Ok, I have a bizarre personal system.
For the weather: for low temperatures (say 10c or below) I use Celsius, but for high temperatures (above 70ishF) I use Fahrenheit. If it's hot and someone says it's 26C, I have to convert to Fahrenheit to make it fit my frame of reference. Vice-versa with low temperatures. I guess I ignore temperatures in between - or at least I don't talk or hear about them. I don't know.
I think this originated because when I was on holiday when I was young I always heard/talked about the temperature in Fahrenheit, so that's how my frame of reference got formed for hot temperatures, and it's stuck.
For cooking, PC component temps, and anything else, I use Celsius.