2002 BMW 745 air conditioning

I own a 745i BMW, when I turn my car on the outside temperature reads (-16F or below). The air conditioner does not start working until it reaches (+15F or Better) during motion. There is never an accurate outside reading it always reads under the temperature outside, mind you it's been over 90 degrees lately. By the way as the temperature rises on the reading the colder the air conditioner gets which leads me to believe its not freon. Please help