barometry

Barometry is the measurement of atmospheric pressure using instruments called barometers. The term combines the Greek words for weight and measure. Barometry is essential in meteorology, aviation, and geophysics, providing data that help forecast weather, calibrate instruments, and determine altitude.

The mercury barometer, invented by Evangelista Torricelli in 1643, is the classic example. It consists of a glass tube filled with mercury inverted into a mercury reservoir. Atmospheric pressure supports a column of mercury, so the height of the column (measured in millimeters or inches) is proportional to the pressure. The space above the mercury is a vacuum, allowing the column to be sustained. Variations in temperature and mercury density can affect readings, and mercury barometers require careful handling.

An alternative is the aneroid barometer, which uses a sealed flexible metal chamber that expands or contracts with changes in pressure. The movement is translated to a pointer or digital readout without a liquid column. Modern barometers are often electronic or digital, employing capacitive, piezoresistive, or resonant sensors and providing direct pressure measurements.

Common units for atmospheric pressure include pascals (Pa), kilopascals (kPa), hectopascals (hPa, also mb), and millimeters of mercury (mmHg) or torr. Standard atmospheric pressure is 101.325 kPa, 1013.25 hPa, or 760 mmHg at sea level.

Barometric data underpin weather analysis, forecasting, altitude estimation in aviation and mountaineering, and calibration of other instruments. Pressure generally decreases with altitude and varies with weather systems, with rapid drops signaling approaching low-pressure conditions.