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UV radiation at the earths surface is measured using two methods
1. The first method measures the amount of energy contained
in the UV radiation that reaches the Earths surface.
The Department of Geography and Environmental Studies at The
University of Tasmania uses this method to measure and record UV information.
The method expresses energy using S.I. (Système International) units. In this
case milliWatts per square meter (ie each square meter of a horizontal surface
receives x milliWatts of energy. The SI symbol is mW m -2
. The graphical display of daily UV shown during TV weather bulletins is based
on this data.
2. The second method of measuring UV is based on the response
of human skin to UV radiation and is recorded as a Global Solar UV-Index.
This Index was developed as part of a plan to standardise UV
radiation reporting around the globe. It is often used as a prediction, as is
the case in Tasmania. The UV Index is determined for the one hour when UV
radiation intensity is at its peak. This will generally occur at solar noon
(approximately 1 pm during daylight saving time). The UV Index directly takes
into account cloud cover and other environmental factors. Of course a predicted
Index value is only that, a prediction, and must be treated as such.
The measurements of UV radiation have been related to
exposure categories based on sun burning times for fair skinned people as
follows;
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milliWatts per square meter
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Index value
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Exposure Category
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< 75
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Les than 3
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Moderate
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75 - 150
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3 - 6
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High
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151 - 275
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6 - 11
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Very High
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> 275
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Greater than 11
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Extreme
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If there is 75 milliWatts of incident UV radiation (UV-Index
of 3), a fair skinned person will experience minimal skin redness after one hour
in the sun. If there is 150 milliWatts of incident UV radiation (UV-Index of 6)
a fair skinned person will experience minimal skin redness after 24 minutes in
the sun. In Tasmania, UV levels
of over 250 milliWatts per square meter are common and will result in a fair-skinned person burning after 15 minutes in the sun. Remember potential long
term health effects from UV radiation do not depend on skin type.
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