above 20°: It’s fairly obvious which of Fahrenheit and Celsius one is talking about, and one doesn’t really need to specify the temperature scale. Raise the border a little higher when in a desert.
between 20° and -30°: It may be unclear which scale one is referring to, so one might want to specify whether one’s talking in Fahrenheit or Celsius when talking about temperatures.
between -30° and -50°: Fahrenheit and Celsius are close enough that one doesn’t really need to specify the temperature scale.
below -50°: One should probably move somewhere more liveable.
(above 200°: someone wants to use Kelvins for temperature without realizing that Kelvins aren’t degrees)
This lack of confusion in some areas can be fixed by reporting your temperatures in Réaumur.