Today I'm walking around in a T-Shirt when everyone has some kind of thin coats on, but when I sit and idle for an hour, or get hungry, I cool down considerably.
My wife is extremely resistant to heat, but this disadvantages her in normal and cold weather severely. Her baseline is 22-23 degrees C to live comfortably, with long sleeved warm clothes.
Also, temperature doesn't always indicate cold's penetration power. If the air is dry, -18 degrees C is survivable with a mid-thickness polar coat. If it's humid, try to survive 2-3 degrees C with a down / wind-resistant coat. It won't be easy.