A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems and frontal boundaries.

Evangelista Torricelli (1608–1647) was an Italian physicist and mathematician, best known for his invention of the barometer, but is also known for his advances in optics. Although Evangelista Torricelli is universally credited with inventing the barometer in 1643, historical documentation also suggests Gasparo Berti, an Italian mathematician and astronomer, unintentionally built a water barometer sometime between 1640 and 1643. French scientist and philosopher René Descartes described the design of an experiment to determine atmospheric pressure as early as 1631, but there is no evidence that he built a working barometer at that time.

Here's a few videos about the atmosphere and barometers: This one is about how to read a barometer. This one is about how to make your very own barometer. This one (starring Bill Nye) is about atmosphere pressure.

Barometer is a term also used when someone is trying to gauge circumstances. For instance, the January Barometer is an indicator of how the US economy will perform for the rest of the year.