Daily Air Water Vapour Pressure

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 It may contain outdated information.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 water vapour pressure from the Bureau of Meteorology's thermometer network across Australia.

Key specifications

Background

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 data

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 observations. 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 a 0.05° x 0.05° grid. The Bureau of Meteorology vapour pressure 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 vapour pressure TERN 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^o (approximately 1km) 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 vapour pressure measured at the contributing stations.

The highest quality products are a least six month old because the vapour pressure products are progressively updated over the following six months after the observation date. Thermometer 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 9am/3pm vapour pressure values have a root mean square error of 1.8 hPa / 2.5 hPa. Details are in Jones et al. (2009).

Product variants

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

9am/3pm Vapour Pressure 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.

Other specifications