UV index and the Weather

The UV index and Weather

Until recently, the methods of calculating and reporting a UV index varied from country to country. Today, a worldwide UV index, standardized by the World Health Organization (WHO), has replaced the inconsistent regional methods. The international UV index not only specifies a uniform calculation method but also standard colours and graphics for printed media.

The UV index is an open-ended scale, with higher values representing the risk level of skin damage due to UV exposure. An index of 0 corresponds to zero UV irradiation, as is essentially the case at night time. While an index of 10 corresponds roughly to mid-day sun and a clear sky, indices greater than 11 are quite common in the southern hemisphere where the Ozone layer is depleted

The UV index announced in weather forecasts is a prediction of how strong the actual UV intensity will be at the sun's highest point in the day, which typically occurs during the four-hour period surrounding solar noon at 12 o'clock midday. The prediction is made by a computer model that accounts for the effects of sun altitude, air mass, and clouds, which increase or decrease, the amount of UV radiation that will reach the earths surface. The calculations are weighted in favor of the UV wavelengths that human skin is most sensitive to eg UVA and B. This provides the public with an easy-to-understand daily forecast of UV intensity and a guide as to what protection is needed from these harmful rays.

The table below from the World Health Organisation gives recommendations for protection when the day's predicted UV index is at various values

UV Index	Description	Media Graphic Color	Recommended Protection
0–2	No danger to the average person	Green	Wear sunglasses; use sunscreen if there is snow on the ground, which reflects UV radiation, or if you have particularly fair skin.
3–5	Little risk of harm from unprotected sun exposure	Yellow	Wear sunglasses and use sunscreen, cover the body with clothing and a hat, and seek shade around midday when the sun is most intense.
6–7	High risk of harm from unprotected sun exposure	Orange	Wear sunglasses and use sunscreen having SPF 15 or higher, cover the body with sun protective clothing and a wide-brim hat, and reduce time in the sun from two hours before to three hours after solar noon (roughly 10:00 AM to 4:00 PM during summer in zones that observe daylight saving time).
8–10	Very high risk of harm from unprotected sun exposure	Red	Wear sunscreen, a shirt, sunglasses, and a hat. Do not stay out in the sun for too long.
11+	Extreme risk of harm from unprotected sun exposure	Violet	Take all precautions, including: wear sunglasses and use sunscreen, cover the body with a long-sleeve shirt and trousers, wear a very broad hat, and avoid the sun from two hours before to three hours after solar noon.

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