Well I have heard of the, "temperature in a jar" setup. But the question is, is it accurate? I mean the temperature in a jar, and the componets that actuall pyranometers and radiometers use are totally different. My UV sensor is a UVN800 for my WMR100. Now when a cloud passes over the sensor, the UV has a down spike. The high UV for the day will show up as a flat line around 11:00AM to 2:00PM for that part of the day. Like for winter you can expect 3 on a sunny day. That's apart from somewhere around 7 and higher during spring, summer, and fall. And the number that the UV dips can give me the cloud thickness. Like if the clear sky UV is at a high of 5 and dips to 4 during certain parts and 3 and so forth on other parts of the day, but comes back up to 5 indicating the UV isn't going down, but being blocked by a cloud or even a bird, trust me thats happened. But this data on sky conditions doesn't show up on Wunderground. Like some other stations in the area that report UV and not solar radiation will have in the tabular little sky condition abreviations like CLR., SCT., BKN. I think It might be the way by software processes and reports the data. But some software programs will base sky conditions off of UV or any other sun data. I will look into this further.