I'd sneak in 'Toggle C/F' somewhere and hook it to the same function you use on the numbers. Fahrenheit goes negative at a measly -18C. Where I live -20 to -30

I'd sneak in 'Toggle C/F' somewhere and hook it to the same function you use on the numbers. Fahrenheit goes negative at a measly -18C. Where I live -20 to -30C is not uncommon. Whether it's 17.5 or 18C typically does not matter, continual changes in wind and other factors will for pretty much all practical purposes quench that difference.