cause sunburn are part of a different spectrum. Ultraviolet (UV) radiation, the true cause of a burn, passes through cloud cover with ease.
Defining UV Radiation
Sunburn and other forms of skin damage are caused by invisible energy waves known as ultraviolet (UV) radiation. This energy is categorized into two primary types that reach the Earth’s surface: UVA and UVB. Both types can penetrate the skin and contribute to long-term health risks, including skin cancer.
UVA and UVB radiation have different characteristics and effects. UVA rays, which make up the majority of UV light reaching the surface, have longer wavelengths that penetrate deeply into the skin, contributing to premature aging and cell damage. UVB rays have shorter wavelengths and are responsible for damaging the outermost layer of the skin, causing painful sunburn. Protection must account for both wavelengths.
Cloud Cover and UV Penetration
Clouds are composed of water droplets and ice crystals that can transmit, scatter, and reflect UV radiation rather than simply blocking it. Depending on their composition, clouds may allow a significant amount of UV light to pass through, sometimes as much as 80% of the total radiation. While thicker, darker storm clouds absorb more UV, they cannot eliminate the risk entirely.
Thin, scattered, or high-altitude clouds pose a unique and heightened risk because they can scatter the radiation in multiple directions, effectively increasing the surface exposure. This phenomenon, known as the broken-cloud effect, can sometimes enhance the UV radiation reaching the ground by up to 25% to 40% compared to a completely clear sky. The perceived safety of an overcast day can lead people to remain outside longer without protection, compounding the risk.
Other Environmental Factors
Cloud cover is not the only environmental factor that increases the risk of UV exposure, even when the sky is gray. Reflective surfaces multiply the amount of radiation that hits your skin. Fresh snow, for example, is an effective reflector, bouncing back between 80% and 90% of incident UV rays.
Other surfaces such as sand and water also increase exposure through reflection, which can be particularly intense at the beach or near a lake. Additionally, altitude plays a role, as UV intensity increases by approximately 10% for every 1000 meters of elevation gained. When these factors combine with thin cloud cover, the potential for sunburn becomes significantly elevated regardless of the temperature.
Daily Sun Protection
Protecting your skin must be a daily practice, independent of the weather or season. The foundation of this defense is using a broad-spectrum sunscreen with a Sun Protection Factor (SPF) of 30 or higher. A broad-spectrum product ensures protection against both the aging UVA rays and the burning UVB rays.
For full coverage, an adult should apply about one ounce of sunscreen—roughly the amount needed to fill a shot glass—to all exposed skin. This should be reapplied every two hours under normal circumstances to maintain effective protection. If swimming or sweating heavily, reapplication is necessary every 40 to 80 minutes, even if the product is labeled as water-resistant. Protective clothing, wide-brimmed hats, and UV-blocking sunglasses should also be used as physical barriers.