Atmospheric pressure is the force per unit area exerted by the atmosphere on any surface in contact with it. If pressure is considered as the weight of a column of air of unit cross-sectional area above a surface, then it can be seen from the diagram that the pressure (weight of the column above) at the upper surface will be less than that at the lower surface.
Thus atmospheric pressure will decrease with an increase in height.
Units of Measurement
The standard unit of force is the NEWTON (N) and an average for atmospheric pressure at sea level is 101 325 newtons per square metre (pascals). For simplicity this is expressed as 1013.25 hectopascals (hPa) because the earlier system of measurement was millibars (mb) and 1 hPa = 1 mb. In some countries millibars are still used. Other units which are still in use are related to the height of a column of mercury in a barometer in inches or millimetres (see overleaf).
Note: mean sea level pressure in the ISA is 29.92 inches or 760 mm of mercury.
The basic instrument used for the measurement of atmospheric pressure is the mercury barometer. The atmospheric pressure is measured by the height of a column of mercury, and this height can be read in terms of any of the units shown above. The USA still uses inches of mercury as their measurement of atmospheric pressure.
A more compact means of measuring atmospheric pressure is the Aneroid Barometer. It consists of partially evacuated capsules, which respond to changes in pressure by expanding and contracting, and a system of levers, these changes of pressure being indicated by a pointer moving over a scale.
To enable a continuous record of pressure changes to be made, a paper covered rotating drum is substituted for the scale and the instrument then becomes a barograph. This instrument is used by the meteorologist to measure what is known as pressure tendency, the rise and fall of pressure over a period of time. Pressure tendency is an important forecasting tool.