Barometers and altimeters perform similar basic functions that are used in different applications. Both instruments measure pressure in the atmosphere, with an altimeter taking that information and performing additional mechanical calculations to display altitude for aircraft.
A barometer measures and displays atmospheric pressure. Typically this is measured in inches of mercury equivalent, and a standard pressure of 29.92 inches is a regular baseline for comparison.
Barometers are consulted in consideration of weather trends. Higher pressure readings indicate good weather; lower pressure readings, poor weather.
An altimeter is an instrument used in aircraft to measure the height above sea level. Incorporated into an altimeter is a pressure-sensing function that works similar to a barometer. The altimeter measures altitude through comparison of air pressure as an aircraft climbs or descends.
While a barometer and an altimeter both measure pressure, an altimeter is further developed to compare barometric (atmospheric pressure) levels and represent the changes as a change in altitude.
An altimeter will typically also have a pressure window in its face (referred to as a Kollsman window). An altimeter can be used as a barometer if it is on the ground by setting the altitude on the altimeter to known field elevations (above sea level) and reading the pressure indication in this window.
