Frost can cause serious losses to agricultural crops and a number of climatic studies have been made in Australia related to specific crops cultivated in local areas. Under calm conditions the overnight temperatures at ground level are often as much as 6Co lower than those measured in the instrument screen which has a base height of 1.1 metres and differences of 10Co have been recorded.
Only a small number of stations measure minima at ground level, the lowest recordings being -15.1 degrees C at Canberra and -11.0 degrees C at Stanthorpe (Q). Lower readings may be recorded in Alpine areas.
Frost frequency depends on location and topography and even on minor variations in the contour of the land. The frost map of Australia is shown below:
FROST PERIOD - MEAN ANNUAL LENGTH DAYS PER ANNUM. A FROST DAY HAS A MINIMUM TEMPERATURE OF <= 2Co
Frost data shows:
The parts of Australia which are most subject to frost are the eastern uplands from northeastern Victoria to western Darling Downs in southern Queensland. Most stations in this region experience more than 10 nights a month with readings of 0 degree C or under for 3-5 months of the year.
On Tasmania's central plateau similar conditions occur for 3-6 months of the year.
Frosts may occur within a few kilometres of the coast except in the Northern Territory and most of the north Queensland coast.
Regions in which frosts may occur at any time of the year comprise most of Tasmania, large areas of the tablelands of NSW, much of inland Victoria, particularly the northeast and a small part of the extreme south west of WA.
Over most of the interior of the continent and on the highlands of Queensland as far north as the Atherton Plateau, frosts commence in April and end in September. Minimum temperatures below 0 degrees C are experienced in most of the subtropical interior in June and July.
The median frost period over the continent varies from 200 days per annum in the southeastern uplands area south of the Hunter Valley to 0 days in northern Australia
In the southern regions the annual frost period generally decreases from about 100 days inland to below 50 days towards the coast. However, there are appreciable spatial variations depending mainly on local topography.
In Tasmania the frost period exceeds 300 days on the uplands decreasing to 100 days near the coast.
The length of the frost period for the year is taken as the number of days between the first and last recording of an air temperature of 2 degrees C or less. The median duration of the frost period in days per annum is shown in the diagram above.
The frequency table below includes the average annual frequency of minima of 2.2Co or less for a wide selection of stations, particularly those prone to frosts:
THE AVERAGE ANNUAL NUMBER OF FROSTY NIGHTS
|ALTITUDE (M)||AVERAGE NUMBER
<=2.2 degrees C <= 0 degrees C
|AVERAGE NUMBER HEAVY FROSTS|
|Adelaide Airport 1956-85||6.0
| Ballan 1957-68
Brisbane Airport 1950-85
| Hobart 1943-85
| Kiandra 1957-68
|Mt Gambier Airport 1943-85|| 63.0
| Walgett 1957-84
The above table shows:
The high spatial variability of frost frequency across Australia shown by the southeastern alpine areas (Kiandra) have a frequency exceeding 200, Kalgoorlie 20.4, Alice Springs 32.7, Charleville 32,.3, Canberra 101.1 and Essendon airport 14.2.
The regions of mainland Australia most prone to heavy frosts are the eastern highlands and adjacent areas extending from Victoria, through NSW to southeastern Queensland.
Stations above 1 000 metres in altitude in the southern parts of the uplands have more than 100 heavy frosts annually.
In the uplands below 1 000 metres the annual frequency ranges from 100 to about 20.
Over the remainder of southern Queensland, NSW and Victoria although there are great variations, the average annual frequency of heavy frost typically ranges from about 20 inland to 10 towards the coast.
The Tasmanian uplands above 1 000 metres have more than 100 heavy frosts annually and in neighbouring areas the frequency is about 100, decreasing to 20 towards the coast. Even some coastal stations have a relatively high frequency for example, Swansea has 15.7.
The southern half of WA, the whole of SA and Alice Springs district of the Northern Territory experience heavy frosts.
Differences in annual frequencies between places are great but in general, the frequency is about 10 inland decreasing towards the coast. Some places average more than 20 heavy frosts annually for example, Wandering (WA) with 21.5, Yongala (SA) with 41.6.