What Is It?
Simply put, ultraviolet radiation (also known as UV radiation or ultraviolet rays)
is a form of energy traveling through space.
Some of the most frequently recognized types of energy are heat and light.
These, along with others, can be classified as a phenomenon known as
radiation. Other types of
electromagnetic radiation are gamma rays, X-rays,
infrared rays, and radio waves. The progression of
electromagnetic radiation through space can be visualized in different ways.
Some experiments suggest that these rays travel in the form of waves. A physicist
can actually measure the length of those waves (simply called their
It turns out that a smaller wavelength means more energy. At other times,
it is more plausible to describe electromagnetic radiation as being
contained and traveling in little packets, called
The distinguishing factor among
the different types of
electromagnetic radiation is their energy content.
Ultraviolet radiation is more energetic than
visible radiation and therefore
has a shorter
wavelength. To be more specific: Ultraviolet rays have
wavelength between approximately 100
includes wavelengths between 400 and 780 nanometers.
Where does it come from?
The sun is a major source of
ultraviolet rays. Though the sun emits all of the different kinds of
electromagnetic radiation, 99% of its rays are in the form of
ultraviolet rays, and infrared rays (also known as heat). Man-made lamps can
also emit UV radiation, and are often used for experimental purposes.
What does it do?
Light enables us to see,
and heat keeps us from being cold. However, ultraviolet rays often carry the
unfortunate circumstance of containing too
much energy. For example, infrared rays create heat in much the same way as rubbing your
hands together does. The energy contained in the infrared rays causes the
molecules of the substance it hits to vibrate back and forth. However, the energy contained in ultraviolet rays
is higher, so instead of just causing the molecules to shake, it actually can
knock electrons away from the atoms, or causes molecules to split. This
results in a change in the chemical structure of the molecule. This change is
especially detrimental to living organisms, as it can cause cell damage and
deformities by actually mutating its genetic code.
What stops it?
Ultraviolet rays can be subdivided into three different wavelength
bands - UV-A, UV-B, and UV-C.
This is simply a convenient way of classifying the rays based on the amount of
energy they contain and their effects on biological matter. UV-C
is most energetic and most harmful; UV-A is least energetic and least harmful.
Luckily,UV-C rays do not reach the earth's
surface because of the ozone layer. When UV-C rays meet
the ozone molecules at high layers of the atmosphere, the
energy inherent in them is enough to break apart the bond of the molecule and
absorb the energy. Therefore, no UV-C
rays from the sun ever come into contact with life on earth, though
man-produced UV-C rays can be a hazard in certain professions, such as welders.
UV-B rays have a lower energy level
and a longer wavelength
than UV-C. As their energy is often not sufficient to split an ozone molecule,
some of them extend down to
the earth's surface. UV-A rays do not have enough energy to break apart the bonds of the
ozone, so UV-A radiation passes the earth's atmosphere almost unfiltered. As
both UV-B and UV-A rays can be detrimental to our health, it is important that we
protect ourselves. This can be done through a variety of ways. The most
obvious is to reduce the amount of time one spends in the sun, particularly
between the hours of 11 am and 3 pm, when the sun is at its highest in the sky.
However, especially during the summer holidays, this does not always work out.
More ways to protect ourselves can be found here.
Variability of UV
UV levels are not constant over the course of a day, or even over the course of a year.
An obvious factor is the position of the sun in the sky. At
noon, for example, the
electromagnetic waves emitted from the sun travel a much shorter path
through the earth's atmosphere then they would at, say, 5 pm, and thus noon-time intensity
is stronger. A second important parameter determining UV
at the ground is the amount of ozone present in the
Low ozone correlates with much UV. However, there are many other features of the environment that
contribute to UV radiation variability. Most important are clouds. On cloudy days, UV levels
are usually lower than during clear skies as clouds
can deflect rays up into space. Clouds can, however, also lead
to increased UV levels. This happens, for example, when the sun
is not obscured by clouds but clouds in the vicinity of the sun
reflect additional radiation to the ground. So a general rule is
not to feel save from UV radiation just because it's cloudy!
The amounts of UV one is exposed to
also varies with altitude. As a rule of thumb, UV levels increase about
4% for every 1,000 foot gain in altitude. This increase has nothing to do with
being closer to the sun - any elevation you might gain would be miniscule in
comparison to the distance from the earth to the sun, and so would have an
insignificant outcome on UV levels. Instead, the increase is the result of a
thinner atmosphere with a smaller number of
molecules being present to absorb or scatter UV. Examples
of such molecules are
tropospheric ozone (commonly associated with smog)
molecules that remain suspended in the air.
Aerosols can be a
multitude of substances - dust, soot, sulfates, etc. These
aerosols absorb and
scatter UV rays, and so cut down on the ultimate UV
Other factors that have an
influence on UV levels are the physical features of the land - sand, snow, and water
all tend to reflect UV rays. This phenomenon is called
albedo. Some of the ultraviolet rays reflected off the
ground encounter scattering by air molecules, aerosols or clouds back down to
the earth, thus increasing the total
irradiance. When there
is snow on the ground the amount of time it takes for sunburn to occur is
Also, the closer one is to the equator, the more ultraviolet rays
one is exposed to. This can be explained by the fact that the
sun is usually higher at the sky at low
latitudes. In addition, the ozone layer is thinner at the equator as it is over, for
example the United States or Europe, and this also contributes
to more UV.
Since the 1980s, polar regions are affected by the ozone hole.
Under the ozone hole, biologically relevant UV levels are 2-3 times as high as they
were before. Learn, based on real data, how UV levels are
affected by the ozone hole by going to the experiments page! Here
you can compare UV radiation measured by the NSF network in
Antarctica with satellite ozone data.
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