A barometer is used to measure atmospheric pressure. Atmospheric pressure is a measure of the amount of force air exerts onto the earth as it is pushed down from the atmosphere. Another term for atmospheric pressure is barometric pressure.
Air pressure is a reliable weather predictor. Lower air pressure is usually an indication of rain. Likewise, the higher the air pressure, the more likely the chance of sun. Since lower elevations tend to have higher atmospheric pressures while higher elevations have lower ones, barometers must be calibrated specifically for the elevations in which they're located. Although mercury was originally used as the weather indicator in barometers, it has been almost completely replaced by aneroid barometers due to the highly toxic properties of mercury.