Mercury Barometer 

Also see READING AIRCRAFT ALTIMETERS

Evangelista Torricelli , an Italian mathematician and scientist, is credited with discovering the principle of the barometer in 1644. He filled a 1 meter column of mercury and inverted it into a container also filled with mercury. The mercury inside the column fell somewhat, creating inside, a Torricellian vacuum. He noted that the height of the column of mercury fluctuated with changing air pressure.

Quite simply put, barometers measure the weight of the air in the atmosphere. As the air presses down on the mercury in the container, the mercury in the column moves up. When air pressure decreases and the mercury in the column weighs more than the air pressing upon it, the mercury in the column drops.

At sea level, this is normally 29.92 inches of mercury high, or about 760 millimeters. 

Mercury barometers are very accurate, delicate, and expensive, so most people use aneroid barometers. 

Aneroid Barometer

The aneroid (without fluid) barometer has one or more metal cells from which air has been removed so that only a small amount remains, along with a spring inside.  Increased air pressure causes the sides of the cells to be pushed closer together. Mechanically, the cells are connected by levers and pulleys which connect to a pointer that indicates the level of pressure on a dial. Often the user will be able to set the current pressure using a second pointer so that trends can be observed.

Barograph

A barograph is another instrument used to report atmospheric pressure. it uses an aneroid barometer mechanism as well, however in the barograph the mechanism moves a pen up and down on a chart which is attached to a slowly rotating drum. This provides a charted record of barometric changes and trends.

Pressure Conversion table

By the way, temperature impacts air pressure readings, so if precise measurements are required, corrections for temperature must be made.  However, if your goal is to spot pressure trends to help you make general forecasts, these corrections are not necessary.

Learn more about how pressure and weather by clicking here

Learn about how meteorologists measure air pressure in the upper atmosphere by clicking here

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