It can be hard to know whether you have a good enough sample-- I'd been assuming that some store staff people were temperamentally grumpy, but the true situation was that I was shopping late in the day. They're much more cheerful if I show up early.

This confuses me. What's the difference between grumpiness and what you updated to after showing up earlier?

I know somebody who is -always-, for lack of a better word, grumpy - what I think you refer to as "temperamentally grumpy". Grumpy doesn't really describe that, though. My internal representation for "grumpiness" as a description of a person isn't "Always a grouch", it's a heavier weight on the rate at which people get grumpy. Same with anger.