ELI5. If you were going to double the temperature of something, what would that be?

Question

If it’s 50 degrees Fahrenheit then it’s 20 degrees Celsius. Double that and it’s 100F, and only 37C.

10C doubled to 20C is 50F and 68F.

On nice spring days I’ve always said “wow, it’s almost twice as hot today as it was last week.” But I don’t think that’s even remotely accurate.

Is Kelvin the only way to accurately measure something like this? If so, 300K is 80.33F and if you were to double it, you get 600K and 620.33F.

Would we ever be able to say “It’s twice as hot today as X time in the past?”

In: 0

Answer 1

Temperature is usually measured on what we call an interval scale. An interval scale is one where the difference between each unit is consistent (adding one degree is the same increase in temperature whether you’re starting at 30 degrees or 500), but the scale is not aligned to any absolute reference point so 0 on the scale is just an arbitrary value. This is why temperatures can go into the negative even though conceptually the idea of having a negative amount of something doesn’t make sense, because the difference between positive and negative values is just which side of the arbitrary 0 point they’re on.

One of the mathematical consequences of an interval scale is that it is meaningless to calculate any ratios on an interval scale. Like, we can say 40 is twice as much as 20 because 40 is twice the distance from 0 as 20, but when 0 is an arbitrary value, that doesn’t make sense anymore. You know, 50 is twice the distance from 36 as 43, but that doesn’t mean 50 is twice as much as 43 because comparing them from their distance to 36 is totally arbitrary.

Kelvin on the other hand is a ratio scale (an interval scale where there is a meaningful 0 point) because it’s essentially just Celsius shifted down to where 0 actually means the total absence of heat, rather than an arbitrary value. So yes, Kelvin is the only way to accurately make claims about one temperature being twice as hot as another.

Fun bonus fact: the lowest natural temperature ever recorded on Earth is about 184 Kelvin and the highest is about 330, so the range of natural temperatures on our planet is too narrow for normal weather to ever actually be twice as hot as some other time.