barometers

A barometer is an instrument used to measure atmospheric pressure. It provides a fundamental environmental variable used in weather forecasting, aviation, and scientific research. Readings are commonly expressed in units such as hectopascals (hPa), millibars, or millimeters of mercury (mmHg, or torr). Pressure readings can also be used to infer altitude or to calibrate other instruments.

Mercury barometer: In the classic form, a glass tube filled with mercury is inverted into a mercury reservoir. The atmospheric pressure presses on the reservoir, supporting a column of mercury whose height balances the air pressure. Readings are taken from the height of the mercury column, typically in millimeters or torr. Mercury barometers are highly precise and stable but are fragile and contain toxic mercury, requiring careful handling and containment.

Aneroid barometer: This type uses a sealed evacuated chamber with a flexible diaphragm that expands or contracts with changes in external pressure. The movement is amplified by gears and springs and displayed on a dial. Aneroid barometers are portable and safer than mercury types, making them common in handheld weather instruments and altimeters, though they can drift over time and are less precise than mercury devices.

Other types and usage: Modern digital barometers rely on MEMS or quartz sensors to measure pressure electronically. Barometric pressure data support weather forecasting, the monitoring of storm systems, and altitude determination in aviation and hiking. The term barometer derives from Greek words for weight and measure, reflecting its function to measure air pressure. Sea-level standard pressure is about 1013.25 hPa or 760 mmHg.