Weather elements

Air pressure 

How to measure air pressure 

Air pressure is the force with which the atmosphere (air) is pressing down on us. It is measured in units called millibars (mb) or hectopascals (hPa) by means of a barometer or a barograph.

Figure 1.1 Aneroid Barometer

The aneriod barometer in Figure 1.1 I a commonly used barometer. On the faceplate of the instrument the units are calibrated (indicated) in millibars. Words like 'RAIN' (low pressure side of scale), 'CHANGE' and 'FAIR' (high pressure side of scale) appear on the faceplate. Inside the instrument you will find a metal container from which the air has been removed (forming a vacuum) and a set of levers. These levers are connected to a pointer on the faceplate. When air pressure rises, the container contracts and the levers force the pointer to move towards the 'FAIR' side. If the pressure becomes lower the container expands and the pointer moves towards the 'RAIN' side. The arrow indicates air pressure in millibars.

At the Weather Bureau in Windhoek, air pressure is usually recorded by using a barograph.

Figure 1.2  A barograph 

A barograph can record atmospheric pressure for up to one week on graph paper. After a week ,the graph paper is replaced and the old paper called a barogram can be filed and kept on record.