When discussing the weather, I am more comfortable discussing the temperature in degrees Celsius (also known by the older name of Centigrade) than in degrees Fahrenheit. As I know that in Britain, many people prefer to use Fahrenheit, I always specify a temprature in both.

Converting from one scale to the other used to be something taught in science classes at school but I meet so many people these days who are igorant of the method for doing it or who have forgotten it. So here is a quick reminder:

If we let F be the temperature in degrees Fahrenheit and C the temperature in degrees Celsius, then we can convert from Celsius to Fahrentheit thus:

Conversion formula: F = ⁹⁄₅C + 32

For example, the boiling point of water is 100°C. In Fahrenheir, therefore, it is:

F = ⁹⁄₅ * 100 + 32 = 180 + 32 = 212°F

At the sort of temperatures we meet in out daily lives, the number of degrees in Fahrenheit is always different from the number of degrees in Celsius. Is it possible, though, for the two ever to be the same or, in other words, is there a temperature at which the number of degrees Fahrenheit is the same as the number of degrees Celsius?

The answer is yes but this poses another question: how do we work out what temperature that is? It’s quite simple, actually, if we use the above formula and a little elementary algebra.

If, as before, we let F = the temperature in Farenheit and C = the temperature in Celsius, then we are looking for a temperature at which:

(1) C = F

Substituting the right-hand side of the conversion formula for F we have:

(2) C = ⁹⁄₅C + 32

Multiply both sides by 5:

(3) 5C = 9C + 160

Subtract 9C from both sides:

(4) -4C = 160

Divide both sides by -4:

(5) C = -40

Therefore both Fahrenheit and Centigrade coincide at a temperature of -40°.

I bet you always wanted to know that! 🙂 Well, now you do know but better still, you know how to prove it!