Otter Valley Weather's guide to ultra violet (uv) light

All about ultraviolet (UV) light

The Electromagnetic Spectrum

Ultraviolet light is emitted by the sun and forms part of the electromagnetic spectrum, of which visible light is included. See the diagram above for the full range of the electromagnetic spectrum that has radio waves (longer waves), going through to the shorter wavelength (more energetic) Gamma and Cosmic rays.

You can see that UV light falls to the left of visible light, therefore each photon contains more energy than say, infrared.

Apart from the border with visible light, UV is invisible to our eyes and covers the wavelengths from 10 nanometres (nm) to 400 nm, and energies from 3 electron volts (eV) to 124 eV.

Nearly all of the UV in sunlight is blocked by the ozone layer, and a good job too as this prevents the shorter wavelengths of UV (and hence most damaging) from reaching the ground.

The sun's disc in UV light

Ultraviolet radiation has been designated as three types: UVA, UVB, and UVC. The most energetic is UVC and this is responsible for the formation of ozone in the atmosphere and hence, does not reach the earth's surface.

What remains of the UV in sunlight, i.e., UVA & UVB (some below 280 nm, and most below 315 nm) penetrates down to the earth's surface. Both types of UV can give a suntan, though UVB is responsible for the deeper, darker, longer-lasting tan. People have evolved this ability to 'tan' over 1000s of years, and it is this tan that can help to reduce the damaging effects that UV can have on our skin.

As a way of giving a forecast of the potential UV strength of sunlight, the UV Index (UVI) was created. This originally was on a scale of 1 to 10, with 1 being minimal strength and 10 being extreme. It was found that UV does exceed the original scale and now UVI levels of 11 or more can be given. The way how UVI is calculated is complex but the scale is linear and can be divided into the following for guidance:

How you respond (sunburn) to the UVI when out in the sunshine will depend on the colour of your skin, with lighter skin burning quicker than darker skin. Also, your skin will adapt to exposure over a longer period of time so you can build up a resistance (suntan) that will allow you to stay out longer in the sunshine before you get sunburnt. In the British Isles, the strongest sunshine is along the south and south-west coasts, and it is these places that also get the most sunshine duration. Here in the Otter Valley, sunburn is a real risk from mid-March until early October with severe sunburn possible from mid-April until mid-September.

Jan		Feb		Mar		Apr		May		Jun		Jul		Aug		Sep		Oct		Nov		Dec
1	2	2	3	3	4	5	6	6	7	7	8	8	8	8	7	6	5	4	3	3	2	2	1
Typical maximum UVI values for here in the Otter Valley throughout the year

	Weather conditions can prevail that cause extreme UV levels. These high levels can occur anytime from mid-May to early August, i.e., when the sun is 60° or above in elevation. Such an event occurred on the 8th June 2008 and can be seen in the HPA graph on the right. In 2011, UVI 9 occurred on the 6th June & the 6th July. UV levels were very high at times during 2014 with UVI 9 being exceeded on 8 days and a maximum UVI of 10.0 being recorded on the 14th July 2014.
Met Office UK Chart 8th June 2008		HPA Graph of the UVI at Camborne, Cornwall on the 7th & 8th June 2008. It shows the UV peaking at over 9 on both days.

The highest UVI levels generally occur in maritime tropical air during May, June, July and early August. A partially cloudy sky with the sun shining will often add 1 to the UVI due to reflection off the clouds. Reflection off the sea and sand on south facing beaches could also add another 1 or 2 to the UVI, often giving our south coast beaches of Devon & Cornwall the highest UVI found in the British Isles.

Here are a few interesting facts about UV:

	The higher the sun in the sky, the higher the UV level. Therefore, UV levels vary with time of day and time of year. Outside of the tropics, the highest levels occur when the sun is at its maximum elevation, at around midday (solar noon) during the summer months. Here in the Otter Valley that occurs at around 13:15.
	At higher altitudes, a thinner atmosphere absorbs less UV radiation. With every 1000 metres increase in altitude, UV radiation levels increase by 10% to 12%.
	Ozone absorbs most of the UV radiation that would otherwise reach the Earth’s surface. Ozone levels vary over the year and even across the day.
	UV radiation is reflected or scattered to varying extents by different surfaces, e.g. fresh snow can reflect as much as 80% of UV radiation, dry beach sand about 15% and sea foam about 25%.