How to Teach Celsius (with Implications for the Teaching of other Metric and Metric-related Units)

Paul S. Boyer, Professor of Geology, Fairleigh Dickinson University

The article by Robert A. Warren ("Algorithms for Mental Conversion between the Fahrenheit and Celsius Scales," The Citizen Scientist, 9 December 2006) gives a handy way of converting between Celsius and Fahrenheit temperature scales.  However, I do not favor learning such conversions. They are largely a waste of time and mental effort, and may delay the use of the preferred conventional scale for international communication of meteorological data.

My position is based on several observations. First, experience shows that the public does not switch to metric or related units (such as the Celsius scale) by making conversions back to the Fahrenheit scale. The public does best when the information is no longer presented in the old units. As with quitting smoking or any other bad habit, "cold turkey" is the best method.

People naturally revert to the familiar, which is not always a bad thing; but it makes it difficult if we wish to learn new ways. This is my experience with students, who frequently ask for conversions whenever I cite values in metric units. As a teacher interested in their welfare, and having an interest in the future of science in this country, I generally refuse to give equivalents. I tell then that they can always look up conversion-factors somewhere (such as in the Handbook of Chemistry and Physics, or on the Internet); but it is not useful to be back-converting from metric to old ("customary") units. 

Conversion the other way (into metric units) should be handled as much as possible by computers. Humans should not worry themselves with conversion formulas and calculations, beyond programming a spreadsheet to do the task automatically.

Instead, students should merely acquaint themselves with the meaning of simple metric units, so that they know them as they know any other units. After all, most people do not know the length of a foot with any great accuracy; nor do they learn more about the meaning of that unit by converting it into some other unit, about which they also have little direct understanding.

The best way to learn what a meter means is to handle a meter stick. Measure a few things with it. Measure the length of a cubit: the distance from your elbow to the tip of your hand. (My cubit is rather close to a half-meter.)  How high is your waste from the floor? (Mine is just about one meter.) How tall are you in meters?

By the same approach, it is not ever really necessary to teach how to convert Fahrenheit to Celsius.  It is better far just to use Celsius. I tell students in meteorology to begin just by remembering three temperatures: 0°C, 20°C, and 37°C.  The first is freezing; the second is a typical room temperature; and the third is normal human body temperature. From these you can relate all weather temperatures.  People who learn this way can understand Celsius air temperatures almost immediately, with no formulas, and no tricks. It is the natural way we judge things in life: by simple comparison.

Readers who wish to comment on this article and Robert Warren's how-to article on Fahrenheit-Celsius conversions may send comments to Backscatter. Editor.