A mercury barometer balances the weight of mercury in a glass tube against the atmospheric pressure above a reservoir of mercury at its base to provide a measurement of atmospheric pressure. This weather instrument works like a set of scales to forecast the weather.
A classic mercury barometer is constructed from a 3-foot glass tube that is open at one end and sealed at the opposite end. The glass tube is filled with liquid mercury and rests upside-down in a reservoir of mercury. As the mercury moves down the glass tube, a vacuum is created. When the atmospheric pressure above the reservoir increases, the mercury inside the tube rises. As atmospheric pressure decreases the mercury moves down the tube, into the reservoir. This type of barometer was first constructed in 1643 by Evangelista Torricelli.
The change in the mercury level in the glass tube is equal to the pressure exerted by the air above the reservoir and is measured using the scale marked on the glass tube. Changes in atmospheric pressure occur prior to weather changes. Sudden drops in atmospheric pressure indicate stormy weather, while sudden rises in pressure predicate brief periods of fair weather. Gradual increases and decreases in atmospheric pressure point to a more sustained weather pattern.