Atmospheric pressure can be described as the weight exerted by air at a certain level. Barometers usually measure the force applied by atmosphere per unit area. Thus, the units used include millibars, inches of mercury and most commonly here in America; pounds per square inch which is expressed as Ib/in2.
Pressure changes under the influence of some parameters, they include altitude or height above sea level and temperature. As we climb up, the force exerted by the atmosphere reduces. At lower altitudes, for every change of one inch of a mercury barometer, there is a change of one thousand feet in altitude. On higher altitudes, the rate of pressure reduction decreases. Like any other matter, air expands upon heating and contracts on cooling. This means that density of air changes as we move from one region to another. From that, it can be concluded that, pressure is not usually the same at a constant altitude.
The altimeter is an onboard aircraft instrument, which is essentially an aneroid barometer, used to measure the plane’s altitude. The altimeter’s scale has been changed to indicate height rather than pressure. In a way, it makes sense since pressure changes with height. On the other hand, temperature changes the air pressure; therefore, altimeters mostly indicate an altitude different from the true altitude. Ideally, the higher we go, the lesser the air pressure; thus, altimeters will indicate high altitudes if they detect a drop in pressure.
Warm air has a lower density than cold air. The thicker the air, the more pressure will be gained as we move upwards. Thus at the same altitude, barometers in cold air will indicate higher pressure than the ones in hot air. As we fly an aircraft from a warm area to a cold area maintaining the same altitude, the atmospheric pressure will drop. This drop in pressure will be interpreted by an altimeter as a rise in altitude. The indicated altitude will be different from the true altitude. On a flat landscape, if the indicated altitude is lower than the true altitude, there is no much problem. On the other hand if a pilot flies in a mountainous region on a cold weather, he should consider flying higher since the altimeter will give a higher altitude reading than the true altitude.
The altimeter can be adjusted to indicate true altitude at various altitudes. Taking off and landing of the planes are the most crucial periods in a flight. Thus, the altitude of an airport is the most meaningful altitude to a pilot. It is advisable for the pilot to make sure that his altimeter reading is at per with other plane nearby. The pilot should regularly set the altimeter in accordance with the nearest tower.
It is not possible, with the current technology, for a pilot to determine the average temperature of the column of air below him. If the technology was available, it could have been the perfect way of determining true altitudes. However, plane computers are able to measure temperatures of air near the plane and estimated the true altitude. The altitude might not be correct but it is usually near the true altitude. By considering this calculated altitude and the one of the control tower, the pilot can calculate altitudes virtually close to true altitude.