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What is a Barometer?

Mercury Barometers - there hasn't been much change 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. 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 (Analog) Barometers - Aneroid (without liquid) accurately describe how this barometer functions. Instead of mercury, these 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.

How to Read a Barometer
Once you know what the average atmospheric pressure should be, following weather is one step closer. Barometers measure pressure in several units, usually inches or millibars. For the sake of efficient demonstration, let's assume the average pressure of a given area is 29 inches. Here's what can happen to that barometer reading:

No Change: If that reading stays at or very near 29 inches, that means the weather in the area is stable. No changes are occurring.

Increase: As the pressure increases, air is becoming heavier and more stable. When barometer readings go above 29 inches, weather generally gets warmer, even muggier. Because the air mass is not going anywhere, urban areas may experience smog.

Decrease: Conversely, as barometer readings fall, fast-moving low pressure air is coming in. Usually associated with winds and storms, low barometers tell a definite change is on the way.