Vapour Pressure Map Information

Vapour Pressure

These vapour pressure 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 vapour pressure across Australia as quickly as possible after the observations are received.

What is vapour pressure?

According to the American Meteorological Society's Glossary of Meteorology, vapour pressure is the pressure exerted by the molecules of a given vapour. For a pure, confined vapour, it is that vapour's pressure on the walls of its containing vessel; for a vapour mixed with other vapours or gases, it is that vapour's contribution to the total pressure (i.e., its partial pressure). In meteorology, vapour pressure is used almost exclusively to denote the partial pressure of water vapour in the atmosphere.

The water vapour pressure is directly related to the number of water vapour molecules in the air. In Australia, the air-masses with the highest water vapour content are tropical in origin, while those with the lowest are sourced from the arid interior or the high latitudes of the Southern Ocean during cold outbreaks.

Humans experience the sensation of "humidity" when the vapour pressure reaches around 18 to 20 hPa, allowing for individual tolerances and acclimatisation to local conditions. Furthermore, the air can feel "humid" despite the fact that the relative humidity doesn't convey it. For example, at 3 pm on an average January day in Broome, the relative humidity would be about 66%, but the vapour pressure would be around 30 hPa. Conversely, on a cool, foggy morning in Hobart with a temperature of, say, 5°C, the relative humidity will be 100% but the vapour pressure would only be around 9 hPa.

Daily vapour pressure maps

At about 3:30 am each day, the 9 am and 3 pm vapour pressure values for the previous day are analysed, using dewpoint temperature data with very limited quality control.

The vapour pressure (in hectopascals) is calculated from the dewpoint temperature (in degrees Celsius), via the equation

vapour pressure = exp (1.8096 + (17.269425 * dewpoint)/(237.3 + dewpoint))

Dewpoint temperatures are measured directly at about 750 sites across the country, and stored in the climate database maintained by the National Climate Centre. 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.

Monthly and multiple-monthly vapour pressure maps

Monthly (9 am or 3 pm) vapour pressures are calculated as the averages of the corresponding daily (9 am or 3 pm) vapour pressures.

The latest vapour pressure 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 1971-2000, 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 vapour pressure anomalies, with excessive rainfall coinciding with above average vapour pressure 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 grid point 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 grid point 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 vapour pressures measured at the contributing stations.

Map formats

Most of these vapour pressure 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. PDF version 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 from the National Climate Centre (webclim@bom.gov.au), but charges may be imposed for their provision.

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.