Mapping the frost risk in WA  

Frost is a significant risk to winter crops over much of Australia, particularly in spring, when they are at their most susceptible stages of flowering and grain filling.

The historical incidence of frost varies strongly across the agricultural regions of Western Australia, with greatest occurrence in the central, eastern and southern regions. Northern and coastal regions in general have a lower risk.

Although rainfall is recorded on a fairly dense network throughout southern Wa, the density of temperature observations traditionally available has been much poorer.

The Department of Agriculture WA (DAWA) has been establishing and expanding its network of weather stations since the early 1980s. This has improved the spatial coverage of temperature observations and now offers an opportunity to study the regional variation of frost risk in greater detail.

A series of maps have been produced that show the average monthly occurrence of low temperature events over the agricultural region of WA during July to October. The maps use data from Bureau of Meteorology observing stations and the DAWA weather stations covering the past 15 years.

They do not show the actual observation of frost, but use the occurrence of a minimum temperature 2°C or less as an indicator of the likelihood of frost. Air temperature is recorded at a height of 1.25 metres, so temperatures of around 2°C generally equate to 0°C or less at ground level if there is minimal air movement.

In general, the maps indicate that the greatest risk from frost occurs in the central and eastern agricultural regions, with a progressive decrease in risk northwards and near the coast. Care needs to be taken in using the maps, as small-scale variations are not shown, such as those on a farm scale.

This year there have been several events of very low temperatures in July, August and September. A significant event occurred on 10 September, with many locations experiencing temperatures below 0°C.

DAWA now has the ability to generate maps of recent events using a combination of Bureau of Meteorology and its own observations. The daily minimum temperature data is available from DAWA"s website, www.agric.wa.gov.au/climate, and from the Bureau of Meteorology"s 9am Bulletin, www.bom.gov.au/weather/wa/observations.shtml.

The number of hours the temperature was below 2°C is also available on the DAWA website. The Department of Land Information provides a daily land surface temperature map at www.rss.dola.wa.gov.au/noaalst/.

For more information:
Dr Ian Foster, DAWA, 08 9368 3642, ifoster@agric.wa.gov.au
Nicolyn Short, DAWA, 08 9368 3448, nshort@agric.wa.gov.au

Region National, North, South, West