Craig C. Freudenrich
A barometer is a device that meteorologists use to measure atmospheric pressure. Understanding atmospheric pressure, or air pressure, is important because the pressure changes with the weather and measuring those changes helps to determine what the weather is like or might do. There are two common types of barometers: mercury and aneroid.
The mercury barometer was invented in 1643 by Evangelista Torricelli. It consists of an inverted, hollow glass tube about 3.3 feet (1 meter) tall. The tube is closed at one end and filled with mercury. The open end of the tube rests in a pool of mercury called a cistern, which is open to the air. The level of mercury in the tube rises or falls to equal the rise or fall of atmospheric pressure. When the atmospheric pressure increases, the air pushes down on the mercury in the cistern, which pushes up on the mercury in the tube. An increasing level of mercury in the tube (rising barometer) indicates increasing air pressure. When the atmospheric pressure drops, the air presses less on the mercury in the cistern, which does not push the mercury in the tube as much.
A decreasing level of mercury in the tube (falling barometer) indicates a decreasing atmospheric pressure. Mercury barometers are calibrated to measure the air pressure in various units depending. Meteorologists generally use millibars on weather maps, and the aviation industry and TV weather reporters use inches of mercury [source: University of Illinois]. There also is an international unit called the hectoPascal; the Pascal is used to measure pressure in the science world and is named after Blaise Pascal. One hectoPascal is equivalent to one millibar [source: National Weather Service]. Mercury barometers sometimes are referred to as wet barometers because they use liquids.
The aneroid barometer was invented in the 19th century by a Frenchman named Lucien Vidie. It is a small metal container (cylinder or box) with a flexible top called a diaphragm; this box is called the aneroid cell. The air inside the aneroid cell is partially evacuated. The diaphragm moves up and down with changes in air pressure: up when the air pressure decreases and down when it increases. Mechanical levers translate the movements of the diaphragm to a needle on a dial. The dial is calibrated with pressure units. Aneroid barometers can be small or large and are easily portable. They are referred to as dry barometers because they don't use liquids.
A barograph is an aneroid barometer that has the needle attached to a pen on a rotating drum of paper (some barographs are attached to digital sensors with LCD readouts and computer recorders). Barographs provide continuous written (or digital) records of changes in air pressure.
Meteorologists use barometric data to predict the weather, depending on the season and conditions. Increasing air pressure generally indicates cold, dry air masses overhead, and decreasing air pressure indicates warm, moist air masses overhead.
Simply put, a barometer is a tool that measures air pressure. A rising barometer means that the surrounding air pressure is increasing, while a falling one indicates it's decreasing. Air pressure differs at different points on Earth, and it can also change over time. Hot air, for example, is less dense than cooler air. These differences in air pressure can have a variety of causes and occur all over the planet.
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