Average mean sea level pressure

At a glance

These mean sea level pressure maps show the average monthly and average annual mean sea level pressure (MSLP) distribution across Australia.

What do the maps show?

These maps show the average annual and monthly mean sea level pressure (MSLP) over Australia, during the period 1979-2000.

Atmospheric pressure recorded at the Earth's surface is caused by the weight of the column of air above it. Pressure data is collected from a number of sources at different altitudes and this data is adjusted to represent the pressure at sea level. The average MSLP maps smooth out the smaller scale features that you see in the daily weather charts, and show a general picture of the monthly MSLP conditions over Australia. These MSLP patterns are, generally speaking, the result of the global atmospheric circulation. If you look at a map of the average MSLP conditions over the whole globe across the whole year, you will notice that there is a band of low pressure at the equator, surrounded by two bands of high pressure at about 40 degrees north and south followed by another two bands of low pressure and an area of high pressure at each of the poles. These alternating high and low pressure bands represent the rising (low pressure) and sinking (high pressure) "arms" of two large-scale atmospheric circulation cells.

Australia's seasonal weather patterns are dominated by these bands of high and low pressure. In summer, the tilt of the Earth means that the sun's rays are focussed on an area approximately 5 degrees south of the equator, over northern Australia. This heat triggers convection, causing a band of low pressure in this area in summer. This area of low pressure brings rain to northern Australia and is also known as the monsoon. At the same time, a band of high pressure can be found over the southern parts of Australia. This high pressure, known as the sub-tropical ridge, is caused by sinking air, and is associated with the descending arm of a large circulation cell.

During winter, the Australian continent is tilted away from the sun so the sun's rays are concentrated just to the north of the equator. This shifts the circulation cell over Australia northwards, so that the low pressure is no longer over northern Australia, and the continent is now dominated by high pressure (the sub-tropical ridge). Areas directly under the high pressure ridge generally experience fine, dry weather. To the south of this ridge, westerly winds (i.e. winds moving in an easterly direction) bring rain to the southern parts of Australia. You can see the influence of these pressure patterns on Australia's seasonal rainfall by looking at the mean rainfall maps for different months and seasons.

How are the values calculated?

These maps are produced based on data processed by a computer model. Current conditions need to be accurately represented in the model (in the form of "analysis" fields) before the model can be used to forecast future weather. Analysis fields are created based upon data from the world's national meteorological services (from ground stations, upper air observations, satellites, ships, buoys). These data are fed into computers, checked for accuracy, and mapped onto a three-dimensional grid. It is these analysis fields which were used to develop the MSLP maps.

Further information

• About the maps - metadata and related information.
• Learn about the atmospheric circulation.
• Find out how climate statistics are calculated.
• Information about the range of gridded climate data we can provide.
• Information about additional climate products and other data available from the Bureau of Meteorology.
• Learn about the influences on the Australian climate.
• Explore other topics relating to climate education.
• View recent weather observations.