How to measure UV index?
Finding 'UV index' at a point might be difficult. UV index is an index created by I think was WHO, and is calculated using 
 - Thickness of ozone above
 - Cloud density of the area
 - Altitude of location
 - Sun's height
more about it here - https://www.who.int/uv/publications/en/UVIGuide.pdf
So I think you are better off finding the UV light intensity instead, using some measurement tools. There's few different options like photo reactive dyes, and digital scales, but probably the most cheap and quantitative data can be obtained using arduino or other similar development board and creating a small module yourself. Good Luck!