Why does the metric system use the Celsius scale rather than the Fahrenheit scale?

Question

Part of why I ask this is because the Fahrenheit scale has a higher degree of precision than the Celsius scale. And the Kelvin scale could just as easily been adapted to be an extension of the Fahrenheit scale(absolute zero is -459.67 degrees Fahrenheit).

Answer 1

If by metric you mean Système international (d'unités) then $\mathrm{^\circ C}$ is not, at least formally, part of it. It's derived unit that has unique properties that makes it so popular.

First: for temperature gradient $\Delta T$ of $1\mathrm{K} = 1\mathrm{^\circ C}$ which makes calculations so much easier as it's one to one identical, only shifted 273,15 degrees.

Second: it's so much more natural to think in Celsius terms where zero is point of freezing and cold and 100 of boiling and burning hot, where with Fahrenheit is 32 and 212 which is almost absolutely illogical.

Answer 2

I don't think what is called the metric system defines degrees Celcius as the default temperature unit anymore. At least, if you by the metric system mean the SI unit system, then the (former) base temperature unit is kelvin, $\mathrm{K}$, and not degree Celsius, $\mathrm{^\circ C}$.

Both degree Fahrenheit, $\mathrm{^\circ F}$, and degree Celcius, $\mathrm{^\circ C}$, (as well as degree Rankine, $\mathrm{^\circ R}$, and degree Rømer, $\mathrm{^\circ Rø}$ as well as other local or past temperature units) are exactly defined from the kelvin scale. There is no "loss in precision" by using one unit instead of another - meaning, you can always convert from one unit to the other without "losing" precision in the number. They are defined in various ways - ways that feel odd for some and intuitive for others - but are nevertheless all equally valid.

Typically, whenever the the metric system or the mks system is mentioned, it typically refers to the modern SI system. But it wasn't always so since the unit systems have undergone quite a clean-up over the centuries. Back in the late 1700s' France where what was called the metric system was invented and defined for the first time, there were a few differences from what we today know as the SI unit system. For instance, volume (then called capacity) was back then originally given the default unit litre, $\mathrm{L}$, which does not match the (former) SI base unit for volume today which is the cubicmetre, $\mathrm{m^3}$. I'm not entirely sure, but it is possible that also the default temperature unit back then was originally defined as degree Celcius, $\mathrm{^\circ C}$. This might be the reason that it is still here and there seen as the default nowadays even though it is an outdated base definition when comparing to SI.

In general when doing scientific work I would advise you to always use SI units by default and never think about it with the term "metric system" so as to avoid ambiguity. So, as long as you don't have specific reasons to go with something else, just go for the kelvin, $\mathrm{K}$, by default for temperature. (Certain scientific fields work in some specific sets of non-SI units per tradition or of practical reasons - such as within chemistry. You might also have local reasons to stick with non-SI units, of course - the Americans and the Brits might never let go of their imperial system...).