Ask yourself when was the last time you cleaned your shield and sensors.
Yesterday I took about 2 hours to remove my Outdoor temp/humidity with Radiation shield. After disassembly I found one lady bug, and a spider had taking up residence in the small bell shaped cover of the sensor itself. The inner plates of the shield had a layer of caked on dust, so I washed those off. The exposed outer portions of the plates had a build up of mildew, which would not come off when using normal dish detergent in the sink. So to get that off, I did the same thing I do when I want to sterilize my fish tank decorations. Immerse the plates in a 5 Gallon bucket of clean hot water and then pour in about a 1/2 cup of household bleach. Let sit for about 30 minutes. Remove and rinse the plates, and they will look sparkly new.
Clean the exposed sensor "bullet" itself with a damp paper towel, being careful not to get any "loose water drops" in the vents/electronics itself. As for the solar panel just use a soapy rag, to wipe it off, and then rinse it in the sink under a light spray(similar to rain), do not immerse it.
I have no "data" to support it, but I would imagine that the dust/pollen/bugs/spider webs/lack of radiative cooling buildup, will eventually cause your numbers to drift off somewhat.
I guess I will tackle the rain gauge sometime in the next few days when I have a little bit more free time.