I disagree. You only really need to take those sources of error into account if you want an absolute measure of error, which as you explain, seems pretty impos

I disagree. You only really need to take those sources of error into account if you want an absolute measure of error, which as you explain, seems pretty impossible. An error for weather only needs to be relative -- for example, if the error for rain today is higher than yesterday, it's not important the exact number higher that it is -- only that it's higher. (Not that I know if this is possible.) It's like how you can't describe how biased a certain news source is or how to read a Yelp or Rotten Tomatoes review -- you just have to read it often enough to get an intuitive sense that a 4.1 star Yelp-rated restaurant with 800 reviews is probably good while a 4.6 star restaurant with 5 reviews is quite possibly terrible.