Does UV change with clouds, shade and altitude?

By Trung Hieu Lam, developer of UV Tracker · Published 1 September 2026

Clouds

Thin or broken cloud passes most erythemal UV through — a hazy or overcast sky can still carry meaningful UV, which is why "it doesn't feel sunny" is an unreliable guide. Counterintuitively, bright cloud edges can briefly reflect extra UV toward the ground, pushing the instantaneous reading above the clear-sky value for a few minutes at a time. Only thick, continuous cloud cover cuts UV substantially.

Shade

Shade blocks direct sunlight but not the diffuse UV scattered across the rest of the sky, so standing under a tree or umbrella meaningfully reduces exposure without eliminating it. How much depends on how much open sky the shade actually blocks.

Reflection

These figures are why sunburn is common at the beach even under a light breeze, and why snow sports carry real UV risk in cold weather.

Altitude

UV intensity rises with altitude — roughly 10% more UV index per 1000 meters of elevation, because there is less atmosphere to absorb it. Mountain UV can be meaningfully higher than the same latitude at sea level even on a similarly clear day.

Glass and cars

Ordinary window glass blocks most UVB but lets a substantial share of UVA through, so a long car ride or a sunny office window is not UV-free, even though the sunburn risk is far lower than direct sun.

Measuring your actual sky

A regional forecast can't account for any of this at your specific location and moment. UV Tracker's sky scan reads the cloud cover actually above you to adjust the number — see how the measurement works, or use the calculator once you have a reading.

UV Tracker measures the UV where you stand — sky scan for local conditions, one photo for your skin type, and your safe sun time on one card. Free forever.