Ultraviolet radiation (UV) comes naturally from the sun.
There are also some manmade lamps and tools (welding tools, for instance)
that can produce UV radiation. For
most of us, however, the sun is the primary source of UV. UV is
divided into at least three different categories based
on wavelength:
-
UVA wavelengths(320-400 nm) are only slightly affected by ozone levels.
Most UVA radiation is able to reach the earth's surface and can
contribute to tanning, skin aging, eye damage, and immune suppresion.
-
UVB wavelengths(280-320 nm) are strongly affected by ozone levels. Decreases in
stratospheric ozone mean that more UVB radiation can reach the earth's surface,
causing sunburns, snow blindness, immune
suppression, and a variety of skin problems including skin
cancer and premature aging.
-
UVC wavelengths (100-280 nm) are very strongly affected by ozone levels, so that
the levels of UVC radiation reaching the earth's surface are relatively small.
The effects of UV radiation on earth's ecosystems are not completely
understood. Even isolating the effects of UVA versus UVB is
somewhat arbitrary. All UV radiation can be damaging. This knowledge has
prompted many manufacturers of sun screen and sunglasses
to offer products that protect against both UVA and
UVB wavelengths.
While humans can choose various courses of protection,
for instance avoiding noon-time sun, plants and animals
are not so fortunate. Studies have shown that increased
UV radiation can cause significant damage, particularly to
small animals and plants. Phytoplankton, fish eggs, and young plants
with developing leaves are particularly suspectible to damage from
overexposure to UV.
Solar UV radiation levels are highest during the middle of the day.
In total, almost half the daytime total UV radiation is received during
the few hours around noontime. Clouds, as well as ozone,
have a tremendous affect on UV radiation levels. However,
cloudy skies generally do not offer significant protection from
UV. Thin or scattered clouds can have minor impacts on
UV and even, for a short time, increase UV above what it
would be on a blue sky day by further scattering the radiation and increasing the
levels that reach the surface.
Looking for more details? See an encyclopedia article on UV, written by Dr. Weatherhead
(PDF format).
Numerous web sites have been developed to provide information on
daily UV levels around the world
and on a variety
of different aspects of UV.
UV monitoring around the world helps scientists
understand how levels are changing and what changes the UV levels.
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