20 isn’t cold at all, it’s perfectly warm.

10 is nippy, but you still warm up quick after a few minutes walking, and get sweaty if you’re working on something.

0 is cold enough for a couple layers. Jumper + jacket so you can take one off if you warm up too much.

-10 doesn’t feel that much different to 0.

-20 is time to put on a thicker coat over the jumper.