Airplanes can find their altitude by using air pressure. The air gets thinner the higher it is (and it has less pressure, because there's less air pushing down on it from above). This pressure can be measured and this can be converted into a height above sea level. The instrument for measuring altitude is the altimeter (from "altitude" and "meter"). The common pressure altimeter is a type of barometer with a dial showing distance (feet or metres) instead of atmospheric pressure.