A barometer is a weather tool that reads air pressure. A change in air pressure causes wind and rain, so a barometer can help forecast changes in the weather. There are two types of barometers -- aneroid or digital. An aneroid barometer includes an airtight cell with needles moving up or down to indicate changes in pressure. A digital barometer displays a graph that shows changes in air pressure. Reading barometric pressure is not as much about measuring the pressure, but rather measuring a change in pressure over time.
Read your barometer at the same time each day. Diurnal pressure changes occur twice a day due to a solar-induced atmospheric tide, so reading at the same time each day will ensure an accurate account of any pressure changes.
Set your barometer by moving the setting hand directly over the measuring hand. A barometer includes two hands. The one on the inside is called the measuring hand, and the one on the outside is called the setting hand.
Determine whether pressure is remaining steady, rising or falling by checking the measuring hand. If it moved to the right of the setting hand pressure is rising. A leftward move means pressure is falling. In general, dropping pressure means a greater chance of precipitation, where rising pressure means a lesser chance.
Read the barometer's dial, which typically expresses mercury in millimetres of mercury (mm) and millibars (mb) or hectopascals (hPa). Standard air pressure at sea level is 760 mm (29.92 inches) of mercury. A mercury reading higher than 767 mm (30.20 inches), with rising or steady pressure generally indicates fair weather, becoming cloudy and warmer with rapidly falling pressure. A mercury reading from 757 mm (29.80 inches) to 767 mm (30.20 inches) with rapidly falling pressure means precipitation is coming. A mercury reading under 757 mm (29.80 inches) with rising or steady pressure means it will start to clear and become cooler, with slowly falling pressure it is an indication of precipitation, and rapidly falling pressure would indicate a storm.