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

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

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.

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.

[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

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.

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.

I want to see this.

I'm gonna be the change I want to see.

I feel very dumb and American for never realizing this was a thing.

I usually do

46°C instead of 46° ???

Sometimes it's easier to type "c" than "°"