Frost is a major risk to vegetable producers. It might be thought that one benefit of increases in the minimum temperature would be to reduce the chance of frost.
Researchers at the Managing Climate Variability have found that the period during which frosts can occur – the frost window – has widened over the last 20 years, particularly in the eastern parts of Australia. This is despite an overall increase in mean minimum temperatures.
The analyses have revealed that in the east the frost window is starting up to 10 days earlier and ending up to 46 days later.
This means that frost will be MORE likely to occur in the future: the risk period will start earlier and finish later.
The pattern of later endings is consistent across much of southern Australia, whereas the earlier starts are more localised to western New South Wales and northern Victoria.
The frequency of extreme cold temperatures is also on the rise across much of southern New South Wales and northern Victoria. An average increase of four frost days and five cold nights each decade has been identified since 1970.
The changes in the frost window, despite increases in mean temperature, are consistent with the expected increases in climate variability. This is due to changes in regional weather patterns. The trend is that the band of high pressure, which normally sits across southern Australia has moved further south and intensified, allowing cold polar air to move onto the continent following the passage of any cold fronts.