Measuring Air Pressure

Air pressure is measured using either a mercury barometer or an aneroid barometer.

Like a thermometer, a mercury barometer contains mercury that rises and falls to indicate rising (high) or falling (low) barometric pressure. Specifically, a standard mercury barometer has a glass column about 30 inches long that is closed at one end. The closed end contains a quantity of mercury. The mercury rises in the tube as the air pressure at the opening of the tube forces mercury out of the reservoir. The mercury moves into a column beside which are marked measurements in either inches of mercury or millibars. High pressure forces the mercury higher in the column; and low pressure allows the mercury to drop to a lower level in the column.

An aneroid barometer, such as the image to the right, uses a small, flexible metal box called an aneroid cell. The box is tightly sealed after some of the air is removed, so that small changes in external air pressure cause the cell to expand or contract. This movement is linked to the needle(s) on the face of the barometer which are calibrated to a labeled dial that lists forecast information as well as the pressure reading.

Even if you do not have a barometer at home, you can usually find the daily barometer reading in the weather section of your local newspaper. The barometer information should list the time of the reading, as well as the reading itself and whether the barometer was rising or falling.