That thermometer in the center, it is 23 cells long, and yet still contains only digits between 1 and 9 inclusive. 23 cells is one of the longest slow thermometers possible within a standard 9x9 Sudoku grid, although I’ve heard rumors of even longer. I think to be longer, the thermo has to cross itself, which can lead to some confusion. There are a few different ways to arrange such long thermometers, and this is one.
I looked up a few temperature scales to use for puzzles involving “slow” thermometers like these, and there are some odd ones. It’s nice to live in a world in which such things are settled, in which we have two settled temperature scales oriented around humans, and easy math to convert between them. Team Fahrenheit and Team Celsius don’t apply here, though. Here, temperatures rise very, very slowly, or sometimes not at all.
The world is full of things to measure, and scales for reporting those measurements. Some are more rational than others, some are more human-oriented than others, and some are older than others, but none of them involve thermometers that sometimes rise and sometimes don’t. This puzzle does, though!