Yes, clouds do block some UV rays, but not all of them. A significant percentage of ultraviolet (UV) radiation can penetrate cloud cover, leading to sunburn even on cloudy days.
How Much UV Radiation Penetrates Clouds?
The amount of UV penetration depends heavily on cloud type and thickness. While clouds generally reduce UV exposure, they don't eliminate it. Studies show that over 90% of UV rays can pass through a thin cloud cover. Source: Medical News Today Thicker, denser clouds offer more protection, but even these can still transmit a substantial amount of harmful UV radiation. Source: American Scientist A phenomenon known as the "broken cloud effect" can even cause higher UV levels under patchy clouds compared to completely clear skies. Source: Dr. Gurgen
Types of UV Rays and Cloud Penetration:
It's also important to note that UVA and UVB rays react differently to cloud cover. While clouds are generally better at blocking UVB rays (the main culprit for sunburn), UVA rays penetrate clouds more effectively. Source: Dignity Health UVA rays are also associated with long-term skin damage and aging.
Practical Implications:
- Always wear sunscreen: Even on cloudy days, applying sunscreen is crucial for protecting your skin from harmful UV radiation. Source: Reddit LPT
- Check the UV index: The UV index provides a daily forecast of UV radiation levels, regardless of cloud cover. Be aware that high UV index days still pose a risk even when it's cloudy. Source: ABC Science
- Be mindful of reflective surfaces: Snow, water, and sand can reflect UV rays, increasing your exposure even on a cloudy day.
