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| Introduction to UV radiation and UV index
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What is UV radiation? |
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The sun emits electromagnetic radiation of different energy or wavelengths. Some of the radiation, such as that making up the colours of rainbow, is visible which our eyes can see. The part of radiation beyond the violet end of visible light is called ultraviolet(UV) radiation which our eyes cannot see. |
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UVA, UVB and UVC |
UV radiation can be classified as UVA, UVB and UVC according to the energy or wavelength. UV radiation reaching the earth's surface consists mostly of UVA and some UVB, as all UVC and most of the UVB are absorbed by the atmosphere. The main characteristics of UVA, UVB and UVC are shown in the following table. |
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UVA |
UVB |
UVC |
Wavelength
(nanometre, nm) |
315-400 |
280-315 |
100-280 |
Absorption by the ozone layer |
Nil, i.e. it goes through the ozone layer |
Mostly absorbed by the ozone layer |
Practically all absorbed by the ozone layer |
Amount reaching the Earth's surface |
> 98% of UV radiation is UVA |
< 2% of UV radiation is UVB |
Negligible |
( 1 nanometre = 0.000000001 metre or 1x10-9 metre) |
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Factors affecting the ground-level UV intensity |
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Factor |
Influence on UV intensity at the Earth's surface |
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Position of the sun |
Varies with time of the year, time of the day and the latitude. The higher the sun's position, the higher the UV intensity. |
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Amount of ozone in the atmosphere |
Ozone absorbs UV radiation. The more abundant the ozone in the upper atmosphere, the less UV radiation reaches the Earth's surface. |
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Clouds, rain, fog and haze |
UV radiation is both absorbed and scattered by clouds, rain, fog and haze. |
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Ground reflection |
Most natural surfaces such as grass, soil and water reflect less than 10% of UV radiation. However, fresh snow strongly reflects around 80% of UV radiation. Sand also reflects 10-25% of UV radiation. |
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Altitude above the sea level |
The higher the altitude, the higher the UV intensity. It is because the atmosphere gets thinner as we go up, and absorbs less UV. |
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Application of UV radiation in daily life |
There are many applications of UV radiation in our daily life. The following presents some examples. |
Application |
Principle |
Type of UV radiation used |
Disinfection of drinking water and sterilization of apparatus |
High energy UV radiation destroys the DNA of bacteria and micro-organism. |
UVC |
Checking of counterfeit banknotes |
Genuine banknotes have fluorescent marks that are visible only under UV light. |
UVA |
Hardening of inks and coatings |
Certain inks and coatings harden when exposed to UV radiation. |
UVA or UVB |
Pest control |
Insects are attracted to the UV light and killed by the electric shock in bug zappers. |
UVA |
Medical applications such as treatment of psoriasis (a kind of disease causing red and scaly skins) |
UV radiation can suppress the immune system and reduce inflammatory responses. |
UVA or UVB |
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UV lamp for picking out counterfeit banknotes |
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A bug zapper |
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