The uv from the sun how is it worked out

how does the met office work out the uv rays from the sun everyday, ive checked on the website and it can be 6 uv on a sunny day and 6 on a cloudy day with loads of rain. is it how far away the sun is from us at the time ? how much we're being exposed to it or what

The amount of energy the surface or the top of the atmosphere gets over a square feet or meter is directly proportional to the distnce of the Sun from the Earth. The distance changes constantly, from somewhat mor thna 91 million miles around January 3 to more than 95 million miles around July 3-4.

http://www.usno.navy.mil/USNO/astronomic…

http://zebu.uoregon.edu/~soper/Light/lum…

the incoming UV is almost constant but it is blocked by clouds rain etc