The UV Index (UVI) is an international standard measurement of the strength of ultraviolet (UV) radiation from the sun at a particular place and time. It ranges from 0 (no UV) to 11+ (extreme UV). The scale was developed by the World Health Organization (WHO) and is used globally to help people protect themselves from harmful UV exposure.

The UV Index is calculated using a computer model that factors in ozone concentration, cloud cover, altitude, solar angle, and ground surface reflectivity. It represents the peak UV radiation level expected during the day, typically around solar noon.

UV radiation is typically strongest between 10:00 AM and 4:00 PM local time, with the peak occurring around solar noon (when the sun is highest in the sky). During these hours, even short exposure can cause skin damage. The UV Index is designed to indicate the maximum level during this peak period.

When the UV Index is 0–2, most people can safely be outside without sunscreen for extended periods. However, individuals with very fair skin, a history of skin cancer, or those taking photosensitizing medications should still consider SPF 15+ protection even on low-UV days.

Yes, but not as much as you might think. Thin or broken clouds can actually enhance UV radiation through scattering. Even under heavy overcast skies, up to 30-40% of UV radiation can still reach the ground. You can still get sunburned on cloudy days if the UV Index is moderate or higher.

Standard window glass blocks most UVB rays (the primary cause of sunburn) but allows a significant portion of UVA rays to pass through. UVA penetrates deeper into the skin and contributes to premature aging and skin cancer risk. Laminated glass and specialized UV-blocking films offer better protection for indoor spaces.

UV radiation increases by approximately 10-12% for every 1,000 meters (3,280 feet) of elevation gain. This is because the atmosphere is thinner at higher altitudes, providing less natural filtration. Skiers and mountaineers face particularly high UV exposure due to both altitude and snow reflection.