Barometers and Barographs (Barograms) are used for Surface weather analysis and Weather forecasting applications. Barograph is a recording aneroid barometer. Barograph produces a paper or foil chart called a barogram that records the barometric pressure over time. Barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Barometer can measure the pressure exerted by the atmosphere by using water, air, or mercury. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems, and frontal boundaries.

Types of Barometers used for Atmospheric Pressure Measurement

  • Water-based Barometers – The concept of Water-based barometers is that decreasing atmospheric pressure predicts stormy weather provides the theoretical basis for a weather prediction device called a “storm glass” or a “Goethe barometer”.
  • Mercury Barometers – Mercury barometer has a glass tube of at least 84 cm in height, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury creates a vacuum in the top of the tube. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir.
  • Aneroid Barometers – Aneroid barometer uses a small, flexible metal box called an aneroid cell. This aneroid capsule (cell) is made from an alloy of beryllium and copper. The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Barograph is a recording aneroid barometer.

Altitude and Temperature Effects on Barometer Pressure Measurement

Altitude and temperature can affect the accuracy of barometer readings. As the air pressure will be decreased at altitudes above sea level (and increased below sea level) the actual reading of the instrument will be dependent upon its location. This pressure is then converted to an equivalent sea-level pressure for purposes of reporting and for adjusting aircraft altimeters (as aircraft may fly between regions of varying normalized atmospheric pressure owing to the presence of weather systems). Aneroid barometers have a mechanical adjustment for altitude that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude.

The density of mercury will change with temperature, so a reading must be adjusted for the temperature of the instrument. For this purpose a mercury thermometer is usually mounted on the instrument. Temperature compensation of an aneroid barometer is accomplished by including a bi-metal element in the mechanical linkages.