Barometer, a device for measuring atmospheric pressure (the pressure resulting from the weight of the air above a given point). Because a change in atmospheric pressure is a sign of changing weather, this instrument is one of the chief tools of the weather forecaster. A falling" barometer, indicating reduced pressure, shows that a low-pressure mass of air is approaching. Such low-pressure systems generally bring storms. A rising" barometer generally means an approaching high-pressure air mass and fairer weather.

Another use of a barometer is as an altimeter, a device that measures heights. As altitude increases, atmospheric pressure decreases because there is less air to press down. A barometer used as an altimeter has a scale marked in feet or meters above sea level.

Two forms of barometers in common use are the mercury barometer and the aneroid barometer.

Mercury Barometer

The mercury barometer consists of a hollow glass tube, about three feet (0.9 m) long, closed at one end and filled with mercury. The open end is placed in an open vessel of mercury called a cistern. The mercury column in the tube will drop until the weight (pressure) of the air pushing down on the mercury in the cistern is equal to the weight of the mercury remaining in the tube. The higher the atmospheric pressure, the higher the mercury will stand in the tube. Similarly, the lower the atmospheric pressure, the lower the mercury will stand.

A change in the height of the mercury, or barometric column, therefore, indicates a corresponding change in the weight of the atmosphere. These changes may be measured by means of markings on the tube. The average height of the barometric column at sea level is 29.92 inches (760 mm). This height indicates a pressure of one atmosphere, or about 14.7 pounds per square inch (101,300 pascals). (On weather maps, barometric pressure is usually expressed in millibars; one atmosphere is equal to about 1,013 millibars.)

The mercury barometer was invented in 1643 by Evangelista Torricelli, an Italian physicist. It is used mainly in laboratories and weather stations.

Aneroid Barometer

A more rugged and easily portable instrument is the aneroid, or dry, barometer. While not as accurate as the mercury type, the aneroid barometer will show much smaller changes in atmospheric pressure.

The aneroid barometer contains a cylindrical container with a flexible top. Air is partially removed from inside the container, and the top, or diaphragm, flexes with each change of atmospheric pressure. Delicate levers transmit these movements to a pointer that moves over a graduated scale.

Aneroid barometers with altitude scales are commonly used for aircraft altimeters. In determining altitude, adjustments must be made to account for changes in air pressure due to the weather. Barometers sold for home use are generally of the aneroid type.