Everyone remember that wonderful conversion from Fahrenheit to Celsius and back again? It went something like this:
F = (9/5)C + 32
So a temperature of 68 degrees Fahrenheit is equal to 68 - 32 = 36 * 5 / 9 = 20 degrees Celsius. Yeah, I know, it usually doesn't work out that neatly each time, but I've got a cure for what ails those of you who travel to Celsius lands and haven't a calculator in your head:
F = 2C + 30
Plug in the 20 degrees Celsius from above and you get 2*20 + 30, which I think most people can handle, and an estimate of 70, close enough to have a feel for what's going on. Fair warning though, the estimator does have its limits. It's equal to the real thing at 10 degrees Celsius, 50 degrees Fahrenheit, and has an error of 6 degrees at 104 and -4 F, so it won't work really well in desert or arctic conditions, and I sure as hell wouldn't cook with it. But in moderate conditions, it's just fine.