One day the weather person on TV said one can get F from C by "doubling the number and adding 30."
This turns out to be an easy and reasonably accurate approximation for temperatures around 10°C, because the 30 offset results in an F approximation within ±1 of actual °F for the range 5-15°C. More generally,
- 32 for -5 - 5°C
- 31 for 0-10°C
- 30 for 5-15°C
- 29 for 10-20°C
- 28 for 15-25°C
offsets results in ±1°F approximations.