By Allison Kubo Hutchison
Scientists have to know how to speak the languages of many units. Improper unit conversions have caused much heartache and suffering in the past, including the loss of a $125 million dollar Mars orbiter. In general, peer-reviewed science journals only accept units that are laid out in the International System of Units (SI). SI lays out seven base units that other derived units are based on. The base unit for temperature is Kelvin (K) which was first laid out by William Thomson also known as Lord Kelvin in On the Absolute Thermometric Scale in 1848. Thomson wanted a scale that started at “infinite cold” or absolute zero where molecules have a minimum vibrational motion which is theoretically the lowest possible temperature. The increments of Kelvin were set to be equal to the increments of Celcius (°C) a system which defined 0 °C to be the freezing point of water and 100 °C. Celsius had been laid out almost 100 years before Kelvin in 1742.
Celsius is widely used in science and across the world in all countries except the USA, the Bahamas, Belize, the Cayman Islands, and Liberia. Celsius and Kelvin are the recognized SI units for temperature. It seems the logical choice for science. But is it actually the best for humans?
Let us consider Fahrenheit. Fahrenheit (°F) was actually proposed before both Celcius and Kelvin in 1724 by Daniel Gabriel Fahrenheit. Fahrenheit ( the scientist not the unit system) designed the first mercury thermometer which was a significant advance over previous thermometer designs since it was more sensitive to temperature changes. He designed a new temperature system to reflect increased sensitivity. He set 0 °F as the lowest temperature he measured in his hometown of Danzig, Poland. And the second point he set as 96 °F the approximate temperature of the human body. It was later redefined with 32 °F as the freezing point of water and 212 °F as the boiling point. A 1 °F change in temperature corresponds to a 0.0001% increase in the volume of mercury. These numbers may seem arbitrary but Fahrenheit was building the most precise thermometer of his time. These numbers seem much less convenient than Celcius’ 0 to 100. But looking closer they are actually more representative in terms of what humans experience.
Firstly, there is a finer gradation in °F than °C. Between the freezing and boiling points, there are 180 steps in °F and only 100 steps in °C. This allows us to pinpoint smaller changes in temperature without having to use decimal points. The truth is that many adults and most children are uncomfortable with decimal numbers. In one study, 40% of participants exhibited poor comprehension of decimal numbers and this test was conducted on doctors, nurses, and laboratory scientists (Sinnot et al., 2013). It has been shown that humans prefer whole numbers to decimals or fractions (Alonso-Diaz et al., 2018). It is very natural to prefer whole numbers since the majority of our counting occurs in whole numbers whether it is shopping at the grocery store or counting sheep. More precision in scale also allows for finer differentiation between temperatures in whole numbers.
Secondly, the range of temperatures we see in everyday life is actually more manageable in Fahrenheit. The main use of temperatures for the majority of people in evaluating the weather. Although 100 °C is an important number for science, we do not experience air temperatures even close to that on Earth. With its preponderance of liquid water and ice, a simple range that spans both liquid and solid water is useful to us. Let us examine the city of Vancouver, Canada. Here we see a histogram of daily temperatures that span the range of -17 to 27 °C while the range in Fahrenheit is 18 to 80 °F. The main argument in favor of Celsius is that it spans 0-100. However, a range of 0°F to 100°F captures more observed air temperatures than the range 0°C to 100°C. The temperature ranges that are important to human life and weather are captured better in Fahrenheit.
Finally, in the face of the global climate crisis perhaps Fahrenheit is a useful unit to communicate rising temperatures to non-scientific audiences and children. Seemingly small changes in temperature can have significant climate effects. Expressing those changes in an accessible and easily relatable way is important to mobilize the public. A change of 5 °C is 9 °F which is equivalent but may seem larger to some. And it is easier to relate that change to how it would affect everyday life in regards to Fahrenheit which is a better measure of air temperature.
Ultimately, units are all equally arbitrary. They are just a way that we try to measure the world around us. It is equivalent to say 70°F, 21.1°C, and 294.1 K. For some, it may be more comfortable to say 70 °F while others prefer 21.1°C. All three have their uses and Fahrenheit is a useful unit for measuring temperature in regards to the human experience.