A barometer is a scientific instrument used to measure atmospheric pressure, also called barometric pressure. Atmospheric pressure changes with distance above or below sea level, so a barometer can also be used to measure altitude. There are two main types of barometers: mercury and aneroid. In a mercury barometer, atmospheric pressure balances a column of mercury, the height of which can be precisely measured. Common pressure units include pounds per square inch, dynes per square centimeter, newtons per square meter (the SI unit called the pascal), inches, centimeters, or millimeters of mercury. A non-liquid barometer called the aneroid barometer is widely used in portable instruments and in aircraft altimeters because of its smaller size and convenience. It contains a flexible-walled evacuated capsule, the wall of which deflects with changes in atmospheric pressure. This deflection is coupled mechanically to an indicating needle.

Barometers are commonly used by meteorologists to predict weather phenomena. Falling barometer readings can signal that bad weather is on the way, while rising barometer readings can indicate clear skies and cooler temperatures. Knowing what the weather might be like can help in planning your day.