A barometer is an instrument used to measure atmospheric pressure.

A standard mercury barometer has a glass column of about 30 inches (about 76 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more downward force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the downward force placed on the reservoir.

The first barometer of this type was devised by Evangelista Torricelli, a student of Galileo Galilei, in 1643. Torricelli had set out to create a perfect vacuum, and an instrument to measure air pressure. He succeeded in creating a vacuum in the top of a tube of mercury. Torricelli also noticed that the level of the fluid in the tube changed slightly each day and concluded that this was due to the changing pressure in the atmosphere. He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".

The mercury barometer's design gives rise to the expression of atmospheric pressure in inches: the pressure is quoted as the level of the mercury's height in the vertical colum. 1 atmosphere is equivalent to about 29.9 inches of mercury. The use of this unit is still popular in the United States, although it has been disused in favor of SI or metric units in other parts of the world. Barometers of this type can usually measure atmospheric pressures in the range between 28 and 31 inches of mercury.

Another type of barometer, the aneroid barometer, uses a small, flexible metal box called an aneroid cell. The box is tightly sealed after some of the air is removed, so that small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives a series of mechanical levers and other devices which are displayed on the face of the aneroid barometer.

A barometer is commonly used for weather prediction, as high air pressure in a region indicates fair weather while low pressure indicates that storms more likely. Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Low atmospheric pressure, on the other hand, represents the path of least resistance for a weather system, making it more likely that low pressure will be associated with increased storm activity.

Water-based barometers

This concept of "decreasing pressure means bad weather" is the basis for a primitive weather prediction device called a weather glass or thunder glass. It consists of a glass container with a spout. The container is filled with water up to about the middle of the spout; some air is left in the main body of the container. The design is such that when the air pressure decreases, the pressure of the air pocket inside the device will push some of the water up the spout. If the air pressure is low enough, some of the water may even drip out of the spout. These devices are essentially a water-based version of the mercury barometer. The "Thunder Glass" is extremely susceptible to the ambient temperature which will alter the height of the water column in the spout.