In its purest form, an altimeter is simply an absolute pressure gauge. This means that it is displaying the pressure being exerted by the atmosphere at its current location.
The earth is surrounded by an atmosphere. This atmosphere is the air that we breathe. The atmosphere is held in place by the earth’s gravity. The atmosphere has a specific weight. The weight of the atmosphere is approximately 14 pounds of weight for every square inch of earth when measured at sea level on an average day.
An accurate method of measuring this weight is to use a barometer. A barometer is a reservoir filled with mercury. The reservoir has two openings; one opening is exposed to the atmosphere and the other empties into a glass tube. The atmosphere pushes down on the mercury within the reservoir causing the mercury to fill up the glass tube. How far the mercury goes up into the glass tube is directly proportional to the weight of the atmosphere pushing it. This is why barometric pressure is normally expressed in terms of “Inches of Mercury (InHg)”.
At sea level, on an average day, the barometric pressure is 29.92 InHg. However, this will vary constantly depending on the weather. Stormy weather tends to pull the atmosphere away from the earth’s surface causing lower pressure. Hot, dry weather pushes the atmosphere down causing higher pressure.
The weight of the atmosphere also changes depending on altitude. The closer to sea level that you are, the more air there will be, consequently the atmosphere will weigh more. As you go higher in altitude, the less dense the atmosphere will be, therefore less weight or pressure is exerted. An altimeter measures this change in atmospheric weight as expressed in terms of pressure or feet of altitude.