The air temperature analyses and associated maps use data contained in the Bureau of Meteorology climate database, the Australian Data Archive for Meteorology (ADAM). The analyses are initially produced automatically from real-time data with limited quality control. They are intended to provide a general overview of air temperature across Australia as quickly as possible after the observations are received.
What is air temperature?
According to the American Meteorological Society's Glossary of Meteorology, temperature is the quantity measured by a thermometer. In gaseous fluid dynamics, temperature represents molecular kinetic energy, which is then consistent with the equation of state and with definitions of pressure as the average force of molecular impacts and density as the total mass of molecules in a volume. Air temperature is measured in a shaded enclosure (most often a Stevenson Screen) at a height of approximately 1.2 m above the ground. Maximum and minimum temperatures for the previous 24 hours are nominally recorded at 9 am local clock time. Minimum temperature is recorded against the day of observation, and the maximum temperature against the previous day.
Mean maximum temperature (°C)
The average daily maximum air temperature, for each month and as an annual statistic, calculated over all years of record.
Highest temperature (°C)
The highest (by month and overall) maximum air temperature observed at the site.
Lowest maximum temperature (°C)
The lowest (by month and overall) maximum air temperature observed at the site.
Decile 1 maximum (or minimum) temperature (°C)
Monthly or annual decile 1 (10th percentile) of maximum or minimum air temperature. The annual decile value must be calculated from yearly data, and cannot be obtained by adding together the monthly deciles. Decile values are used to give an indication of the spread of the observations over the period of record; in this case, daily observations within a month. To determine decile 1of a series of observations, they are first arranged in order from lowest to highest, and then divided into 10 equal groups. Decile 1 is the value at the top of the 1st grouping. Over the long term about one day in ten can be expected to have a (maximum or minimum) temperature below the decile 1 value.
Decile 9 maximum (or minimum) temperature (°C)
Monthly decile 9 (90th percentile) of maximum or minimum air temperature. The annual decile value must be calculated from yearly data, and cannot be obtained by adding together the monthly deciles. Decile values are used to give an indication of the spread of the observations over the period of record; in this case, daily observations within a month. To determine decile 9 of a series of observations, the data are first arranged in order from lowest to highest, and then divided into 10 equal groups. Decile 9 is the value at the top of the 9th grouping. Over the long term about one day in ten can be expected to have a (maximum or minimum) temperature exceed the decile 9 value.
Mean minimum temperature (°C)
The long-term average daily minimum air temperature observed during a calendar month and over the year.
Lowest temperature (°C)
The lowest recorded temperature observed at the site, calculated over all years of record.
Highest minimum temperature (°C)
The highest recorded minimum temperature observed at the site, calculated over all years of record.
Daily air temperature maps
At 1:00 pm (AEST) each day, the daily maximum/minimum air temperature values for the previous day are analysed. Temperatures are measured directly at about 750 sites across the country. These station data are then analysed onto 0.25x0.25 and 0.05x0.05 degree grids.
The national map shown on the web is based on the 0.05x0.05 degree grid, sub-sampled at every fifth point to give an effective resolution of 0.25x0.25 degrees. The regional maps are based directly on the 0.05x0.05 degree grids, so there may be some differences in the fine detail between the national map and the regional maps.
All analyses and maps are progressively updated over the following six months, as new data becomes available and as the data in the climate database are improved through quality control. The schedule of updates is available here. Subsequent versions will tend to be more accurate, as they will be based on larger quality-controlled input datasets. A date stamp at the bottom right-hand corner of each map indicates when the analysis was produced.
9am to 3pm maximum temperature maps
From about 4pm local time each day, the highest temperatures recorded between 9 am and 3 pm at sites across Australia are analysed onto grids and displayed as maps. This method allows for maximum temperature maps to be generated as close to real-time as possible.
The highest temperature recorded at stations between 9am and 3pm is a good guide to the maximum temperature recorded for a day. However, the maximum temperature can occur later than 3pm, which must be considered when interpreting the 9am to 3pm maximum temperature maps. This is particularly the case for southern Australia during the summer months when it is not unusual to have the highest temperature of the day as late as 6pm.
Due to the multiple time zones across the country regional maps will appear first for the eastern states and two to three hours later for WA (this will also depend on whether or not your state observes daylight saving). The national map is the last map to be generated and is only generated once all 3pm temperature data from all states and and the Northern Territory has been received. This means that as a guide, while the regional maps will appear for each state or territory around 4pm local time, the national map will not appear until as late as 7:30pm eastern daylight savings time during summertime.
The state maps are updated on the website every fifteen minutes through this period, so that any new data that is received is analysed onto the grids and viewable on the website as soon as possible. A time stamp will appear on each map with the analysis generation time in the top right hand corner of the map. The time will be given in both standard local time and UTC time. UTC stands for coordinated Universal Time and is equivalent to GMT time, more information on how to convert from UTC time to local time can be found here. The conversion table below can also be used to convert between local time and UTC.
UTC and local time conversion table (ST = standard time, DT = daylight time)
Monthly and multiple-monthly maximum and minimum temperature maps
Monthly maximum and minimum temperatures are calculated as the averages of the corresponding daily maximum or minimum temperature.
The latest maximum and minimum temperature maps, for periods of one or more months, are usually produced on the first day of the following month, with further updates according to the schedule of updates available here. 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.
Analyses over 3, 6 and 12 months are based on the average of the one-month grids which comprise the period in question.
The anomaly maps show the departure from the long-term climate average calculated over the period 1961-1990, with the daily anomalies calculated with respect to the monthly average for the relevant month. There would normally be some correlation between rainfall anomalies and maximum and minimum temperature anomalies, with excessive rainfall coinciding with above average maximum and minimum temperature values (positive anomalies) and drought conditions coinciding with negative anomalies. During droughts, soil and vegetation become drier thereby reducing the amount of water available for evaporation and transpiration.
The analyses are computer generated using a sophisticated analysis technique described here. This method uses an optimised Barnes successive correction technique that applies a weighted averaging process to the station data. Topographical information is included by the use of anomalies (departures from average) in the analysis process. On the maps each gridpoint represents an approximately square area with sides of about 5 kilometres (0.05 degrees). The size of the grids is limited by the data density across Australia.
This gridpoint analysis technique provides an objective average for each grid square and enables useful estimates in data-sparse areas such as central Australia. However, in data-rich areas such as southeast Australia or in regions with strong gradients, 'data smoothing' will occur resulting in gridpoint values that may differ slightly from the exact maximum and minimum temperatures measured at the contributing stations.
Most of these maximum and minimum 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. Portable Document Format (PDF) versions of the images are also generated for high-quality printing. Please note however that the PDF version is not archived for reasons of space. PDF version of older maps may be obtained via feedback form, but charges may be imposed for their provision.
The map projections used are either Cylindrical Equidistant (CE) or Lambert Conformal (LC). The Lambert Conformal projection takes three parameters; the central longitude (in degrees east of the Greenwich Meridian) and two standard parallels of latitude (in degrees south of the equator).
|Map projection||LC 134° 10°, 40°||CE||CE||LC 140.8° 10°, 40°||LC 146.5° 10°, 44°||CE||CE||CE|
The Victoria and Tasmania maps are based on a finer resolution analysis than the remaining maps. Consequently there may be slight inconsistencies in the detail represented on the Vic./Tas. maps as compared against the Aus./SA/NSW maps.
Daily, weekly and monthly temperature grids may be downloaded from the Bureau's website. These grids are in an ASCII format suitable for ingesting into geographic information systems (GISs), compressed using the UNIX compress utility. The ASCII grids have appended to them their original AIFS ASCII grid header (a Bureau of Meteorology grid format), to provide additional grid metadata. Note that some GISs may require the user to change the grid file extension from '.grid' to '.txt' prior to ingestion into the GIS.
The analyses use data collected through electronic and paper communication channels. These data have been screened for errors, using an automated technique, and make use of quality control which has been undertaken on the climate database. Full quality control is completed some weeks to months 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, observational errors may enter the analyses because they cannot be detected by comparison with other reports. In these instances, the erroneous maps will be amended as soon as is practicable.