4 ms·

If you want "twice as hot" to be meaningful, you have to use an absolute scale, either Kelvin or Rankine. (1 K = 1.8 °R) Doubling absolute temperature doubles

If you want "twice as hot" to be meaningful, you have to use an absolute scale, either Kelvin or Rankine. (1 K = 1.8 °R) Doubling absolute temperature doubles the pressure (at constant volume) or volume (at constant pressure) of an ideal gas. This gets really important when it comes to compressing and expanding air and other gasses. Room temperature is approximately 293 K, twice that is 586 K / 595 °F / 313 °C. Hotter than your typical oven cooking temperature. We often don't realize how warm the world we live in truly is, from a physics standpoint.

benhurmarcel 3y ago
The Python package "pint" (to manage units) implements that with different units for absolute temperature vs delta temperatures. https://pint.readthedocs.io/en/stable/user/nonmult.html https://pint.readthedocs.io/en/stable/user/nonmult.html
usui 3y ago
Kelvin doesn't use degrees
- psychoslave 3y ago
  Why do we use degrees for other temperatures by the way? Is that related to some kind of absolute vs relative metric? (Kelvin, I guess, is based on absolute zero as a reference point, doesn't it?) Incidentally, why is the degree immune to disappearance in division unlike all other units? That is, if I divide a distance (m) by a speed (m/s) I get a duration (s). From sheer memory, degree doesn't work like that. Any clue why?
  - rocqua 3y ago
    Degrees aren't a 'unit' indeed because the zero point isn't a real zero. Hence you don't get correct scaling.
    - js8 3y ago
      Except for the degrees that measure an angle.
      - Arch485 3y ago
        No - An angle can be measured from an arbitrary point, and thus "0°" is not a true zero. "0° on a circle" (for example) doesn't make sense. It has to be 0° _from somewhere._
        its-summertime 3y ago
        A length can be measured from an arbitrary point too, no? And in some cases must be. The perimeter of a circle for example. - - - Further, Radians is another measurement of angle, but it is not "degrees radian"
        tinus_hn 3y ago
        180° on a circle makes a lot of sense, it’s half the circle. Just like a meter makes sense, it’s a unit of distance.
    - bloak 3y ago
      That is true, but if you're talking about a change in temperature (the temperature is rising by 2 degrees per hour, or the temperature falls by so many degrees per km of altitude) then there's a real zero, of course.
  - mananaysiempre 3y ago
    > Incidentally, why is the degree immune to disappearance in division unlike all other units? I understand why one could say that, but I’d say it’s more misleading than helpful. (And the reason why is directly connected to the problem TFA discusses.) Instead of temperature, let’s look at time first. If you’re measuring the temporal extent of an event, then it absolutely makes sense to divide 30s by 15s or 6 days by 3 days and obtain a dimensionless 2: one thing takes twice as long as another, and the factor 2 that doesn’t depend on the choice of units, so it’s dimensionless. Now consider dates. The year AD 2022 denotes (not very precisely) a point in time by specifiying its distance from a fixed reference in years; as another example, astronomers use the “Julian day”, which is a (possibly non-integer) number of days from a different reference point. And you could technically divide AD 2022 by AD 1011—the ratio between the distances to the reference point is indeed a dimensionless 2, and if you counted days from it instead of years it’d still be the same. But it’s just not a very useful statement. And if you divide the Julian days instead, you’ll get a different number, because the reference point has moved 4712 years. So you can divide the time coordinates as long as the chosen origin stays in place, but you can’t really divide points in time. Similarly, when you’re talking about differences in temperature, degrees work like any other unit, but temperatures themselves cannot be divided in the standard scales (without tying yourself to the scale): you’ll get the same result in Celsius and Réaumur (using the same origin) but a different result in Fahrenheit (using a different origin). Of course, the silly part is that there is a natural origin for temperature, it’s just that the usual scales don’t use it. But then there’s a natural origin for time as well (and you’ll occasionally see it used in cosmology), it’s just that it’s metrologically inconvenient (being known only very approximately) in addition to being very far away. (Library assignment: what defines a coordinate system on an affine line?)
  - Karellen 3y ago
    The root of the word "degree" basically means "units" or "subdivisions" or "measurements" or "amounts". We didn't really have a better word for units of temperature, as "Celsiuses" or "Farhenheits" didn't become a thing, so "degrees (of) Celsius" or "degrees (of) Fahrenheit" is kind of what we're left with. We could have chosen to say "degrees (of) kelvin", but we decided to go with "kelvins" (like "meters" or "seconds") instead. It's just a choice of language. Temperature does have dimensionality, but it's weird because it's a bulk property of a particular material. If you consider two stars, you can add their masses, or their volumes, and the resulting values are meaningful. But you can't add their temperatures. You can't even average their temperatures meaningfully without knowing the relative sizes of the stars and a bunch of other information either. That said, I can't think of a situation where degrees (of temperature?) would disappear in a mathematical operation. Can you give a specific example?
    - trepanne 3y ago
      > That said, I can't think of a situation where degrees (of temperature?) would disappear in a mathematical operation. I regret that the decades that have elapsed since I studied physical chemistry & thermodynamics don’t permit me to make a proper answer, but if you want this to make sense I believe you should look many orders of magnitude smaller- Boltzman’s derivation of the ideal gas law from Newtonian mechanics using statistical methods. Temperature is basically thermal density; it’s not really a fundamental unit. Chemical engineers use steam tables all the time to convert temperature to heat or other quantities.
  - dan_hawkins 3y ago
    > Incidentally, why is the degree immune to disappearance in division unlike all other units? It's not. If you divide temperature by temperature (T/T) you get unitless value. But it doesn't make sense (what does it mean Kelvins per Kelvin?) Your mistake is to take a scalar value for temperature: 300 K / 2 = 150 K (T/scalar = T).
  - GuB-42 3y ago
    Degrees are used for many things besides temperature. Angles obviously, but is also common in some countries for alcohol concentration (same as %ABV) and other specialized units. Essentially, "degree" means "unit", and "degree Celsius" means "the unit of Mr.Celsius". SI dropped it for Kelvin probably for consistency and because it is redundant information, we already know that Kelvin is a unit, no need to specify "degree". It also adds a funny character to the abbreviation, and would be the only unit in two words. It isn't because it is a relative scale, Rankine uses degrees and it is an absolute temperature scale just like Kelvin. The difference, I'd say, is that the Rankine scale didn't go though the SI standardization process. There is nothing special with division, "degree Celsius" is the unit, you can't separate the "degree" from the "Celsius". You can measure a thermostat dial in degrees Celsius per degree (of angle). The resulting unit is just that, not "Celsius".
- kergonath 3y ago
  You’re right in modern usage, but it is common to see °K in older documents.
mr_mitm 3y ago
You don't need an absolute scale, you just need a sensible origin. I suggest taking human perception into account. When talking about the weather or everyday objects, one could argue that the border between "hot" and "cold" is somewhere at room temperature, so around 21°C. At that temperature, it's neither hot or cold. If it's 30°C, it's 9°C of "hotness", so 39°C would be twice as hot. I don't know if it feels twice as hot, as that's quite subjective, but if you want more precision we should switch to another vocabulary entirely.
- lostmsu 3y ago
  36.6C?
- throwaway290 3y ago
  How given temperature feels depends on humidity, airflow, local culture, individual human and specific condition of that human (sitting/moving/sleeping, caffeinated, just ate a meal, bored, enthusiastic). - I know someone who wears a puffy jacket indoors if it's 24 degrees. - You will find sitting in 20 degrees with 88% humidity much more miserable than 20 degrees and 33% humidity. - 42deg with 20% humidity and slight breeze is fine, 38deg with 90% humidity can make you pass out in minutes if you need to walk anywhere in the sun.
- Joker_vD 3y ago
  Water-freezing temperature is an absolutely sensible zero. The difference between e.g. 2 °C and -2 °C is really quite obvious for the exposed skin: while it's merely "yeah, it's cold" feeling when it's above zero, it almost feel like the frost is biting, almost literally, when it's below zero.
  - bangkoksbest 3y ago
    This is not true, and it's why we developed separate scales for wind chill (which factors in wind) and heat index (which factors humidity). Looking at the wind chill chart[0] for frostbite times, there is nothing particularly special around the freezing point temperature, even at absurd wind speeds. Ironically, actually, the 0 point for Fahrenheit is far more relevant threshold for frostbite. [0] https://www.weather.gov/safety/cold-wind-chill-chart https://www.weather.gov/safety/cold-wind-chill-chart
ginko 3y ago
Dealing with any temperature difference requires an absolute scale as well. The difference between 20°C and 25°C is 5K.
lynguist 3y ago
Twice as hot is also useful in Celsius. For every increase of 10C chemical reactions happen twice as fast, it will literally, thermodynamically be twice as hot. Which will probably even match your subjective feeling. Celsius scales thermodynamically logarithmically. You can’t just double the numbers. (The same with sound etc.)
- Certhas 3y ago
  Reaction rates are approximately exponential in temperature, but there is nothing special about 10C. The doubling temperature difference depends on what reaction you look at. This is as opposed to decibel for sound, where +10 decibel means X10 power exactly.
  - InitialLastName 3y ago
    To be clear, that's true of power decibels for everything, not just sound.
- thfuran 3y ago
  >For every increase of 10C chemical reactions happen twice as fast Except for the ones that go at different rates.
- _Microft 3y ago
  The Celsius scale is an "interval scale" while the Kelvin scale is a "ratio scale". One cannot take ratios of Celsius temperature values as ratios are not even defined for that. It’s possible for differences of temperatures on the Celsius scale, though. https://en.wikipedia.org/wiki/Level_of_measurement https://en.wikipedia.org/wiki/Level_of_measurement
aqme28 3y ago
> We often don't realize how warm the world we live in truly is, from a physics standpoint. Or cold? There is a big ball in our solar system sitting at tens of millions of degrees K, and you think 273 degrees is warm?

benhurmarcel 3y ago

The Python package "pint" (to manage units) implements that with different units for absolute temperature vs delta temperatures. https://pint.readthedocs.io/en/stable/user/nonmult.html https://pint.readthedocs.io/en/stable/user/nonmult.html

usui 3y ago

Kelvin doesn't use degrees

psychoslave 3y ago
Why do we use degrees for other temperatures by the way? Is that related to some kind of absolute vs relative metric? (Kelvin, I guess, is based on absolute zero as a reference point, doesn't it?) Incidentally, why is the degree immune to disappearance in division unlike all other units? That is, if I divide a distance (m) by a speed (m/s) I get a duration (s). From sheer memory, degree doesn't work like that. Any clue why?
- rocqua 3y ago
  Degrees aren't a 'unit' indeed because the zero point isn't a real zero. Hence you don't get correct scaling.
  - js8 3y ago
    Except for the degrees that measure an angle.
    - Arch485 3y ago
      No - An angle can be measured from an arbitrary point, and thus "0°" is not a true zero. "0° on a circle" (for example) doesn't make sense. It has to be 0° _from somewhere._
      - its-summertime 3y ago
        A length can be measured from an arbitrary point too, no? And in some cases must be. The perimeter of a circle for example. - - - Further, Radians is another measurement of angle, but it is not "degrees radian"
      - tinus_hn 3y ago
        180° on a circle makes a lot of sense, it’s half the circle. Just like a meter makes sense, it’s a unit of distance.
  - bloak 3y ago
    That is true, but if you're talking about a change in temperature (the temperature is rising by 2 degrees per hour, or the temperature falls by so many degrees per km of altitude) then there's a real zero, of course.
- mananaysiempre 3y ago
  > Incidentally, why is the degree immune to disappearance in division unlike all other units? I understand why one could say that, but I’d say it’s more misleading than helpful. (And the reason why is directly connected to the problem TFA discusses.) Instead of temperature, let’s look at time first. If you’re measuring the temporal extent of an event, then it absolutely makes sense to divide 30s by 15s or 6 days by 3 days and obtain a dimensionless 2: one thing takes twice as long as another, and the factor 2 that doesn’t depend on the choice of units, so it’s dimensionless. Now consider dates. The year AD 2022 denotes (not very precisely) a point in time by specifiying its distance from a fixed reference in years; as another example, astronomers use the “Julian day”, which is a (possibly non-integer) number of days from a different reference point. And you could technically divide AD 2022 by AD 1011—the ratio between the distances to the reference point is indeed a dimensionless 2, and if you counted days from it instead of years it’d still be the same. But it’s just not a very useful statement. And if you divide the Julian days instead, you’ll get a different number, because the reference point has moved 4712 years. So you can divide the time coordinates as long as the chosen origin stays in place, but you can’t really divide points in time. Similarly, when you’re talking about differences in temperature, degrees work like any other unit, but temperatures themselves cannot be divided in the standard scales (without tying yourself to the scale): you’ll get the same result in Celsius and Réaumur (using the same origin) but a different result in Fahrenheit (using a different origin). Of course, the silly part is that there is a natural origin for temperature, it’s just that the usual scales don’t use it. But then there’s a natural origin for time as well (and you’ll occasionally see it used in cosmology), it’s just that it’s metrologically inconvenient (being known only very approximately) in addition to being very far away. (Library assignment: what defines a coordinate system on an affine line?)
- Karellen 3y ago
  The root of the word "degree" basically means "units" or "subdivisions" or "measurements" or "amounts". We didn't really have a better word for units of temperature, as "Celsiuses" or "Farhenheits" didn't become a thing, so "degrees (of) Celsius" or "degrees (of) Fahrenheit" is kind of what we're left with. We could have chosen to say "degrees (of) kelvin", but we decided to go with "kelvins" (like "meters" or "seconds") instead. It's just a choice of language. Temperature does have dimensionality, but it's weird because it's a bulk property of a particular material. If you consider two stars, you can add their masses, or their volumes, and the resulting values are meaningful. But you can't add their temperatures. You can't even average their temperatures meaningfully without knowing the relative sizes of the stars and a bunch of other information either. That said, I can't think of a situation where degrees (of temperature?) would disappear in a mathematical operation. Can you give a specific example?
  - trepanne 3y ago
    > That said, I can't think of a situation where degrees (of temperature?) would disappear in a mathematical operation. I regret that the decades that have elapsed since I studied physical chemistry & thermodynamics don’t permit me to make a proper answer, but if you want this to make sense I believe you should look many orders of magnitude smaller- Boltzman’s derivation of the ideal gas law from Newtonian mechanics using statistical methods. Temperature is basically thermal density; it’s not really a fundamental unit. Chemical engineers use steam tables all the time to convert temperature to heat or other quantities.
- dan_hawkins 3y ago
  > Incidentally, why is the degree immune to disappearance in division unlike all other units? It's not. If you divide temperature by temperature (T/T) you get unitless value. But it doesn't make sense (what does it mean Kelvins per Kelvin?) Your mistake is to take a scalar value for temperature: 300 K / 2 = 150 K (T/scalar = T).
- GuB-42 3y ago
  Degrees are used for many things besides temperature. Angles obviously, but is also common in some countries for alcohol concentration (same as %ABV) and other specialized units. Essentially, "degree" means "unit", and "degree Celsius" means "the unit of Mr.Celsius". SI dropped it for Kelvin probably for consistency and because it is redundant information, we already know that Kelvin is a unit, no need to specify "degree". It also adds a funny character to the abbreviation, and would be the only unit in two words. It isn't because it is a relative scale, Rankine uses degrees and it is an absolute temperature scale just like Kelvin. The difference, I'd say, is that the Rankine scale didn't go though the SI standardization process. There is nothing special with division, "degree Celsius" is the unit, you can't separate the "degree" from the "Celsius". You can measure a thermostat dial in degrees Celsius per degree (of angle). The resulting unit is just that, not "Celsius".
kergonath 3y ago
You’re right in modern usage, but it is common to see °K in older documents.

mr_mitm 3y ago

You don't need an absolute scale, you just need a sensible origin. I suggest taking human perception into account. When talking about the weather or everyday objects, one could argue that the border between "hot" and "cold" is somewhere at room temperature, so around 21°C. At that temperature, it's neither hot or cold. If it's 30°C, it's 9°C of "hotness", so 39°C would be twice as hot. I don't know if it feels twice as hot, as that's quite subjective, but if you want more precision we should switch to another vocabulary entirely.

lostmsu 3y ago
36.6C?
throwaway290 3y ago
How given temperature feels depends on humidity, airflow, local culture, individual human and specific condition of that human (sitting/moving/sleeping, caffeinated, just ate a meal, bored, enthusiastic). - I know someone who wears a puffy jacket indoors if it's 24 degrees. - You will find sitting in 20 degrees with 88% humidity much more miserable than 20 degrees and 33% humidity. - 42deg with 20% humidity and slight breeze is fine, 38deg with 90% humidity can make you pass out in minutes if you need to walk anywhere in the sun.
Joker_vD 3y ago
Water-freezing temperature is an absolutely sensible zero. The difference between e.g. 2 °C and -2 °C is really quite obvious for the exposed skin: while it's merely "yeah, it's cold" feeling when it's above zero, it almost feel like the frost is biting, almost literally, when it's below zero.
- bangkoksbest 3y ago
  This is not true, and it's why we developed separate scales for wind chill (which factors in wind) and heat index (which factors humidity). Looking at the wind chill chart[0] for frostbite times, there is nothing particularly special around the freezing point temperature, even at absurd wind speeds. Ironically, actually, the 0 point for Fahrenheit is far more relevant threshold for frostbite. [0] https://www.weather.gov/safety/cold-wind-chill-chart https://www.weather.gov/safety/cold-wind-chill-chart

ginko 3y ago

Dealing with any temperature difference requires an absolute scale as well. The difference between 20°C and 25°C is 5K.

lynguist 3y ago

Twice as hot is also useful in Celsius. For every increase of 10C chemical reactions happen twice as fast, it will literally, thermodynamically be twice as hot. Which will probably even match your subjective feeling. Celsius scales thermodynamically logarithmically. You can’t just double the numbers. (The same with sound etc.)

Certhas 3y ago
Reaction rates are approximately exponential in temperature, but there is nothing special about 10C. The doubling temperature difference depends on what reaction you look at. This is as opposed to decibel for sound, where +10 decibel means X10 power exactly.
- InitialLastName 3y ago
  To be clear, that's true of power decibels for everything, not just sound.
thfuran 3y ago
>For every increase of 10C chemical reactions happen twice as fast Except for the ones that go at different rates.
_Microft 3y ago
The Celsius scale is an "interval scale" while the Kelvin scale is a "ratio scale". One cannot take ratios of Celsius temperature values as ratios are not even defined for that. It’s possible for differences of temperatures on the Celsius scale, though. https://en.wikipedia.org/wiki/Level_of_measurement https://en.wikipedia.org/wiki/Level_of_measurement

aqme28 3y ago

> We often don't realize how warm the world we live in truly is, from a physics standpoint. Or cold? There is a big ball in our solar system sitting at tens of millions of degrees K, and you think 273 degrees is warm?