air pressure The atmosphere is made up of a mechanical mixture of gases, which all have mass. Thus a column of air has a weight. If this weight is measured over a unit surface area, it is called
pressure and may be measured by the force that the column exerts on a column of mercury trapped inside a calibrated tube, otherwise known as a barometer. The unit for measuring air pressure in atmospheric science is typically a millibar (mb) but increasingly the metric equivalent, the hectopascal, hPa, is used (1 mb = 1 hPa). Measurements of air pressure are useful since they relate to the behaviour of the weather. High surface pressure refers to an anomalously large amount of mass above a point with surface values in the range of 1020 to 1040 hPa. Low pressure refers to an anomalously low mass of air above a point, and is characterized by values in the range 970–1010 hPa. Areas of high and low pressure in the mid-latitudes tend to be cellular and a few hundred kilometres in size. High-pressure cells are characterized by air diverging or moving away from the centre of pressure. To compensate, air sinks in the core of the cell, heating adiabatically as it descends. High-pressure areas are thus generally warm and cloud-free. In contrast, areas of low pressure tend to drive airflow towards their centres. This convergence of air leads to ascent, adiabatic cooling, and often rainfall.
Since air pressure refers to the mass of the column of air above a point, it follows that pressure decreases with altitude to zero at the top of the atmosphere. Most weather features occur below the 300 hPa level, which in the mid-latitudes is roughly 8000 m above sea level (corresponding to the highest mountains on Earth). Since air is compressible, the rate of decrease of pressure with altitude is non-linear.
Pressure is inversely related to temperature. Regions which undergo intense solar heating generally experience low pressures (for example, thermal low pressures over Spain and North Africa in July), whereas regions of intense cooling experience high-pressure systems (for example the Siberian High in January). These thermal effects are generally confined to the lowest few kilometres of the atmosphere. A useful meteorological quantity is therefore the thickness of atmosphere between two standard pressure levels (for instance 1000 hPa and 500 hPa), since this reveals the three-dimensional structure of the atmosphere. The thickness is usually expressed in metres, where relatively small values indicate cold air and larger values warmer air.
In countries characterized by marked relief, such as the escarpment of South Africa, a map of surface pressure would simply provide a map of altitude rather than an indication of weather features, since pressure decreases sharply with altitude. In these circumstances it is desirable to plot the altitude (in a unit approximating to metres) at which a certain pressure (such as 850 hPa) occurs. Maps of the topography of the pressure surface are analogous to maps of pressure, where low values indicate low pressure.
R. Washington
Bibliography
Meteorological Office (1991) Meteorological glossary. HMSO, London.
