On Solar Radiation:

The average incident solar radiation outside of the earth's atmosphere is 1367 watts per square meter (W/m^2). At noon on a cloudless day nearly one quarter of the solar radiation is scattered and absorbed as it passes through the atmosphere, leaving roughly 1000 W/m^2 of the incident solar radiation reaching the earth's surface.

Check out the University of Oregon's Solar Radiation Monitoring Laboratory website for more information.

 

UV Index Scale

The UV Index scale used in the United States conforms with international guidelines for UVI reporting established by the World Health Organization. Following is a description of each UV Index level and tips to help you minimize harmful exposure to UV radiation.

2 or less: Low
A UV Index reading of 2 or less means low danger from the sun's UV rays for the average person. Wear sunglasses on bright days. In winter, reflection off snow can nearly double UV strength. If you burn easily, cover up and use sunscreen.

3- 5: Moderate
A UV Index reading of 3 to 5 means moderate risk of harm from unprotected sun exposure. Take precautions, such as covering up, if you will be outside. Stay in shade near midday when the sun is strongest.

6 - 7: High
A UV Index reading of 6 to 7 means high risk of harm from unprotected sun exposure. Apply a sunscreen with a SPF of at least 15. Wear a wide-brim hat and sunglasses to protect your eyes. Protection against sunburn is needed. Reduce time in the sun between 10 a.m. and 4 p.m. Cover up, wear a hat and sunglasses, and use sunscreen.

8 - 10: Very High
A UV Index reading of 8 to 10 means very high risk of harm from unprotected sun exposure. Minimize sun exposure during midday hours, from 10 a.m. to 4 p.m. Protect yourself by liberally applying a sunscreen with an SPF of at least 15. Wear protective clothing and sunglasses to protect the eyes. Take extra precautions. Unprotected skin will be damaged and can burn quickly. Minimize sun exposure between 10 a.m. and 4 p.m. Otherwise, seek shade, cover up, wear a hat and sunglasses, and use sunscreen.

11+: Extreme
A UV Index reading of 11 or higher means extreme risk of harm from unprotected sun exposure. Try to avoid sun exposure during midday hours, from 10 a.m. to 4 p.m. Apply sunscreen with an SPF of at least 15 liberally every 2 hours. Take all precautions. Unprotected skin can burn in minutes. Beachgoers should know that white sand and other bright surfaces reflect UV and will increase UV exposure. Try to avoid sun exposure between 10 a.m. and 4 p.m. Seek shade, cover up, wear a hat and sunglasses, and use sunscreen.

For more on the UV Index, see the World Health Organization's website.

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