Most days, the radio transmitter from my iPhone to my car stereo works perfectly with no interference from outside signals. Other days, it’s highly static-y and sometimes I even get interfering stations, particularly when driving over high bridges or under large pieces of conductive metal like road signs. It seems to happen more on clear days than cloudy days. Why is this?
Radio waves are more sensitive to the air than most people think. Humidity, thunderstorms, even night and day can make a difference (not sure if this applies to FM band).