Altitude
At high altitude, atmospheric pressure is lower than that at sea level. This is because of two effects. These effects are gravity and the heat in the air. Gravity makes the air to be as close as possible to the ground. The amount of heat in the air makes the molecules move quickly and push against each other. This makes the air expand.  So most of the air will be in the lower atmosphere, closer to sea level, and all the air pushing against each other is called air pressure.  So air pressure is less the higher you go.