There are several ways of determining the barometric pressure. It used to be measured most commonly with a mercury barometer, with a column of mercury (Figure 1.6) replacing a similar device using water, invented by Evangelista Torricelli in 1643. (Mercury is 13.6 times more dense than water and as a result a more convenient column only about 760 mm

Figure 1.6 A mercury barometer, in which the atmospheric pressure on the leather washer is balanced by that exerted by the column of mercury.

tall is needed to measure a typical sea-level atmospheric pressure, instead of about 10 m of water.) The mercury is surmounted by a vacuum, inside a vertical tube sealed at the top. Thus, only the weight of the mercury presses down and is balanced by the atmospheric pressure, so the height of the column is a measure of that pressure. This kind of barometer should be mounted on a solid wall in a place of constant temperature, with protection from sunlight and damage, but with good light.

The aneroid barometer (i.e. one without liquid) is more usual nowadays. It was invented in principle by Gottfried Liebniz around 1700, and constructed by Lucien Vide in 1843. In this instrument, the pressure being measured is opposed by the elasticity of a partially evacuated metal cylinder, and the movement of the ends of the cylinder is amplified by levers to show any pressure change (Figure 1.7). Aner-oid instruments have the advantages of lightness, portability, fast response to rapid changes of pressure and easy adaptation to the recording

Aneroid Barometers Flexible Membrane
Figure 1.7 The operation of an aneroid barometer. A partially evacuated box with a flexible end is attached to an indicator arm. An increase of external atmospheric pressure compresses the top of the box and moves the arm.

of data. On the other hand, they do need regular calibration. Both types of barometer can measure pressure changes as small as 0.3 hPa, corresponding to an altitude change of about 3 m near sea-level. As a rule-of-thumb, the pressure drops by approximately 1 hPa per 10 metres elevation near sea-level (Note 1.G).

Air pressure varies horizontally also (and we shall be concerned with that in Chapters 12 and 13) but there are much greater variations with height. (The distinctions between 'height', 'altitude' and 'elevation' are detailed in Note 1.H.) To determine the much smaller horizontal differences of pressure which are important in meteorology, it must be expressed in a standard manner. This is usually done in terms of the pressure at the mean level of the sea, after averaging tidal fluctuations over a few years. That pressure is called the Mean Sea-Level Pressure (i.e. the MSLP) of the atmosphere.

Deriving it necessitates correcting surface measurements of pressure for the effects of height (Note 1.G) and making small adjustments for the latitude, gravity, temperature and the time of day. For instance, a measurement of 850 hPa at Johannesburg (at 1,665 m altitude) implies about 1,017 hPa at sea-level (i.e. 850+1,665/ 10).

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