UV Index

Rules of thumb

Sun angle and latitude set the baseline (see the charts above). Altitude, clouds, and reflection then scale how much UV you actually get. These are rough multipliers — useful for planning, not exact forecasts.

Altitude

Thinner air blocks less UV. A common rule: about +10% per 1,000 m (≈3,300 ft).

• Sea-level UV 8 → roughly 9 at 1,000 m → 10 at 2,000 m
• A mountain town at 1,500 m can feel a full UVI step stronger than the coast on the same clear day

Cloud cover

Clouds usually lower UV, but they don’t shut it off. Heavy overcast can cut a lot; thin or broken cloud often barely helps — and scattered cloud can even bump UV slightly by reflecting extra light.

• Clear-sky UV 10 under thick storm clouds might fall to ~2–4
• Hazy or “bright overcast” UV 8 might only drop to ~6–7 — still High
• Cool + cloudy ≠ safe: you can burn when the air feels mild

Reflection

Surfaces bounce UV back at you, so your face and eyes get more than the “flat ground” index alone suggests.

• Fresh snow — can reflect ~80% and roughly double exposure (UV 6 feeling more like 12 on skin/eyes)
• Dry beach sand — about +15% (UV 8 → closer to 9)
• Sea foam / bright water — about +25%; open water itself reflects less than snow but still adds glare
• Grass and dark soil reflect little (<10%)

Stacking matters: high altitude + snow is why spring skiing burns so fast even when it’s cold.

Why UV index matters

The UV index is a simple 0–11+ scale for how strong the sun’s ultraviolet radiation is at the ground. Higher values mean skin and eyes can burn faster — even on cool or partly cloudy days, because UV is about sunlight intensity, not air temperature.

Checking it helps you time outdoor plans, shade, clothing, and sunscreen before damage adds up. Over years, repeated UV exposure raises the risk of premature aging and skin cancers; a few bad burns early in life matter too.

Who should care most

• Anyone outdoors for more than a short stretch around midday
• Kids and teens — burns early in life raise lifetime risk
• People with fair or sun-sensitive skin, or a family history of skin cancer
• Outdoor workers, athletes, gardeners, and beach or snow travelers (snow and water reflect UV)
• Anyone taking medicines that increase sun sensitivity

WHO bands: Low 0–2 · Moderate 3–5 · High 6–7 · Very High 8–10 · Extreme 11+. Protection still helps at Moderate and above — especially from late morning through afternoon.