The UV index (UVI) is an international standard measure of the strength of ultraviolet radiation from the sun at a given place and time. Developed by the World Health Organization and the WMO, the scale is linear and open-ended, though values on Earth typically range from 0 (night) to 11+ (tropical noon, high altitude). Categories are: Low (1–2), Moderate (3–5), High (6–7), Very High (8–10), and Extreme (11+).
UVI depends on several factors: solar elevation (highest at solar noon and in summer), latitude (higher in the tropics), altitude (increases ~10 % per 1,000 m), cloud cover (thick clouds reduce UV by 70–90 %, but thin clouds let most through), and ozone column depth (the ozone layer is the primary natural UV shield). Reflective surfaces like snow (80 % reflection), sand (15–25 %), and water (10–20 %) can significantly increase UV exposure.
In Spain, summer UVI routinely reaches 9–11 in central and southern regions, requiring sun protection from 10:00 to 16:00. AEMET publishes daily UV forecasts for 59 provincial capitals. At UVI ≥ 6, health authorities recommend sun cream (SPF 30+), hat, sunglasses, and shade during peak hours. Prolonged unprotected exposure causes sunburn, accelerates skin ageing, and increases skin cancer risk — the incidence of melanoma has risen dramatically in recent decades. The UV index is an essential tool for public health, outdoor sports planning, and occupational safety.