Understanding weather and what’s happening to the climate requires an understanding of temperature. In turn, understanding temperature requires a definition, a device to record it, a temperature scale, and a standard that works anywhere in the world.
We use a device called a thermometer and one of three scales: Fahrenheit, Celsius, and Kelvin. At least four other scales have been proposed and used historically.
Most of them depend on the freezing and boiling points of water, which differ with altitude and air pressure. Accurate air temperature measurement, then, requires a very accurate thermometer and an international standard for interpreting the reading.
Let’s define temperature and look at a little history.
What is temperature?
We understand temperature as the degree of hotness or coldness. That, in turn, depends on the vibration of atoms and molecules. Molecules constantly vibrate, and the faster they vibrate, the hotter the temperature.
In physics, everything influences everything else, so such aspects as atmospheric pressure and volume also affect temperature.
A brief history of the thermometer
The ancient Greeks may have had something like a thermometer, but for practical purposes, we can begin with Galileo’s invention of the thermoscope in the early 17th century. It involved suspending a glass tube in a liquid and noticing how the liquid rose and fell within it. It could indicate that something was hotter than something else, but it lacked a scale for measuring it.
Ferdinand II, Grand Duke of Tuscany, improved the thermoscope when he sealed liquid in a bulb at its base. Understanding temperature required being able to measure it and assign numbers to it.
The earliest thermometers amounted to Ferdinand’s thermoscope with some kind of scale that could assign numbers to temperature. Thermometer makers used a variety of liquids, including alcohol and mercury.
We still use liquid thermometers, but technology has given us other options.
John Harrison, a clockmaker, invented the dial thermometer in 1759. It uses strips of two different metals bonded together in a coil. They respond differently to changes in temperature.
One end of the coil is anchored in place. Because the metals expand and contract differently, the coil straightens out or coils tighter as the temperature changes. The other end attached to a needle, which points to numbers on the dial.
Digital thermometers use a resistor called a thermistor. An electric current flows through it. As temperature changes, the resistance changes. Therefore, it displays temperature based on measurement of the strength of the current. Digital thermometers can be as simple as the kind you buy in a drug store to take your body temperature or as sophisticated as those used in weather stations.
Each manufacturer of the earliest thermometers devised his own scale, probably based on the hottest and coldest days of the year at the place of manufacture. They could give a reading, but no two thermometers measured temperature the same way. It was impossible to compare temperature readings from different thermometers. Understanding temperature still had a way to go.
Isaac Newton worked for the Royal Mint. He needed to understand the melting point of the various metals he worked with. For his own purposes, in about 1700, he established 0 degrees as the melting point of water. His scale had no upper fixed point. He arbitrarily called the boiling point of water 33 degrees.
Newton’s linseed oil thermometer could measure no higher temperature than the melting point of tin. To estimate higher temperatures, he heated iron until it turned red hot. He put various metals on it until they melted, then he exposed it to wind to cool it off. The time it took the various metals to solidify enabled him to determine degrees of heat. Determining the melting point of tin with both methods enabled him to use the same scale.
Newton apparently devised his scale for his own purposes and did not attempt to establish a standard.
In about 1701, Danish astronomer Ole Christensen Rømer developed the first practical temperature scale. He established two fixed points that any lab could duplicate. A slurry of ice, salt, and water provided the lowest easily achievable temperature. Rømer called that 0 degrees. He called the boiling point of unsalted water 60 degrees. On the Rømer scale, the freezing point of unsalted water is 7.5 degrees.
Any two thermometers calibrated to this scale would provide the same degrees of temperatures at any point in between. Understanding temperature became a practical possibility.
Daniel Fahrenheit, who was born in Poland and spent much of his life in Amsterdam, visited Rømer and learned of his scale. He thought that increasing the number of degrees between the salt-water slurry and the boiling point would enable more precise measurement of temperature. It would also get rid of the fraction that, in Rømer, expressed such an important point as when water freezes.
Fahrenheit introduced his scale in 1724. It uses three fixed points. Like Rømer, he started with a slurry of salt, ice, and water. He defined the freezing point of water as the 32nd degree. Tripling that, he arrived at 96 degrees for human body temperature.
After Fahrenheit’s death, other scientists modified his scale. They added 180 degrees to Fahrenheit’s freezing point of water to arrive at 212º for the boiling point of water. In this form, the Fahrenheit scale eventually found worldwide acceptance.
In 1730, French entomologist René de Réaumur established a scale that defined the freezing point of water as 0º and its boiling point as 80º. This scale found worldwide acceptance somewhat earlier than Fahrenheit’s and remained in use into the 20th century. Some candy makers and dairies still use it.
French astronomer Joseph-Nicolas Delisle spent many years in Russia at the invitation of Tsar Peter the Great. He built a mercury thermometer in 1732. Unlike any of the other scales, he defined 0º as the boiling point of water and measured how the mercury contracted as it cooled. His original scale had 2,400 gradations.
Josias Weitbrecht recalibrated Delisle’s scale in 1738. He defined the freezing point of water as a second fixed point, 150º D. In this form, the Russians used the Delisle scale for about a century.
Weitbrecht sent his version of the Delisle scale to Swedish astronomer Anders Celsius. Celsius redefined the freezing point of water as 100º. He also published an influential paper that described his ideas.
Not long after his death, other scientists turned his scale around. With freezing defined as 0º and boiling as 100º, they had defined the familiar centigrade scale, which was renamed the Celsius scale only in 1948.
The Réaumur scale was well entrenched in France until the French Revolution. The government tried to convert everything to decimal scales, to the point of trying to replace weeks of seven days with decades of ten days. It naturally adopted the centigrade scale because 100 degrees seemed more logical than 80.
Most of the Revolution’s true innovations have fallen by the wayside. Centigrade, however, found worldwide acceptance and displaced all other scales except Fahrenheit.
Most of these scales serve everyday purposes well enough. Scholarship found them difficult. They could easily enough express temperature in the scale in use in their country. But to read a paper published elsewhere, they had to calculate how to convert from one temperature scale to another.
Also, no one of them is truly adequate for science. They all arbitrarily chose to use water for their scales because it was convenient. Among other problems, the boiling point of water is not a fixed temperature. It varies with altitude and atmospheric pressure.
The suggestion of a truly fixed point had come in the late 17th century. French physicist Guillaume Amontons studied the relationship between temperature and pressure in gases. He found that pressure and temperature rise and fall together. That fact suggested to him that there must be a temperature so cold that air would have no pressure at all. It would be impossible to achieve a lower temperature than that.
Amontons’ insight led to the concept of absolute zero. In 1848, William Lord Kelvin established absolute zero as -273º C. He proposed a new temperature scale where 0º meant absolute zero. Higher temperature would rise in increments equal to 1º C. But scientists who use the scale do not measure in degrees but Kelvins. Water freezes at 273 K and boils at 373 K.
The Kelvin scale, indispensable to science, is not practical for day-to-day use.
Scottish physicist William John Macquorn Rankine proposed another scale that starts at absolute zero. But instead of rising in increments according to Celsius, his scale rises in Fahrenheit degrees. Absolute zero equals -459.67º F.
Scientists who use the Rankine scale likewise express temperature in Rankines, not degrees on the Rankine scale. Water, then, freezes at about 491.67 Rankines or 491.67 R. But since the names of three of the men who devised temperature scales begin with the letter R, Rankine is sometimes abbreviated Ra. Water boils at 671.67 Ra.
If you want to convert a temperature from any one scale to all of the others, check out this handy calculator:
The problem with thermometers for weather and climate
According to an old saying, if you have a clock, you know what time it is. If you have two clocks, you can never be sure. It’s the same with thermometers.
One winter, I stood on a street corner in Iowa City, Iowa, where two banks had signs that displayed time and temperature. One showed both Fahrenheit and Celsius: 32º F and something other than 0º C. The other showed only Fahrenheit but something other than 32º. Two thermometers displaying three temperatures!
Neither thermometer expressed the official temperature in Iowa City that day. Official temperatures require not only a more accurate thermometer but a temperature standard.
International temperature standards
The first attempt at a standardized mercury-in-glass thermometer came in 1878. It was intended by the Bureau International des Poids et Mesures (BIPM, International Bureau of Weights and Measures) to correct for thermal expansion of the bar used as the official meter. Therefore, it had to be accurate within a few thousandths of one degree centigrade.
By that time, historical records in locations all over the world documented daily air and ocean temperatures. They necessarily used thermometers no more accurate than those at the Iowa City banks.
All of the pre-Kelvin temperature scales were based on phase transitions as fixed points that defined temperatures between them and could be used to define temperatures above or below them. But, as I showed earlier, the boiling point of water turns out not to be a fixed point. Absolute zero as a fixed point is of no use for meteorologists or climate scientists.
The first international temperature standard was based on constant gas volume thermometry and defined by the first Conférence Générale des Poids et Mesures (CGPM, General Conference on Weights and Measures) in 1889.
Hugh Longbourne Callendar and Ernest Howard Griffiths developed a platinum resistance thermometer, which is more accurate than the best mercury thermometer. In 1913, the fifth CGPM proposed development of a thermodynamic International Temperature Scale (ITS). The First World War interrupted its development, but the standard was eventually adopted by the seventh CGPM in 1927.
The ITS has been revised twice since then. Each time, the BIPM published transformation tables to convert between the old and revised standards. The calculations require complicated mathematics.
It follows that it is impossible to convert pre-1927 weather and climate records to any international standard. Fortunately, understanding temperature does not require such exactitude. Scientists have attempted to find useful and reasonably accurate approximations.
Daniel Fahrenheit, Anders Celsius left their marks / Ned Rozell, University of Alaska at Fairbanks Geophysical Institute. December 24, 1996
History of temperature scales and their impact on the climate trends / Peter Pavlásek, Variable Variability. 
Kelvin: history / National Institutes of Standards and Technology. May 14, 2018, updated September 16, 2022
Weather thermometers: what they are and how they work / Wessel Wessels, Own Your Weather.