A common exposure scale
The UV Index expresses the strength of sunburn-producing ultraviolet radiation at the surface. Values 0–2 are low, 3–5 moderate, 6–7 high, 8–10 very high and 11 or more extreme.
It is not a temperature scale. A cool or windy day can still have a high UV Index because thermal comfort does not reveal how much ultraviolet radiation reaches the surface.
Time, cloud and surroundings
Exposure usually peaks around solar noon. Thin cloud may reduce UV less than people expect, while particular cloud arrangements can maintain strong diffuse radiation.
Exposure rises with altitude. Sand, water and snow reflect radiation and increase total dose, including close to shade or beneath a beach umbrella.
Using the forecast
When the Index is 3 or higher, the World Health Organization recommends protective measures such as shade, clothing, a hat, sunglasses and sunscreen. Sensitive groups should follow individual medical advice.
Check the hourly peak rather than only the daily maximum. This makes it easier to choose a morning or later-afternoon period with lower exposure.
This guide is educational. For safety-critical decisions, use current observations and follow the competent official authorities.