There are so many ways to know that it is -25c or -13f.

1) If your car was not plugged in over night, you are not going to work in the morning.
2) When you get up in the morning and look out the window to see what kind of day it is and you see someone walking down the sidewalk. Unfortunately you can not tell if that person is male or female. They are dressed for function not fashion. After all the layers it becomes that cold androgynous look.
3) There are sun dogs around the sun
4) The air sparkles. The moisture in the air freezes and reflects the sun and makes the air sparkle.
5) There are more ads on the radio for vacations to Southern destinations.

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"In my sex fantasy, nobody ever loves me for my mind."
Nora Ephron

“Clinton lied. A man might forget where he parks or where he lives, but he never forgets oral sex, no matter how bad it is.”
Barbara Bush

"You know that look women get when they want sex? Me neither!"
Drew Carey