Map information - maps of recent Australian climate

Contents

These rainfall and temperature analyses are produced automatically from preliminary data with limited quality control. They are intended to provide a general overview of rainfall and temperature distribution across Australia as quickly as possible after the observations are received. For example, daily rainfall maps are available at around 2pm (Eastern Standard Time) which is only a couple of hours after the morning observations from all time zones are received in the central database. Daily temperature maps for the previous day are available at around 12.30pm (Eastern Standard Time).

Daily rainfall maps

As indicated in the previous paragraph, the daily rainfall maps are initially generated using real time data, by the National Meteorological and Oceanographic Centre, with limited quality control on the data.

Monthly and multiple-monthly rainfall maps

The latest rainfall maps for periods of one month and longer are usually produced within the first few days of the new month and updated several times at later dates. The first version of the one month rainfall analysis normally appears on the last day of the just completed month. Subsequent versions will be more accurate, as they will be based on larger and more accurate input datasets. A date stamp at the bottom right-hand corner of each map indicates when the analysis was produced. Generally the accuracy of the analysis increases as the time between the end of the analysis period and the date the analysis is produced increases.

Monthly and multiple-monthly temperature maps

The latest temperature maps for periods of one month and longer are likewise usually produced within the first few days of the new month. If the first version of these temperature maps appears on the last day of the just completed month, then the maximum temperatures for the last day of the analysis period (whether monthly or multiple-monthly) will not be included in the calculation, though being unavailable at that time. When the maps are subsequently regenerated, all the available data will be included.

Analysis technique

The analyses are computer generated using the Barnes successive correction technique which applies a weighted average to data reported within set grids across Australia. On most maps (both rainfall and temperature), each gridpoint represents a square area with sides of approximately 25 kilometres. The size of the grids is limited by the relatively coarse average data separation throughout Australia. Rainfall maps for South Eastern Australia are able to be analysed at finer resolution with approximately 10 kilometre grid spacing, due to greater data density in this region. Finer resolution maps are able to show greater analysis detail so may differ slightly from maps analysed at the coarser resolutions. For example, South Eastern Australia rainfall maps will be able to show greater detail than the corresponding Murray Darling Basin maps.

This gridpoint analysis technique provides an objective average for each grid square and enables useful estimates of rainfall and temperature in data-sparse areas such as central Australia. However, in data-rich areas such as southeast Australia or in regions with strong rainfall or temperature gradients "data smoothing" will occur resulting in values at point locations which may differ slightly from the exact rainfall or temperature recorded.

Consequently, these area-weighted maps may be different from maps produced by hand. Manual analyses can allow for greater quality control of the input data but are time consuming, difficult to reproduce and will differ between analysts. Automatic area-weighted analyses are appropriate for the purpose of providing an objective Australia-wide analysis in a timely manner.

Map formats

Most of these rainfall and temperature maps are produced as both colour and black/white GIF images, with low and high resolution versions available in each case. The low resolution colour GIF images are the ones usually displayed, with links to the other three types placed under the main image. Place names are generally to be found on the high resolution versions. PostScript and Portable Document Format (PDF) versions of the images are also generated for high-quality printing. Please note however that the PostScript and PDF versions are not archived for reasons of space.

Quality control

The Bureau of Meteorology's rainfall and temperature analyses use data collected through electronic communication channels. These data have been screened for errors, but not yet fully quality controlled. Full quality control is completed some weeks after the end of the most recent month when (a) extreme values are confirmed by written reports, and (b) data more generally are compared with those of nearby stations so that values and dates of occurrences are similar.

Occasionally in the data-sparse areas, gross observational errors will slip through the limited automated quality control applied to the shorter period analyses. In these instances, the erroneous maps will be amended as soon as is practicable.

Rainfall variables

For information on the various rainfall analysis variables mapped, use the following links - amounts, deciles, drought, percentages and anomalies. The rainfall deciles and drought maps based on a reference period comprising all gridded analyses from 1900 to the present available at the time the deciles are calculated. The rainfall percentages and anomaly maps are calculated with respect to the reference period 1961-1990.

Daily rainfall observations are made at 9am local time, and the daily rainfall for a particular day denotes the rainfall which falls during the 24 hours to 9am on that day. Hence, for example, a June rainfall total denotes the rainfall which fell between 9am on the 31st May and 9am on the 30th June.

Temperature variables

The temperature anomaly maps are likewise calculated with respect to the reference period 1961-1990.

Daily minimum temperature observations follow the practice described above for daily rainfall - for example, the daily minimum temperature recorded for the 20th of the month denotes the minimum temperature observed between 9am on the 19th and 9am on the 20th, local time. In contrast, the daily maximum temperature is measured from 9am local time - for example, the daily maximum temperature recorded for the 20th of the month denotes the maximum temperature observed between 9am on the 20th and 9am on the 21st, local time.