Two Types of Barometers

One indispensable tool of weather forecasters is the barometer An accurate instrument for measuring atmospheric pressure, barometers come in two distinct types.

Mercury Barometers
Mercury barometers haven't changed much since their invention in 1643. A column of mercury is inside a glass tube sealed at one end. The other end rests in a small cup of mercury, called a cistern. Starting from an average day, the phenomenon works like this:

Initial reading: The barometer shows the mercury level at 29 inches, which is about average pressure for barometers at sea level. Storm comes in: Storms are low-pressure systems. As they come into an area, there is less atmospheric pressure on the cistern. The barometer in turn shows the mercury column is falling. Storm goes out: As the storm passes, the atmosphere is replaced with a calmer high-pressure system. Since the air is heavier, the barometer goes back up.

These old-fashioned barometers are fun and educational, but do so safely. Mercury is toxic and must be handled with care. If the barometer breaks, do not touch any mercury spill. Keep children out of the area. For more information on mercury, check out the EPA website for comprehensive clean-up instructions.

Aneroid Barometers
Aneroid (without liquid) accurately describe how this barometer functions. Instead of mercury, these barometers use a small metal box called an aneroid cell. This cell is actually a bellows and springs system which expands or contracts as air pressure changes. This style is also known as an analog barometer.

The Same, Just Different
Although mercury and aneroid barometers work on the same principles, they do so in different ways. Mercury barometers are relatively simple (though accurate) devices. Aneroid barometers employ a complex mechanism to record minute changes in air pressure.