An altimeter is an instrument used in an airplane to tell the pilot how high above sea level he is flying. All dry land has to be above the level of the sea, or it would be covered with water, but flat land near the ocean may be only a few feet or a few hundred feet above sea level, while mountains are thousands of feet above sea level. An airplane pilot must know how high he is. Otherwise, he might hit a mountainside at night or in cloudy weather. The altimeter used in most airplanes is a kind of barometer. This is an instrument used to measure air pressure.
The article on AIR tells how the weight of the air presses down on the ground, so that at sea level the pressure of the air is more than fourteen pounds on every square inch of the earth. The higher you go into the air, the lower the air pressure is. An aneroid barometer is a device to tell what the air pressure is. (See the article on BAROMETER.) An altimeter uses this air pressure to show how high the plane is. But the altimeter does not show how high the plane is above the ground.
A plane may be flying at an altitude of 15,000 feet, but it would be only a thousand feet or so above the ground if the land in that district happened to be 14,000 feet above sea level, as much of the mountainous part of the United States , in the region of the Rocky Mountains, may be. Therefore a pilot is always careful to fly higher than any land or mountains that can be in the territory over which he is passing.