Seasonal Climate Outlook Message for April to July 2004

The bottom line

Frost Update 20/04/04

With the arrival of cooler weather and the winter cropping season, many producers are seeking information on the likely length and severity of the frost season.

Frost occurs with a combination of still cold air, clear skies and a dry atmosphere. In Australia, these conditions usually occur when our weather is dominated by high pressure systems during winter. The clear skies and dry atmosphere usually associated with high pressure systems allow the heat generated during the day to escape at night.

The clear skies associated with a negative Southern Oscillation Index (SOI) pattern increase the chance of early and late frosts and the number of frosts. Years with a positive SOI pattern are associated with more moisture and cloud and are more likely to have a shorter frost season and fewer than normal frosts.

The SOI phase in January/February and April/May and historical frost records are used when developing frost forecasts. At the end of February this year, the SOI was in a "Rapidly Rising" phase.

Using a severe minus 2oC frost season at Roma for this example, (and keeping in mind that ground temperatures will be even lower) a "Rapidly Rising" SOI phase at the end of February means that there has historically been a later average start and earlier average finish to the severe frost season at Roma.

However a "Negative" SOI phase at the end of May will increase the chance of an earlier than average start and later than average finish (mid to late August) to the severe frost season at Roma.

Given the recent fall in value of the SOI there is the potential that a "Negative" SOI pattern will exist at the end of May.

A number of local features also impact on the likelihood of frosts. Frosts are more likely to occur in the bottom of valleys because cold dense air drains downhill. Black soils lose heat at night more quickly than lighter coloured soils and so tend to be colder. Plant cover will reduce the heat lose from soils, but plants tend to lose more heat than bare soil. So with bare soil the coldest air will be found just above the soil but with a good plant cover the coldest air will be found just above the plant level.

Some of the decisions affected by frost risk include whether to plant on frost prone sites, the selection of crop varieties, the timing of planting, choosing to insure against damage and the adjustment of stock numbers etc.

The 30day average of the SOI as the 20th of April is minus 23.7. For more information call me through the DPI Call Centre on 132523.