Despite being metric, Celsius is barely used with a prefix when describing temps high enough to warrant it

Because physics uses Kelvin for high temperatures, and electron volts for really high temperatures.

And Kelvin for really low temperatures, too. mK used a lot more than MK in many a physics lab...

Because kilodegrees sounds funny. But megadegrees really sounds volcano lair evil.

Megadegrees sounds like something graduates from Trump University got for finishing a retreat. They are the highest quality degrees - so good they deserve to have their own name!

Going back to temperature though, it would be odd-sounding to say the Sun can get as hot as 15 megadegrees at it's core.

Give it a few kilomonths

First of all, the °C is not the metric SI unit for temperature. K (Kelvin) is.

Second, even with Kelvin, nearly all temperatures that matter for normal human issues happen to be below 4000K, usually way below that mark. And with most of those temperatures, about all digits usually count. A core body temperature of 310K or 313K makes a BIG difference for the person involved.

Celsius is the SI unit of temperature. Kelvin is the SI unit of thermodynamic temperature. They're both defined in SI.
- You can say anything with confidence and people will believe it
I've seen mK used numerous times, but I haven't seen, like MK for internal temperatures of stars or things. I imagine because those are more "for fun" numbers while the precise temperatures in a low temperature physics lab are four technical purposes.
Isn't Kelvin just Celsius+273.15?
- Celsius uses an arbitrary reference point (freezing point of water). Kelvin uses the same sized units, but is referenced from absolute zero. While this seems just as arbitrary, it actually makes some scientific calculations a lot easier.
  
  Basically, scientists have been working to slot the various base units together in a neat and orderly manner. Kelvin fits this far better than Celsius, and so became the baseline SI unit.

Would just be confusing. Temperatures above a few hundred degrees have no place in most people's daily lives, so that would be mostly for scientific notations, and scientists use Kelvin anyway for precision.

The use of kelvin over Celsius has nothing to do with precision. They're the same thing, with different offsets.
- Technically yes and no. Kevin is absolute temperature, since the offset is zero it measures the total temperature. Celsius is relative, since the offset places its zero at a conventionally useful place it measures deviation from that baseline. That's why you have temperatures always in K and never °K, but always in °C and never just C. But yes, the sizes of the units are the same.
Fahrenheit solves this
- How exactly?

[Edited because of weird auto-formatting. Edit 2 added more pedantry. Edit 3+ is because I lost the plot and had to bring it back.]

Because the SI unit for temperature is the Kelvin, which has already been stated. It has also been mentioned that K and °C are the same but with different offsets. It has not been mentioned that °C is to K as Degrees Fahrenheit (°F) is to Rankine ( R). It would be similarly inappropriate to say "millidegrees Fahrenheit" or "kilofahrenheit". I have no idea if mR or kR would be appropriate, though.

I would offer that there are two ways to look at SI ("metric") prefixes, and these can be thought of similarly with the multipliers they represent: as a prefix to the unit, by definition; or as a suffix to the value. Let me illustrate with an example.

38,000 K could be expressed 38 kK, or "thirty-eight kiloKelvin". It could also be spoken "thirty-eight thousand Kelvin" (or Kelvins, idfk). This isn't normally important for the layperson, but suppose you have a temperature meter (and, literally, I do not mean "thermometer") that has only 4 digits of resolution. 38.00 k ("38,00 k" for the Europeans?) would be how it reads out the value in question. This would be 38 kK, certainly, due to the position of the decimal.

Now suppose that temperature meter read out in °C. 38.00 k °C would, in fact, denote "thirty-eight thousand degrees Celsius" for the reasons mentioned above.

So, because Degrees Celsius is not an SI unit, in the technical sense...

Btw, I have been explicitly using upper case letters when spelling out the units. This is incorrect. The symbols for SI (International System of Units) units should be capitalized when they respect a person (K, A). The names of the units should be all lower case because you are not naming the person, but the unit named after them (kelvin after Lord Kelvin, and ampere after Andre-Marie Ampere).

Yeah, I know. I'm being pedantic. It's literally my job. I really should be sleeping right now. Here's a source: https://www.bipm.org/en/measurement-units/si-base-units

There's no way someone would use something as logical as "Millifahrenheit".

It's be 143 Fahrenheit in a Blurgenfurl, 2 Blurgenfurl in a Whatjamagick and 19003 Whatjamagick in a Plenderboing.
Dude, did you run out of hot water while having this thought?
- Lol. Nah, my brother woke me up in crisis to have a conversation in text instead of over the phone, so my wife left to sleep in her own bed in a huff, and I just started new meds ...

Guess there's not much need. Most of the prefixes used are 1000 (kilo, mega, etc.) or 1/1000 (milli, micro, etc). The tens and hundreds are a bit odd to use and imo shouldn't be used. So there's no need to use prefixes until you're into Star temperatures or really extreme experiments.

Centicelsius has a nice ring to it.
- It would be centidegree.
- Centicentigrade
- centi- actually means 1/100th in the metric system
What do you mean? The high today was 1.3 decicelcius where I’m at

Edit: decacelcius
- You mean decacelsius?
Where I think they could be used is for in between temps. 1 degree centigrade covers a wide range when it comes to precision cooking like sous vide. Would be nice to drop to a smaller unit, but since metric can only work in multiples of ten, going down a level becomes overly precise.

The result is that I tend to prefer Farenheit for cooking, especially for sous vide. Unless you're doing molecular gastronomy shit, converting between units isn't that useful; you don't need to worry about how many Jules it's going to take to boil a given volume of water.

Conversely, grams are way nicer for measuring most things in the kitchen.

It's really when you get into the thousands though that SI prefixes generally start to be used, you don't see deca or hecto used that often. It's mainly because we're usually happy keeping three digits of precision in general conversation (185 degrees C, 250 metres, etc). After that we get a bit sloppy and start rounding, and that's where kilo comes in and we start talking about "1.25 kilometres" and such.

Add in the fact that people rarely need to describe temperatures higher than 1000 degrees C with any precision, (they'll just round to hundreds/thousands/millions usually) and that's why SI units feel weird with temperature.

Interestingly, I hear people use terms like millikelvin and microkelvin often enough, but never kilokelvin. In fact, there are some hilariously impractical ways to avoid large scientific notation for Kelvin. There's T4, which is the temperature in kelvin divided by 10^4, and there's electron volts, which is almost the same value, but preferred by different fields.

It's probably more common that scientific notation is used. So 3.2 *10^4 or simply 3.2e4. From the little physics I had, you often used kilometers instead of something like megameters. Or used just lightyears when you got on a big enough scale.