Daily Air Temperature

Disclaimer: This page is a copy of the record for the BOM AGCD dataset once held by TERN AusCover. It may contain outdated information. Users should verify any key information with the BOM.

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Overview

These datasets are gridded analyses of daily air temperatures from the Bureau of Meteorology's thermometer network across Australia.

Key specifications

Background

What is Temperature?

According to the American Meteorological Society's https://glossary.ametsoc.org/wiki/Temperature, 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 Data

Temperatures are measured directly at about 750 sites across the country, and stored in the climate database maintained by the National Climate Centre. Maximum and minimum temperatures for 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. Air temperature is measured in a shaded enclosure (most often a Stevenson Screen) at a height of approximately 1.2 m above the ground.

At about 1:30 am each day, the daily maximum/minimum air temperature values from sites across the country for the previous day are analysed onto a 0.05° x 05° grid. The Bureau of Meteorology air temperature products are progressively updated over the following six months, as new data become available and as the data in the Bureau of Meteorology climate database are improved through quality control. Hence, the highest quality products are a least six months old because subsequent versions will tend to be more accurate, as they will be based on larger quality-controlled input datasets and will contain more data from non-real-time reporting sites. For Air Temperature AusCover provides the data after 3 months worth of updates, to ensure stable values and good quality, and so the most recent data is 3 months before the present.

Algorithm

The analyses (grids) are computer generated using a sophisticated analysis technique described in Jones et al. (2009). It incorporates an optimised Barnes successive correction technique that applies a weighted averaging process to the station data. The meteorological variable being analysed is decomposed into a long-term average (climatological) component and an anomaly component.

Quality

Quality and limitations

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.

The observational (station) data on which the analyses were based have an associated positional accuracy of the order of 0.01° (approximately 1 km) or better.

The grid-point analysis technique provides an objective average for each grid square and enables useful estimates in data-sparse areas such as central Australia. The size of the grid is limited by the data density across Australia. However, in data-rich areas such as southeast Australia or in regions with strong gradients, "data smoothing" will occur resulting in grid-point values that may differ slightly from the exact air temperature measured at the contributing stations.

The highest quality products are a least six months old because the temperature products are progressively updated over the following six months after the observation date. Temperature site stations can take time to report, hence, as new data become available the product quality control improves.

Data completeness

Temporal coverage is complete.

The grids are derived only from land-based observations. Although the entire grid domain is populated with values, over ocean the values are valid only up to one or two grid cells beyond the coast.

Validation

The accuracy of the spatial analyses has been determined through a cross-validation procedure which repeatedly deleted 5% of the stations at a time and calculated the errors in an analysis of the remaining stations. The daily maximum/minimum air temperature values have a root mean square error of 1.2^o/1.7^o. Details are in Jones et al. (2009).

Product variants

Gridded air temperature data are available back to 1900 from the Bureau of Meteorology's Temperature web page.

Maximum/Minimum/Mean Temperature Anomaly

The anomaly data are the departure from the long-term climate average calculated over the period 1971-2000. The daily anomalies are calculated with respect to the monthly average for the relevant month.

Highest Maximum/Minimum Temperature

The highest (by month and overall) maximum/minimum air temperature observed at the site.

Lowest Maximum/Minimum Temperature

The lowest (by month and overall) maximum/minimum air temperature observed at the site.

Mean Anomaly

Anomalies denote the departure of an element from its long-period average value for the location concerned.

Mean Temperature

The average daily air temperature, for each month, 3 months, 6 months and as an annual statistic, calculated over all years of record.

Maximum/Minimum/Mean Decile

Monthly, 3 month, 6 month or annual decile 1 (10th percentile) or 9 (90th percentile) of maximum or minimum air temperature.

9am-3pm Temperature

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. The highest temperature recorded at stations between 9am and 3pm is a good guide to the maximum temperature recorded for a day.

Other specifications