Many people get tanned or sunburned on cloudy days, despite the sun feeling less intense. A clear scientific explanation involves ultraviolet (UV) radiation and its interaction with our atmosphere. Understanding this clarifies why cloudy conditions do not offer full protection.
The Science of Sun Tanning
Tanning is skin darkening from UV radiation, caused by melanin production from melanocytes. Melanin absorbs UV rays, protecting deeper skin layers.
UV radiation includes UVA and UVB rays, relevant for skin tanning and damage. UVA rays penetrate deeper, contributing to long-term tanning and aging by activating existing melanin for an immediate, short-lived tan. UVB rays affect outer skin layers, causing sunburn and stimulating new melanin for a delayed, longer-lasting tan.
How Clouds Affect Ultraviolet Rays
Clouds do not block all UV radiation; their primary effect is on visible light, making the day appear less bright. While thick clouds significantly reduce UV levels, thinner or scattered clouds allow substantial UV rays to pass. Up to 80% of UV radiation can penetrate clouds, reaching the Earth’s surface.
Scattered or broken cloud cover can intensify UV exposure. This “broken cloud effect” occurs when UV radiation is scattered and reflected by cloud sides, directing it towards the ground from multiple angles. This can lead to higher UV levels than on a clear day, sometimes increasing UV-B rays by up to 25%. The deceptive feeling of coolness on a cloudy day can also lead individuals to underestimate the need for sun protection.
Other Factors Boosting UV Exposure
Several environmental factors influence UV intensity. Higher altitudes experience increased UV exposure because the atmosphere is thinner and absorbs less radiation. UV levels can increase by 10% to 12% for every 1,000 meters of elevation gain.
UV radiation is strongest between 10 AM and 4 PM when the sun is highest. Reflective surfaces amplify UV exposure: fresh snow reflects 50% to 88% of UV, sand 15% to 18%, and water 5% to 10%. UV intensity is also higher closer to the equator and during summer.
Staying Safe from UV on Cloudy Days
Protecting skin from UV radiation is important regardless of weather. Apply broad-spectrum sunscreen with an SPF of 30 or higher, which blocks approximately 97% of harmful UVB rays and protects against UVA. Reapply every two hours, or more frequently after swimming or sweating.
Wearing protective clothing offers defense against UV exposure. Long-sleeved shirts, pants, and wide-brimmed hats made from tightly woven fabrics shield skin. Many garments have a Ultraviolet Protection Factor (UPF) rating.
Seeking shade during peak UV hours (10 AM to 4 PM) reduces direct sun exposure. Wearing UV-protective sunglasses that block 100% of UVA and UVB rays protects the eyes.