First off, I basically agree with Chris Sunami's answer; the worldbuilding is primarily for you as the author or storyteller, not for the reader.
However, and I know that this is repeating an old cliché; do the specifics truly matter? (Chekhov's gun applies.) Does it make a great deal of difference if the temperature is -20°C or -25°C? Really, show, don't tell!
Let me give you an example.
I have a story that I work on from time to time; basically, this is a hobby project for when the mood strikes. One scene that I really like in that one has the main, point-of-view character dealing with the cold, in a stiff wind and snowfall. The temperature is far below freezing even before you account for windchill. However, nowhere is the temperature explicitly stated. The POV character just has no way to know what the temperature is (only that it is cold). What is described is the effects that the character can observe. How their glasses fog over when going inside from having been outside. The crystals of frozen water vapor in their face from their breathing. The sound of the snow on the ground as they walk. That sort of things. If you've ever been in serious or even moderately cold weather, then you'll be able to put two and two together; if you haven't, then stating that the temperature is -25°C (or whatever) will be just a number anyway, and it won't add appreciably to the reader's experience of the story except perhaps to convey the notion that "okay, that's pretty cold".
Of course, if the specific temperature somehow does matter, or if it is relevant before the character can experience it directly, then it makes more sense to mention it. Maybe your character is looking at the outside thermometer and contemplating on how it is -25°C and hoping that the power doesn't go out just before the lights start to flicker; or just how glad they are to be able to stay indoors in such temperatures even if the sky is clear, the sun is shining, and the weather is calm, a moment before someone gets hit by a skidding car on the otherwise empty street before it takes off leaving your dear protagonist to rush out and take care of the pedestrian.