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Solar radiation

Ultraviolet (UV) Radiation & The Electromagnetic Spectrum

Ultraviolet Radiation is a form of radiant energy that is part of the Electromagnetic Spectrum. Other forms of radiation making up the Electromagnetic Spectrum are Radio Waves, Microwaves, Infrared, Visible Light, X-rays and Gamma Rays.

Electromagnetic Radiation can best be described in terms of a stream of mass-less particles that travel at the speed of light in wave-like patterns. Each particle contains a certain amount of energy, referred to as a photon. Electromagnetic Radiation types are distinguished by their wavelength, their frequency and the differing levels of energy found in their photons. This energy is measured in cycles per second (hertz), electron volts and metres respectively. Most Electromagnetic Radiation, apart from Visible Light, is invisible to the human eye.

As can be seen from the diagram below, Radio Waves (at one end of the spectrum) have a long wavelength, low frequency and low energy. At the other end of the spectrum are Gamma Rays which have a very short wavelength, high frequency and high energy. In the Electromagnetic Spectrum Ultraviolet Radiation occurs after Visible Light and just before X-rays.

Ultraviolet Radiation (UV) falls into the range from a few electron volts (eV) to about 100 eV. The principal natural source of UV Radiation occurring on Earth is from the Sun. The Sun’s Electromagnetic Spectrum shows that most of the Sun’s radiation is emitted in the Ultraviolet range.

Scientists classify UV Radiation in three types or bands:
UVA (which has a wavelength of 315-400nM);
UVB (280-315nM) and
UVC (1-280nM).

The Earth’s stratospheric Ozone layer absorbs some but not all of these types of UV Radiation.  

UVA is not absorbed by the Ozone layer.
UVB is partially absorbed by the Ozone layer
UVC is completely absorbed by the Ozone layer. 

The level of UVA & UVB Radiation that reaches the Earth’s surface varies depending on a variety of factors. Whilst some radiation is healthy, higher levels of radiation can cause significant changes to the environment and have a notable effect on our health. These changes range from effects on plant growth, fisheries, aquatic food and disease resistance to human health issues such as skin cancer, suppression of the human immune system’s ability to respond to infections and disease, cataracts of the eye & premature ageing of the skin.

The main factors determining the UVA & UVB exposure we receive are as follows: 

Stratospheric Ozone
This absorbs most of the Sun’s UV rays however the amount of absorption depends on the time of year and other natural phenomena. Absorption levels are now decreasing as the Ozone layer has thinned due to the release of Ozone depleting substances that have been widely used in industry. 

Time of Day
The Sun is highest in the sky around noon. At this point the Sun’s rays have the least distance to travel through the atmosphere and UVB levels are at their highest. In the early morning and late afternoon the Sun’s rays pass through the atmosphere at an angle and thus their intensity is greatly reduced. 

The Sun’s rays are strongest at the equator where the Sun is almost directly overhead. Ozone is naturally thinner in the tropics compared to mid and low latitudes. At higher latitudes the sun is lower in the sky and therefore the rays must travel a greater distance through the Ozone rich portions of the atmosphere thus reducing the UV radiation reaching those latitudes. 

UV intensity increases with altitude because there is less atmosphere to absorb the UV rays. 

Weather Conditions
Cloud cover reduces UV levels to varying degrees depending on its thickness, however this reduction is never complete. 

Surfaces such as snow, sand, water and even grass can reflect much of the UV radiation that reaches them. As a result of reflection UV intensity can be deceptively high, even in shaded areas. 

UV Radiation awareness
Due to the major health and environmental concerns now associated with UV Radiation the World Health Organisation has launched a Global UV Research Project called Intersun. As a result of this many countries have now introduced a UV Index to report daily levels of UV Radiation. See UV Index.