Listen to Those Rails A-Thrumming All Aboard
To convert from Fahrenheit to Celsius and back again is often a pain in the neck. Do you multiply by 5/9 first, and then subtract 32? The other way around? Add 32? How does it go, again?
Most people know that water freezes at 32˚F / 0˚C, and boils at 212˚F / 100˚C (at standard pressure).Why was 212˚F chosen as the boiling point? Well, freezing and boiling are in some sense opposite, and they are separated by 180˚…
This information is enough to convert. But most people don’t want to solve two equations in two unknowns on the fly. So people make up rules of thumb.
A rule of thumb that isn’t so bad at temperatures near freezing is that each degree Fahrenheit is half a degree Celsius. It is only off by 1 part in 18 so that it’s off by only 1˚C at 50˚F. But the error gets worse as you get further from freezing.
Luckily, I know another rule of thumb that stays within 1˚C of the true value. It has to do with the 6 train, also called the Lexington Avenue local train, in New York City. The rule is this: the stop at 33rd street is 0˚C, each stop (thought of as ˚F) farther up town is another 5˚C.
That is:
Stop | Exact Conversion | Rule of thumb |
---|---|---|
[Street or ˚F] | [˚C] | [˚C] |
33rd | 0.555 | 0 |
42nd | 5.555 | 5 |
51st | 10.555 | 10 |
59th | 15 | 15 |
68th | 20 | 20 |
77th | 25 | 25 |
86th | 30 | 30 |
96th | 35.555 | 35 |
103rd | 39.444 | 40 |
which is good to 5/9˚C all the way through 103rd street. After that, it sort of… goes off the rails.