Thanks Kenneth,

However, I couldn't find any reference to UV index in your graphs.

KIPP & ZONEN CUV5 UV radiometer measures both UV-A and UV-B. Its signal is dominated by UV-A as irradiance is higher there than in UV-B. UV index is based on erythemal action spectrum that is mostly UV-B. You need column ozone to estimate clear sky erythemal UV and then you can use your measurements as a proxy for the cloud cover. Or, you can just use a different sensor: KIPP & ZONEN  UVS-E-T UV. See http://www.kippzonen.com/ProductGroup/11/UV-Radiometers