Air temperature
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. Subsequent analysis in following days make use of late-arriving observations and more comprehensive quality control.
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.
Maximum, minimum or mean temperature maps
For a daily period, the mean maximum or mean minimum temperature maps are calculated from the analysed station maximum or minimum observations for that day. For a weekly period, the mean maximum or mean minimum temperature are averaged from the applicable daily temperature grids over the past week. At the monthly scale, the mean monthly maximum or mean monthly minimum temperature maps are calculated as monthly means from the daily station maximum or minmum observations for the month, which are subsequently analysed to a grid. Monthly mean temperature is the average of the mean monthly maximum and mean monthly minimum temperature grids. Multi-month period grids (including annual) are calculated from the average of the corresponding monthly grids in that period.
Anomaly maps
Anomaly maps are used to compare the departure of the observations from a climate normal (or reference) period. The reference period is used to calculate a climate average using similar months associated with the observation period. The WMO defines climatological standard normals for use by the international community in order to maintain consistency in the calculation of climate statistics across the world and can be used as an indication of conditions most likely to be experienced in a given location. The reference period for supporting long-term climate change assessments is 1961 to 1990, and is commonly used in our climate maps, climate statistics and is the base period for most climate change studies. The next standard reference period is expected to be 1991 to 2020.
For instance, the anomaly map for the period December 2018 to February 2019 (summer), would be making reference to the monthly December, January and February grids over the 1961 to 1990 standard reference period.
Highest or lowest maximum or minimum temperature
The highest or lowest temperature, across the corresponding time period, calculated from the applicable daily maximum or minimum temperature grids.
Decile maps
Decile maps are used to give an indication of how the observation values for that period sit relative to the full history of record. The maps show if an observation is average (middle decile bands 4 to 7), below average (decile bands 2 to 3), very much below average (decile band 1), above average (decile bands 8 to 9) or very much above average (decile band 10). The extreme ends of the distribution are the lowest on record and highest on record, and each can also be considered within decile bands 1 and 10 respectively.
To determine which decile band an observation over a particular time period (e.g. monthly, annual or any multi-month period) falls into, the following calculation is carried out. First all data since the beginning of the record are collated over similar time periods, then values at each grid point are sorted in order from lowest to highest, and divided into ten equal groups, with the threshold value for each of the eleven decile band divisions used to categorise the relative rank for the current observation period.
For example, if a calculation on 2019 annual temperature data was performed during 2020, using the full record of the 110 years of sorted (or ranked) data since 1910, there would be 10 equal groups of 11 years within each. The lowest on record would be the smallest (first) value. Temperatures from that value to the top of the first grouping would be classified as decile 1 (or very much below average). This continues through to the last grouping of years, and temperatures between the top of the ninth grouping and the highest value would be decile 10 (or very much above average).
Historical decile maps do need to be recomputed from year-to-year, as addition of recent years changes the reference period of the full history of record, and perhaps some parts of a map that were highest on record in one historical period may have since been exceeded with the most recent data in the latest year. Due to the lengthy computation time required to re-run the historical decile analysis, the historical maps will not be updated in real-time.
Daily air temperature maps
At 1:30 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 (see the update schedule) over the following six months, as new data becomes available and as the data in the climate database are improved through quality control. 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)
| UTC | EST | EDT | CST | CDT | WST |
|---|---|---|---|---|---|
| 05:01 | 15:01 | 16:01 | 14:31 | 15:31 | 13:01 |
| 05:16 | 15:16 | 16:16 | 14:46 | 15:46 | 13:16 |
| 05:31 | 15:31 | 16:31 | 15:01 | 16:01 | 13:31 |
| 05:46 | 15:46 | 16:46 | 15:16 | 16:16 | 13:46 |
| 06:01 | 16:01 | 17:01 | 15:31 | 16:31 | 14:01 |
| 06:16 | 16:16 | 17:16 | 15:46 | 16:46 | 14:16 |
| 06:31 | 16:31 | 17:31 | 16:01 | 17:01 | 14:31 |
| 06:46 | 16:46 | 17:46 | 16:16 | 17:16 | 14:46 |
| 07:01 | 17:01 | 18:01 | 16:31 | 17:31 | 15:01 |
| 07:16 | 17:16 | 18:16 | 16:46 | 17:46 | 15:16 |
| 07:31 | 17:31 | 18:31 | 17:01 | 18:01 | 15:31 |
| 07:46 | 17:46 | 18:46 | 17:16 | 18:16 | 15:46 |
| 08:01 | 18:01 | 19:01 | 17:31 | 18:31 | 16:01 |
| 08:16 | 18:16 | 19:16 | 17:46 | 18:46 | 16:16 |
| 08:31 | 18:31 | 19:31 | 18:01 | 19:01 | 16:31 |
| 08:46 | 18:46 | 19:46 | 18:16 | 19:16 | 16:46 |
| 09:01 | 19:01 | 20:01 | 18:31 | 19:31 | 17:01 |
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.
Analysis technique
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.
Map formats
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.
Map Projections
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).
| Region | Aus. | Qld | NSW | Vic. | Tas. | SA | WA | NT |
|---|---|---|---|---|---|---|---|---|
| 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.
Grid format
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.
Quality control
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.
