# Meteorological instruments -- Pressure

A barometer should be hung or placed in a room in which the temperature is constant or changes only slowly. It should be shielded from direct sunshine at all times and should not be placed near any heating apparatus or where there is a draught.

In order to compare pressure levels measured made at weather stations at different altitudes, they are usually corrected to the mean sea level pressure.

## Measurement of Atmospheric Pressure

### Atmospheric pressure is generally measured by a barometer:

Mercury Barometer

Electronic Barometer

### Definition of atmospheric pressure

The atmospheric pressure on a given surface is the force per unit area exerted by virtue of the weight of the atmosphere above.  The pressure is thus equal to the weight of the vertical column of air above the surface, taken horizontally, and extending to the outer limit of the atmosphere.  Analysed pressure fields are fundamental requirements of the science of meteorology.   It is imperative that these pressure fields be accurately defined as they form the basis for all subsequent predictions of the state of the atmosphere.

### Meteorological requirements

A barometer is an instrument for measuring atmospheric pressure.  Mercury barometers, electronic barometers and aneroid barometers are commonly used for meteorological purposes.

A barometer should be hung or placed in a room in which the temperature is constant or changes only slowly.  It should be shielded from direct sunshine at all times and should not be placed near any heating apparatus or where there is a draught.

In order to compare pressure levels measured made at weather stations at different altitudes, they are usually corrected to the mean sea level pressure.

### Units of measurement

hectopascal (hPa)
millibar (mbar)
millimetres of mercury (mm Hg)

 Conversion factors: 1 hPa = 1 mbar           =  0.750 mm Hg 1 mbar = 1 hPa             = 0.750 mm Hg 1 mm Hg = 1.333 hPa                = 1.333 mbar