Barometers are instruments that measure the pressure of air in a specific environment; barometer measurements are used to forecast weather and measure altitude.

Atmospheric pressure also depends on the planet’s mass, its surface radius, the gas concentrations and composition, and the gases’ vertical distribution in the atmosphere. It is also affected by the planet’s rotation and other local conditions including wind velocity and temperature-dependent density and gas composition variations.

Common pressure units include:

• Pounds per square inch (psi)
• Dynes per square centimeter
• Newtons per square meter (equivalent to the pascal, the SI unit)
• Inches, centimeters, or millimeters of mercury
• Millibars (1 millibar equals 1,000 dynes per square centimeter, 100 pascals, or 0.75 millimeters of mercury)

The pascal (Pa) or kilopascal (kPa) has worldwide use as a unit of pressure and has mostly replaced the unit of pounds per square inch. Exceptions are countries that use the imperial measurement system or the U.S. customary system.

In general, better or drier weather is indicated by a rising barometer; a falling barometer indicates worse or wetter weather.

Several types of quantitative or semi-quantitative barometers exist. Torricellian or mercury barometers use a column of mercury to balance atmospheric pressure. The height of the mercury is pre-measured, and the change in height is used to calculate the atmospheric pressure.

Nonliquid or aneroid (dial) barometers are used in portable instruments and aircraft altimeters because their smaller size offers convenience. A flexible-walled evacuated capsule moves with changes in atmospheric pressure – the movement mechanically moves an indicating needle. Recording aneroid barometers are called barographs.

Digital barometers contain a pressure transducer that converts the air pressure into an electrical signal.