Monitoring the weather

Pressure

Air pressure is probably the single most important weather element, because pressure variations drive the winds. Daily weather map sequences show a series of high pressure ('highs') and low pressure ('lows') systems moving across the continent.

The lines on the map show variations in the pressure from place to place at a horizontal level, usually mean sea level. These 'isobar' lines ('iso' means equal) join places having the same atmospheric pressure.

Air movement and pressure systems
Exactly how does air move in relation to these systems? Outside the tropics, the broadscale wind flow is closely related to the distribution of atmospheric pressure.

Some general rules are:

• In the southern hemisphere, winds flow clockwise around low pressure systems (and slightly inward) and anticlockwise (and slightly outward) around high pressure systems.
  
  
• Where isobars are closer together the wind will be stronger than in areas where they are further apart.
  

Inflowing air associated with a low pressure system results in convergence and ascent of air often resulting in deep cloud formations and rainfall. Conversely, outflowing air associated with a high pressure system results in divergence and descent of air and more stable weather.