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A pressure barometer is an instrument used to measure atmospheric pressure at a particular location, and there are several different measurement units that might be used. It is useful for predicting short-term weather and finding high and low pressure systems. Traditionally, a pressure barometer is made using water or mercury, and the air pressure changes can be measured with the height of the substance as it rises and falls. There is also a more modern kind, called an aneroid barometer, which shows pressure changes with a needle on the instrument’s face pointing to the amount of measured air pressure.
Atmospheric pressure is the weight of air molecules above a particular point that are pushing down on it. As altitude increases, air pressure lowers because there is less air exerting force on the Earth’s surface. There are several different measurement units common for air pressure, including inches or millimeters of mercury, millibars, and kilopascals. The type of pressure barometer used will usually determine in which units it measures air pressure.
Changes in atmospheric pressure, shown with a pressure barometer, can help forecast short-term weather. When the air pressure is high, it generally indicates fair, dry weather. On the other hand, when pressure is low, it typically means rain, wind, or stormy weather. Rapid changes in pressure usually mean more extreme weather than when the changes are slow. Also, barometric measurements can help find high and lower pressure systems as well as frontal boundaries because air tends to move from high to low pressure areas.
One kind of pressure barometer is made with water, sometimes called a Goethe barometer. It has a sealed glass container of water with a spout that is connected to the main body of the container below the water line. When air pressure is low, water will go up the connected spout so it is above the level of water in the body of the barometer. During high pressure, water in the spout will fall below the level of water in the body.
A mercury pressure barometer, also known as a Torricelli barometer, is made with a tube of glass 33 inches (84 cm) long that is closed at one end, and the open end of the tube is placed vertically in an open reservoir of mercury. This creates a vacuum in the upper part of the tube, so mercury rises in the tube column until the weight of the mercury balances with the air pressure pushing on the open reservoir. When the atmospheric pressure is high, it pushes the mercury up higher in the column. With low pressure, the mercury drops lower in the column. This type of pressure barometer will measure atmospheric pressure in terms of inches or millimeters of mercury, which generally ranges from 28 to 31 inches (71-79 cm).
Another pressure barometer is the modern aneroid barometer. It uses an aneroid cell made from flexible metal that expands or contracts as atmospheric pressure changes. The adjustments in the cell are transmitted to levers that make a needle move on the face of the instrument. The needle points to the measured amount of air pressure, often labeled in millibars.