I left my birth control in car over night in 30f degree weather. will the cold 30 degree weather make birth control less effective?

Still effective...: A night in the car at 30 degrees won't make your birth-controll pills less effective. Most pill makers state that the pill needs to be kept between 68 & 77 degrees. They won't "guarantee" a pill's integrity outside this range. In reality its heat that will denature a pill rather than short exposure to cold such as you described. Best wishes!