☀️ UV strength
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The UV Index is a standardized international scale, developed by the World Health Organization and the World Meteorological Organization, that describes the strength of ultraviolet radiation from the sun at a particular place and time. It runs from 0 upward, with higher numbers meaning a greater risk of harm to unprotected skin and eyes. The scale is linear: a UV Index of 8 represents twice the UV intensity of a 4, and roughly twice the speed at which skin will burn.
The sun emits energy across the electromagnetic spectrum, including ultraviolet (UV) light at wavelengths shorter than visible light. UV is divided into UVA (315-400 nm), UVB (280-315 nm) and UVC (100-280 nm). UVC is entirely absorbed by the atmosphere and never reaches the surface. Most UVB is absorbed by the stratospheric ozone layer, but the fraction that gets through is the main cause of sunburn and skin cancer. UVA passes through more freely and penetrates deeper into the skin, driving ageing and contributing to long-term damage.
UV intensity depends on the angle of the sun above the horizon. When the sun is high — around solar noon, in summer, and near the equator — sunlight travels through less atmosphere, so less UV is absorbed before reaching you. Early morning and late afternoon sun passes through far more air at a shallow angle, which is why the index peaks in the few hours around midday and falls to zero at night.
UV rises by roughly 10-12% for every 1,000 metres of altitude because there is less atmosphere overhead. Surfaces also reflect UV back at you: fresh snow can almost double exposure, and sand and water add a meaningful amount. Cloud is unreliable protection — thin or broken cloud can let through 80% or more of the UV, and scattered cloud can even briefly increase it.
Ground stations measure UV with instruments called broadband radiometers and spectroradiometers, which weight each wavelength by its biological harm using the erythemal (skin-reddening) action spectrum. Satellites estimate UV from ozone, cloud and aerosol measurements. Forecast models like the one behind this page combine sun angle, ozone, altitude and cloud cover to produce the clear-sky and all-sky values shown above.
Yes. Thin and broken cloud transmits most UV, and up to 80% can pass through light cloud cover. People are often caught out because it feels cooler, but the burning radiation is still present.
No. UV measures ultraviolet radiation, not heat. A cold, clear day at altitude or in spring snow can have a high UV Index, while a hot, hazy summer evening can have a low one.
An index of 0-2 is considered low risk for most people, though those with very fair skin may still wish to protect themselves. At 3 and above, protection is recommended.
Altitude increases UV, and fresh snow reflects up to 90% of it back at you. Combined, these can produce surprisingly high effective exposure even when temperatures are below freezing.