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What the FAQ are Celsius and Fahrenheit?

Most people think Anders Celsius and Daniel Gabriel Fahrenheit invented the temperature scales bearing their names. Most people would be wrong!

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I grew up in England where the weather forecasters informed us what to expect temperature-wise in units of degrees Celsius (°C). I vaguely recall that, when I was a kid, we also talked about degrees Centigrade (°C). In fact, we used the terms Celsius and Centigrade interchangeably and I never wondered why. When I moved to America, it was a bit of a shock to my system to see the weather forecast being presented in units of degrees Fahrenheit (°F). For quite some time, I had to perform a °F to °C calculation in my head to work out whether it was going to be hot or cold (I’ve now been in America so long that I have to perform °C to °F calculations in my head when I return to England to visit my dear old mom). Recently, I saw a funny skit that purported to be a conversation between the German physicist Daniel Gabriel Fahrenheit (1686–1736) and the Swedish astronomer Anders Celsius (1701–1744). It was Fahrenheit who proposed the original temperature scale upon which what we now call the Fahrenheit scale was based, while Celsius created a temperature scale that evolved into what we now call the Celsius scale. The idea behind the skit was that Daniel Fahrenheit and Anders Celsius are at a cocktail party having a cozy chat. We can imagine their conversation as being something like the following: C: “Greetings Daniel old chap. I hear you’ve created a new temperature scale. Funnily enough, I’ve been working on something similar myself.” F: “Felicitations Anders my dear fellow. You’re looking well. Yes, I have indeed devised a new temperature scale.” C: “So, upon what did you base your zero-degree point?” F: “I used the freezing temperature of a solution of brine made from equal parts of ice, water and a salt (ammonium chloride).” C: “Hmmmm” (said in a dubious tone). F: “Why are you ‘hmmmming’ like that? Can you think of anything better?” C: “What about something everyone is familiar with, like the freezing point of water, for example?” F: “Well, I suppose that would be an alternative” (said in a begrudging way). C: “And what value does the boiling point of water have in your scale.” F: “Ah, that’s the clever part, water boils at 212 degrees on my scale.” C: “Hmmmm” (said in a dubious tone). F:You are ‘hmmmming’ again. Can you think of something better?” C: “What about a nice round number like 100?” F: “Well, I suppose that could have been another possibility” (said in a somewhat brusque tone). Of course, nothing like this actually took place, but it’s still amusing the think about. The thing is, as we shall see, nothing about temperature is simple, least of all measuring it.  

What is Temperature?

The simplest definition of temperature is that it’s a measure of how hot or cold something is, but that really doesn’t help us as much as we might wish. As Piotr Małek and Álvaro Díez say on OmniCalculator.com: “[To define temperature] we have to turn to physics, in particular to thermodynamics and statistical physics, which is like thermodynamics meets quantum physics.” Now, I love thermodynamics and statistical physics as much as the next man, which is to say, “Not a lot,” so I think we will kick this particular question down the road for a future discussion.  

Why Measure Temperature?

As to why we want to measure temperature, different people have different reasons. Chemists need to know the temperature to control their reactions, scientists need to be able to measure temperature as part of their experiments, and so on and so forth. For most of us, in addition to cooking (i.e., setting the desired temperature on the oven), our main interest in measuring temperature is in knowing what to expect weather-wise so we can make an informed decision as to whether shorts and a T-shirt are a good idea, or if woolen undergarments and furry hats (or furry undergarments and woolen hats) will be the order of the day.  


If you ask most people how many senses they have, their knee-jerk reaction will be to respond, “Five of course: Sight, Hearing, Touch, Taste, and Smell.” Would you believe that these are the same five senses that were first described by the ancient Greek philosopher and scientist, Aristotle (384–322 BC), but we now know that humans have at least nine senses, and possibly as many as twenty or more. In particular, in the context of our discussions here, as I wrote in How Many Senses Do Humans Have?: “Thermoception, also known as thermoreception, is the sense by which we perceive temperature. Even if you are blindfolded, for example, if you hold your hand close to something hot, you can feel the heat in the form of infrared (IR) radiation. Similarly, if you hold your hand over a block of ice, you can detect the lack of heat.”  

Easy Peasy Lemon Squeezy

Apparently, the “official” antonyms to “easy peasy lemon squeezy” are “hard hard lemon hard” and “difficult difficult lemon difficult,” but I find both of these to be less than satisfying. I prefer “stressed depressed lemon zest,” but this in no way relates to what we are talking about here. To be honest, the sad thing about measuring temperature is that — outside of laboratories with specialist equipment — we really don’t seem to be tremendously good at it. Recently, for example, items of food in our fridge didn’t feel as cool as I was expecting, so I decided to purchase a special fridge/freezer thermometer from Walmart. In fact, I ended up buying two because (a) I no longer trust a single thermometer reading and (b) there were only two on the shelf. The reason I no longer trust a single thermometer is that, a couple of years ago, our HVAC system died and we had to buy a new one. As part of this, we ended up with a new thermostat. Even though we set the new thermostat to the same temperature as the previous unit, the house felt hotter. In order to wrap my head around this conundrum, I ambled down to The Home Depot to pick up an outside thermometer. On the bright side, they had about 30 of the little rascals. On the downside, even though these devices were on a shelf under a shaded area of the garden center, they all presented different readings with a spread of about 10 degrees. As a result, I spent a happy few minutes adding all the readings together, taking the average, and then determining which of the thermometers was closest to that average. The only reason I mention this here is that I was surprised to discover that, almost 1/5th the way through the 21st century, it appears that we still find it less than easy peasy lemon squeezy to tell the temperature.  

Who Invented the First Thermometer?

This is a bit of a tricky question because there have been so many people stomping through history with hob-nailed boots. First, let’s define what a thermometer is. For this, I quite like the Wikipedia definition, which reads as follows:
A thermometer is a device that measures temperature or a temperature gradient. A thermometer has two important elements: (1) a temperature sensor (e.g., the bulb of a mercury-in-glass thermometer or the pyrometric sensor in an infrared thermometer) in which some change occurs with a change in temperature; and (2) some means of converting this change into a numerical value (e.g., the visible scale that is marked on a mercury-in-glass thermometer or the digital readout on an infrared model).
The precursor to the thermometer was the thermoscope, which is essentially a thermometer without a scale. Although a thermoscope couldn’t indicate a specific temperature, it could be used to show differences in temperature, thereby allowing its users to determine if something was getting hotter or colder. Two contenders for the creator of the first thermoscope are Hero of Alexandria (10–70 AD) and Galen of Pergamon (129– ~208 AD). Work on thermoscopes continued all the way to the early 1600s, with the most famous practitioner being the Italian astronomer, physicist, and engineer, Galileo Galilei (1564–1642). Somewhere around 1610, the first person to put a scale on a thermoscope, thereby turning it into a thermometer, may have been Francesco Sagredo (1571–6620), who was a Venetian mathematician and close friend of Galileo, or it might have been the Venetian physiologist, physician, and professor Santorio Santorio (1561–1626), or it could have been someone else — who really knows?  

What the FAQ is Fahrenheit?

This is a bit of a tortuous tale, but we can summarize it as follows. Sometime circa the 1700s, the famous Danish astronomer Ole Christensen Rømer (1644–1710) came up with his own temperature scale. As an aside, there were about 30 other temperature scales in use at that time, one of which was based on the melting point of butter (I’m going to take a wild guess that the proponent of this one was French). The ancient Babylonians have a lot to answer for, not least that there are 6 x 60 = 360 degrees in a circle, 60 seconds in a degree, 60 minutes in an hour, and 60 seconds in a minute Since, as an astronomer, Rømer was used to dividing things by 60, he decided to use 60 as the temperature of the boiling point of water. By dividing this into eighths, Rømer ended up with values of 0, 7.5, 15, 22.5, 30, 37.5, 45, 52.2, and 60. For reasons I don’t fully understand, he set 7.5 as representing the freezing point of water. If you are interested, you can find out more in this excellent video by Veritasium.  
The bottom line is that Fahrenheit met Rømer and was introduced to his temperature scale. Since he didn’t like all the fractions, Fahrenheit made a number of adjustments, eventually ending up with the values of 32 °F and 212 °F for the freezing and boiling points of water, respectively. Now, these values may seem a little arbitrary, but there are all sorts of hidden tidbits of trivia and nuggets of knowledge here, such as the fact that 212 – 32 = 180 (half the number of degrees in a circle). Also, that Fahrenheit was using mercury in his thermometers, and a change of one degree in the Fahrenheit scale results in a 1/10,000th change in the volume of mercury. Coincidence? I think not!  

What the FAQ is Celsius?

Most people assume that Anders Celsius took the temperature at which water freezes and said, “let’s call this 0 degrees.” Also, that he took the temperature at which water boils and said, “let’s call this 100 degrees.” In reality, although Celsius did come up with the idea of separating the freezing and boiling points of water by 100 degrees, he initially started off with 0 °C representing the boiling point of water, while 100 °C represented the freezing point of water. It wasn’t until a year after his death that other users decided to swap them over. The reasoning behind all of this is presented in another awesome video by Veritasium.  
The really interesting thing is that, in 1743, a year before Celsius died, the French physicist, mathematician, astronomer and musician Jeane-Pierre Christin (1683–1755) also came up with the idea of using 0 and 100 degrees to represent the freezing and boiling points of water, respectively. So, why don’t we say things like “100 degrees Christin”? In reality, for the longest time, neither Celsius nor Christin’s names were used. Instead, based on the fact that there were 100 steps between the freezing and boiling points of water, most people used the term “Centigrade.” The problem here is that the word “centigrade” has multiple meanings in different languages. Thus, in 1948, the International Bureau of Weights and Measures decided to rename this scale after a scientist like other temperature scales, and Celsius “won the toss,” as it were.  

Example Values

In addition to the freezing and boiling points of water, another value in which people are often interested is that of normal human body temperature. Also known as normothermia or euthermia, this is the typical temperature range found in humans, which is 36.5 to 37.5 °C (with an average of 37 °C) or 97.7 to 99.5 °F (with an average of 98.6 °F).
Comparison of Celsius and Fahrenheit (Click image to see a larger version — Image source: Max Maxfield)
Just to provide a sense of scale (no pun intended), the sublimation temperature of a block of dry ice is –78.5 °C (–109.3 °F), while the boiling point of liquid air is –194.35 °C (–317.83 °F). The lowest limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reach their minimum value, is known as absolute zero. This occurs at –273.15 °C (–459.67 °F).  

Conversion Formulas

Fahrenheit to Celsius: Subtract 32, then multiply by 5, then divide by 9. Celsius to Fahrenheit: Multiply by 9, divide by 5, then add 32. As a point of interest, the Celsius and Fahrenheit scales converge at minus 40 degrees, so –40 °C and –40 °F represent the same temperature.  

But Which is Best?

Everyone thinks that whatever they grow up with is the best way to do things, and this applies to measuring things in Celsius and Fahrenheit, but can we really say that one is better than the other? On the one hand, dividing the span between the freezing and boiling points of water into 100 degrees is easy to wrap our brains around, but do we really care? The vast majority of us are interested in expressing temperature only in regard to our comfort, for which purpose we use the temperature of the surrounding air. When inside our homes, offices, or other buildings, we set the temperature we desire; when outside, we use temperature as one criterion in our decision-making process as to what clothes to wear. Humans are very sensitive to temperature; a small difference in temperature can have a large effect on one’s comfort level. In the case of the Fahrenheit temperature scale, we have 1.8x the integer resolution to express and communicate temperature without being obliged to use fractional values. On this basis, one could argue that the Fahrenheit temperature scale is better suited to our everyday requirements.  

But Wait, There’s More

Some people say that 0 °C (32 °F) is defined as the temperature at which water freezes; others say it’s defined as the temperature at which ice melts. Is there a difference? In fact, the terms “freezing point” and “melting point” describe the same intermediate point in the transition of matter from liquid to solid (freezing) or from solid to liquid (melting). While water is in the process of freezing or melting, its temperature is not changing – it remains at 0 °C (32 °F) throughout the entire freezing or melting process. Similarly, when the temperature of water rises, it will start to boil at 100 °C (212 °F), and it will remain at this temperature until all the water has boiled away. Having said this, the freezing, melting, and boiling points of water are defined at a pressure of one standard atmosphere, which itself is defined as being the mean atmospheric pressure at sea level. The thing is that the boiling point of water is the point at which vapor pressure equals atmospheric pressure, and atmospheric pressure decreases as elevation increases. This means that, if you are climbing a mountain, for example, then as your elevation increases and the atmospheric pressure decreases, so too will the boiling point of water decrease. As a result, a cup of tea brewed on top of mount Everest won’t be nearly as hot and tasty as one brewed in the awesomely beautiful dales of Yorkshire, England, which — by some strange quirk of fate — happens to be the county of my birth (what are the odds?).  

Over to You

Did you already know all the above, or did you learn something new? Either way, I’d love to hear what you think (at the very least you could say something nice about my spiffy thermometer diagram, which took a lot longer to draw than you might think). Also, it would be great if you have any additional nuggets of knowledge or tidbits of trivia regarding temperature in general, Celsius (Centigrade), or Fahrenheit that you’d care to share.

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Mark Rackin

Nicely amusing, Max, as most of your writings are! I started reading science books and magazines soon after my family moved into their first house (a shortish walk from the house to the city library). I was 6 1/2 at that time. By the time I was about 8 or 9, my parents signed a permission slip for me to get full adult status at the library including access to the reference section. That was obviously a major turning point in my education! Since I wasn’t on the mailing list for the international standards bodies (yet), I wasn’t made aware of the change in the “scientific” standard name for the temperature scale from Centigrade to Celsius. Also, most of the science books in the library used Centigrade, so I was quite familiar with that until university when the professors insisted we use the (new-fangled to me) term Celsius. I didn’t know much about the “temperature wars” until today.

Mark Rackin

I forgot to mention that the thermometer diagram is quite aesthetic; unfortunately, even with my best glasses and my 27-inch monitor, I can’t quite make out the text.

Mark Rackin

Works for me!

Mark Rackin

It’s pretty good. Does the error have anything to do with dry ice??? I didn’t do the calculation, but the 40 degree difference seems odd.

Aubrey Kagan

Growing up in the backwoods of Africa, back then we used Fahrenheit Actually everything was the FPS system along with pounds, shillings and pence, slugs, ergs etc.
Most importantly though, if the temperature was over 100F we were supposed to get off school. Never happened! But on one day when the temperature was in the late 90s at assembly the Headmaster said we could take our ties off (school uniform and all that) which we did with gay abandon until we got to Latin. The teacher, Mr Suttle (a most unsubtle gentleman) had not been in assembly and asked about our attire. He took a deep breath and said “I am wearing a tie- you will wear a tie. Put them on!”

Since then measurement regimes have changed and I am so confused I no longer have any idea what relative temperature means. In Canada, temperature is given in Celsius, but for some reason pool temperatures are in Fahrenheit. I never have an idea what to expect when getting into a pool.

Aubrey Kagan

I have a practical example of where Fahrenheit is better than Celsius. National Semiconductor (now part of TI) have two analog parts that measure temperature. The LM34 (http://www.ti.com/lit/ds/symlink/lm34.pdf ) provides an output of 10mV per degree F. The LM35 (http://www.ti.com/lit/ds/symlink/lm35.pdf ) generates an output of 10mV per degree C.

In their simplest configuration (just power and ground- see the data sheets) the LM34 will operate from 5F to 300F. The LM35 will work from 2C to 150C. To go lower than these temperatures the circuit needs a resistor and a negative supply i.e. extra cost of components. Depending on the requirements, if you use the LM34, system will operate from -15C (5F) without the need for the extra components.

Also the dynamic range of the LM34 is nearly double that of the LM35 (9/5) so you may get away without an amplifier ahead of the analog input of the ADC built into your micro.

All it needs is some software conversion.

Aubrey Kagan

To all of you who dare to question Max’s authority, when he said “As a result, a cup of tea brewed on top of mount Everest won’t be nearly as hot and tasty as one brewed in the awesomely beautiful dales of Yorkshire, England, which — by some strange quirk of fate — happens to be the county of my birth (what are the odds?).” I discovered this bit via the BBC…

“Ms Kelly reveals how “strong” Yorkshire tea helped to recreate a replica of the royal christening gown which was first used for the christening of James, Viscount Severn, in 2008.

“To make sure it looked authentic we dyed it in Yorkshire tea (the strongest, as we all know),” Ms Kelly wrote of the piece.”

See “Five royal secrets revealed by the Queen’s dresser Angela Kelly” (https://www.bbc.com/news/uk-50209441)


I have a picture of my father, while teaching Chemistry in Uganda, at the top of Mount Elgon on a school expedition. They dutifully fired up the Primus stove and measured the boiling point of water at the top!

Mike Anderson

An entertaining read, Max. When I grew up, we always referred to it as “centigrade” although we never new why. But, isn’t Celsius now actually defined on the Kelvin scale? I actually saw a dewar of liquid helium at the Naval Research Lab that was 4. That is, 4 degrees Kelvin!. Amazing to think that they could maintain that temp. They actually used liquid nitrogen to help keep the helium from boiling off so quickly.


To add another wrinkle…note that there is no such thing as a “degree Kelvin”. It is simply 4K. The Kelvin is a primary unit. It just so happens that a temperature difference of 1K is the same as a difference of 1 degree Centigrade.

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