I found that there is a very simple way to convert from C to F and get a fairly good "ish" idea of the temperature by doubling the number and adding 30 to give you a very rough approximation of what it is in Fahrenheit.
So, 10C would roughly be 50F, 20C would be 70F (where actually in an accurate conversion it is 68F). It is not exact but it's close enough to know how to dress.
Below zero it still approximates somewhat OK. If it's -10 C, you double it (-20) add 30 and you get 10F ... in an accurate conversion it would be 14F. Again close enough.