Arbitrary: Based on random choice or personal whim, rather than any reason or system.
I can see no reason to use the freezing point of brine as 0℉, and the average temperature of a person as 100℉.
That is completely arbitrary.
Using the freezing point of water for 0℃, and the boiling point for 100℃ is anything but arbitrary.
You can argue that this is not as accurate as it should be, but that does not make it an arbitrary choice.
Hahahahaha. I was really just trolling, and I'm pretty sure you are too. But, this honestly gets to exactly my point.
0°C is only the freezing point of water (give or take a fraction of a degree) with a particular, arbitrarily chosen, kind of water (the standard isn't actually 100% pure water because that's essentially impossible to reproduce, and there are all kinds of isotopes of water in different concentrations) at a particular, arbitrarily chosen, altitude.
100℃ is only the boiling point of water given the same arbitrary preconditions.
The boiling point of the same exact chemical composition of water in Denver is around 95℃. At the top of Mt. Everest, water will boil at about 65°C. If water is exposed to outer space, it flash-boils instantly (after which it will either freeze solid or stay vapor, depending on if it is in sunlight or not). We only think that "water boils at 100°C" because that is close to the altitude where most people on the planet, or the powerful ones anyway, happen to live right now. Give us 100 years and sea level will be different and the rules will have all changed.
We don't actually encounter pure water (or the close-to-pure water they set as the benchmark) very often in real life, nor do we encounter surface temperatures of 100°C in everyday experience (if we did, we'd be very sad).
So, it is based on something. But not necessarily something useful.
Fahrenheit is based on the (rough) freezing point of brine and the (rough) body temperature of a human. They're not right, of course, and the amount of salinity in the brine is pretty random. But the two items chosen do relate pretty well to average human experience.
100°F is roughly the core average body temperature (I know, they say 37°C/98.6°F, but that is also arbitrarily chosen, because it varies by as much as 10°F from person to person and based on the time of day, how much the subject has eaten, and a wide variety of other factors). In any case, 100°F, because it is roughly our body temperature, is also roughly the air temperature at which you transition from "uncomfortable" to "dangerous". A normal, healthy human can easily withstand 92°F air all day long and (given no other external factors) they might be uncomfortable, but not in serious danger. Above 100°F, and you head into danger, because your core body temperature will start to rise uncontrollably.
Likewise, it is around 0°F that exposed skin freezes, not 0°C. Because we are
bags of mostly salty water. And, of course, that worked pretty well when you were mostly concerned about navigating the oceans and avoiding pack ice, and didn't care so much about slick roads from freezing rain (which, by the way, doesn't happen at 0°C air temperature either).
They're both arbitrary, but Fahrenheit (for all its flaws) does
happen to relate to the human experience of air temperature fairly well. If not by design, then by accident. Celsius related pretty well to doing basic chemistry and other science back when the standard was invented, though most modern, interesting science today requires using vastly different scales.
And don't even get me started on using so-called absolute zero (which is mythical) as a starting point.
