Pressure systems

Atmospheric pressure varies with both height above mean sea level, and from place to place at mean sea level (that is, horizontally).

We measure mean sea level pressure (MSLP) in units called hectopascals (hPa). The lowest MSLP ever recorded (870 hPa) was in typhoon Tip in 1979; the highest ever recorded (1084 hPa) was in Siberia in 1968.

Lines on a weather map joining places of equal mean sea level pressure are called isobars.

At the broadest scale, the equatorial region tends to be an area of relatively low mean sea level pressure. The mid-latitudes tend to be areas of relatively high pressure (typically around 30°N, 30°S) with another belt of low pressure further poleward (40°- 60° latitude), and relatively high pressure over the poles.

On a daily weather map, such as the one shown here, there will be meandering patterns of relatively high and low pressure systems over Australia. It is these systems that generate variations in wind, temperature and rainfall. In particular the direction and speed of the wind is closely related to the distribution of mean sea level pressure.

For reasons explained later, in the southern hemisphere the wind flows clockwise around low pressure systems and anticlockwise around high pressure systems. The strength or speed of the wind is closely related to the spacing of the isobars: closer spacing means stronger winds.