Selecting a Barometer on Amazon.com
A barometer measures atmospheric pressure, also called air pressure or barometric pressure, which is the weight of the Earth's atmosphere at a specific point on the planet's surface, in the ocean, or in the atmosphere itself. Changes in atmospheric pressure signal changing weather: Rising pressure indicates clearing skies, falling pressure signals approaching rain or storm.
Mercury barometers are very accurate, but use is restricted because of the mercury content. The rising or falling level of mercury in a vertical tube indicates changing air pressure. The aneroid type of barometer has a sealed capsule of copper and beryllium that contracts or expands as air pressure changes, often using a pointer and dial to display readings. Aneroid barometers often have a second pointer that is compared to the current reading to indicate the direction of pressure change. They may be compact and portable, or wall-mounted. A barograph is an aneroid barometer that records changing pressure on a graph. Other barometer types use liquids such as water or oil. A digital barometer uses an electronic sensor, and because it can take multiple readings, can forecast with improved accuracy.
Barometric pressure readings may be given in inches of mercury (inHg), millimeters of mercury (mmHg), or millibars (mbar), also called hectopascals (hPa). Even a barometer that uses no mercury may use the traditional inHg or mmHg measurements. Air pressure at sea level is typically 29.92 inHg, or 1013 mbar. A barometer may be designed for use in specific ranges of altitude, and may need to have altitude set at the time they are installed.
Barometers are used for weather forecasting and research in homes, industrial sites, laboratories, and weather stations. Because air pressure changes with altitude, a barometer can also be used as an altimeter, an instrument that measures height above sea level. Aneroid barometers are widely used as altimeters in aviation.