Have you guys ever noticed why airplane sound low at night!

Aeroplanes make sound all the time! The intensity of the sound vary with temperature. During day time the static temperature of the atmosphere is more than that of the night. The air molecules are at a higher temperature at the day time, and hence posses high energy and can vibrate faster. Thus the sound can reach more distance in the day time than at the night. But sound will be produced even at the night time.