Thanks for the explanation.  That said, I am confused by this concept.  To put this another way, a person can walk in a slightly different path for whatever reason, but this whole concept of inherent variability sounds an awful lot like randomness, and like it does not have a cause.  And, this confuses me. (As well, it kind of bothers me because, if it was random, it would be impossible to make good weather forecasts.)

Or, maybe I just don't understand it well enough.

So, I was wondering if you could tell me what causes this "inherent variability"?  The sun (i.e. changes in the length of daylight or the angle of the noontime sun)?  Mountains / other geographical features?  Whether or not there is snow on the ground?  Something else?  A combination?

Thanks for reading this and for answering my question.