Extreme cold and hair transplant. I am 4 months post-op.

Extreme temperatures should not affect your surgical results unless you are  exposing your body to the cold to cause harm to other parts of your body.

After 4 months post op everything is safe , you don't have any risk with extreme cold or extreme hot weather. 

Skin directly exposed to temperatures of -30 to -40 will be at risk of frost nip and frostbite. If freshly transplanted hairs were directly exposed to these sorts of extremely cold temperatures in the first few days it may have an impact on their survival as the blood supply to the skin is significantly reduced. In the 4th month post op, the transplants hairs will be at no greater risk than the natural hairs.