When Celsius and Fahrenheit used?

Answer 1

Fahrenheit is used for non scientific work almost exclusively in the USA, and a few other countries including Belize and somewhat in the UK and Canada.

Celsius is used for virtually all scientific work.

Both scales have their benefits and shortcomings.

Celsius is intuitive with the freezing point of water being 0°C, and the boiling point of water being 100°C. And, it is helpful when all the units are interrelated, so it takes 1 calorie to raise 1g (or cc or ml) of water 1 degree Celsius. Kelvin uses the same scale, but uses "Absolute Zero" as the starting point. It is particularly useful to think of temperatures in Kelvin when dealing with the Ideal Gas Law.

However, the big advantage of Fahrenheit is that it deals with temperatures that are "normal" for our environment. So, 0°F is awfully cold. 100°F is awfully hot. And for much of our country the temperatures rarely go out of that range. 100°F is also considered a cutoff for being febrile...

I suppose it is all what one gets used to using. And, one could learn to change over relatively quickly (except that all the thermometers I have already read in Fahrenheit).