BMW in cold weather

Hi can anyone give me advice on my car. It's a BMW and every time the
weather gets colder the car wont start. Once I get a jump start it seems
fine for the rest of the day until it sits for a few hours, then wont start
again.

Say, before I forget, thanks for not stating the model or year or anything. It's always nice when the question is so open ended that any answer will work.
Anyway, if it's a modern car, the battery voltage needs to be very high, sometimes 11 VDC while the engine is cranking so that the electronics can work properly and allow the start. Strangely enough, the starter motor doesn't care so much if the voltage drops to 9VDC while turning becaue it just draws more amps. Therefore, a person can sit there cranking their car over and wondering why it won't start when it's turning so well. A jump start with the correct voltage will work in such cases and once warmed up it takes very little to restart with the old battery.
It would be real interesting to put a meter across the battery as you cranked and see what the voltage dropped down to.