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
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
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.
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.