# Twice as hot / cold ?

1. Sep 29, 2008

### Nick89

Hi,

I was just wondering, if you say "The temperature today was twice as hot as it was yesterday", in which units are people usually referring?

Degrees Celsius doesn't make any sense, since it is perfectly possible for something to be twice as hot or cold as 0C... For the same reason I don't think degrees Fahrenheit make any sense...
So that leaves Kelvin I suppose? Twice as cold as 0 K would still mean 0 K since you can't go any lower, right?

So if today the temperature is 20 degrees C (roughly 293 K) and tomorrow it's twice as hot, would that mean it would be 293 * 2 = 586 K = 313 degrees C...? This doesn't really make sense to me either.

I suppose it's all about how humans physically interpret temperature? If we sense something to be twice as hot that doesn't mean the temperature is twice as hot in any scale we use, right?

So yeah, I was just wondering, is there any 'convention' for this, or does nobody care what twice as hot means? :tongue:

2. Sep 29, 2008

### Staff: Mentor

When folks use that expression, I don't think it's intended to be taken literally.

3. Sep 29, 2008

### gmax137

Next time someone tells you "it's twice as hot as it was yesterday," ask them what they mean.

4. Sep 29, 2008

### Staff: Mentor

Here - and we are using Celsius scale only - that will mean something like "it was 10 yesterday, it is 20 today". Or "it was -5 but it is -10". Not exactly, but something like that. No idea how it works if you use Fahrenheit.

5. Sep 29, 2008

### Staff: Mentor

No reason why you couldn't do the same with Fahrenheit. It was 32 degrees yesterday; today it's 64 degrees. So--even though the units are arbitrary and the zero point is arbitrary--I guess it's OK to say the temperature has doubled! :rofl:

(It would be more meaningful if you used the Kelvin scale.)

6. Sep 29, 2008

### Staff: Mentor

That it can be used same way with F is obvious, it is just beyond my experience - and cultural/language reflexes often cripple logic down

7. Sep 29, 2008

### mgb_phys

It's meaningless if you aren't on an absolute scale otherwise it would double from 10-20C in Canada but only go up by a third 50-68F in the USA.

8. Sep 29, 2008

### Staff: Mentor

For most people this meaniglessness doesn't matter, only some eggheads wring hands

9. Sep 29, 2008

### mgb_phys

Thats me!
Our low temperature physics group used to have a sign over the door saying "1million times colder than room temperature" - which always slightly annoyed me as well.

10. Sep 29, 2008

### Nick89

But that's exactly me point

If you wouldn't have any thermometer and someone asks you something like "how hot do you think it is", and you reply "it's nearly twice as hot as yesterday", then I can't imagine people using the celsius scale mean something completely different than people using the fahrenheit scale... While for your feeling, the temperature doubled, in reality, the temperature (depending on which units used) did not double at all...

I suppose you could look at the energy of the heat, just like you can look at the intensity of a soundwave (if the volume of sound doubles for your feeling, the soundlevel (in dB) has not nearly doubled)... Oh well, I think it's just one of those human things that don't really make sense :P