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.  

Thermoception

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.