Breaking Down the UV Index

Weather Knowledge

LITTLE ROCK, Ark. – You may have heard a number reflecting how high the UV Index is for a certain day or time of day, or perhaps which actions you should take to keep safe from UV radiation, but what does that all really mean? We’re here to break it down for you.

The UV Index represents the amount of skin-harming ultraviolet (UV) radiation reaching earth’s surface from the sun.

It is calculated using a variety of factors including the total ozone in the atmosphere, forecast parameters from the National Weather Service’s (NWS) forecast models, sun angle at solar noon over a designated location, UV radiation for a range of wavelengths and human skin response to UV radiation at each given wavelength, to name a few.

UV radiation has been linked to many health conditions. The most popular effects of harmful sunlight rays include skin cancer, sunburn, sun sensitivity, eye damage, premature wrinkling and immune system suppression or disease.

In order to help Americans stay safe while planning or participating in outdoor activities that could inhibit their health from exposure to UV radiation, the NWS and United States Environmental Protection Angency (EPA) developed the UV Index in 1994.

The basic UV Index forecast is provided for solar noon, the time at which the sun is highest in the sky and which UV radiation is elevated under a clear sky.

The index is broken down into 5 categories, as follows:

Low: 1-2
Moderate: 3-5
High: 6-7
Very High: 8-10
Extreme: 11+

There are several precautionary actions people can and should take to protect themselves from UV radiation given each UV Index category.

Low UV Index: wear sunglasses on bright days. If you burn easily, cover up and wear sunscreen.

Moderate UV Index: cover up skin and wear sunscreen if you’ll be outside. Stay in the shade near midday hours when the sun is the strongest.

High UV Index: protection against a sunburn is needed. Reduce time in the sun between 11 a.m. and 4 p.m. Cover exposed skin and wear a hat, sunglasses and sunscreen.

Very High UV Index: unprotected skin will be damaged and can burn quickly – extra precautions needed. Avoid sun between 11 a.m. and 4 p.m. Cover exposed skin with clothing, wear a hat, sunglasses and sunscreen. Seek the shade.

Extreme UV Index: take all precautions as unprotected skin can burn in minutes. Bright surfaces will reflect the UV radiation and increase exposure. Avoid sun between 11 a.m. and 4 p.m. Seek shade, cover exposed skin with clothing, wear a hat, sunglasses and use sunscreen.

Clothing made from tightly woven fabrics will limit radiation from penetrating into the skin. Hats with a wide brim will protect not only the eyes but also face and neck area. Proper eye-ware should provide 100% protection from UV radiation. Broad spectrum SPF sunscreens are recommended and should be regularly reapplied to exposed skin.

The UV Index varies on location depending on a multitude of factors that will impact the strength of the sun’s rays.

  1. Time of Day: the UV Index is lower in morning and evening because the sun angle is low. When the sun is at its highest point in the sky, the UV Index will be at its highest.
  2. Cloud Cover: if clouds are present, widely scattered across the sky, they will help block UV radiation from making it to the earth’s surface. However, fair weather clouds such as cumulus, can reflect the radiation and increase its presence in the atmosphere resulting in a higher UV Index.
  3. Land Cover: trees and structures will help reduce exposure to UV radiation. Sparse infrastructure over land will allow more UV radiation make it to the surface level of the earth, resulting in a higher UV Index.
  4. Ozone: Ozone in the atmosphere absorbs UV radiation. The higher ozone, the less UV radiation reaching the surface of earth.
  5. Latitude: UV radiation is stronger at the equator than towards the poles due to the sun’s angle relative to earth and the equator receiving more daylight than locations to the north and south.
  6. Altitude: UV radiation increases about 2% for ever 1,000-foot increase in elevation due to thinner air.
  7. Seasons: UV radiation peaks in spring and summer months, declines in fall and is at its lowest in winter.
  8. Earth Surface Characteristics: the makeup of land, water or other bright surfaces on earth can result in reflection or scattering of UV radiation. Of incoming UV radiation, snowfall may reflect up to 80%, sand up to 15% and water around 10%.

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