The proper way to read a mercury or aneroid barometer is to calibrate it for the altitude at your location, adjust the set needle to align with the pointer, and then observe the degree and direction of the pointer relative to the set needle to track change in the barometric pressure. Clockwise movement indicates increasing pressure and improving weather, while counter-clockwise movement indicates decreasing pressure and worsening weather.
To calibrate a barometer, use a small screwdriver to adjust a small screw, typically located on the back of the barometer, until the pointer indicates the current barometric pressure. To determine the current barometric pressure, type your ZIP code in the search box near the top-left corner of the home page at Weather.gov. It is best to perform the calculation outdoors in a spot sheltered from wind. After calibrating the barometer, manually adjust the set needle so that it aligns with the pointer. Periodically observe how the pointer has moved relative to the set needle to determine whether the ambient barometric pressure is increasing or decreasing.
Modern electronic barometers are calibrated at the factory, and then internal sensors recalibrate automatically for changes in altitude as the unit ships to a different location. These barometers often include a graphical display of recent changes in barometric pressure.