The big bust—southerly busters explained!
30 October 2011
A 'southerly buster' (or 'southerly burster') is the term Sydneysiders have adopted for an abrupt southerly change that can charge up the New South Wales coast, mostly between October and February. Read on to find out more about this weather phenomenon!
What are southerly busters?
Contrary to popular opinion, this fast-moving southerly change is not unique to New South Wales (Tasmania, New Zealand and Argentina experience similar conditions), however in southeast Australia the proximity of the Great Dividing Range to the coast helps create these particularly fierce winds—often gusting well over 60 km/h.
Varying definitions of the southerly buster exist; we adopt the criteria of southerly winds gusting in excess of 29 knots (54 km/h) and a three-hour temperature drop of at least five degrees (during a 2010 study).They travel from the south coast of NSW to the mid-north coast, generally reaching their maximum intensity between Nowra and Newcastle.
Sydney receives an average of about five southerly busters a year, with the stronger busters usually reaching Sydney in the late afternoon or early evening after several days of hot weather. Temperature changes can be dramatic, with falls of 10–15 °C often occurring in less than one hour.
The leading edge of a buster is sometimes marked by a spectacular roll cloud running perpendicular to the coast—known as an ominous portent among Sydney's yachting community.
However, on most occasions there is initially little or no cloud, and consequently little visual indicator of the buster's onset.
How do they form?
Southerly busters typically form when a cold front passes over southeast Australia and cool air becomes trapped against the mountains of the Great Dividing Range, often in the Gippsland area of Victoria.
While the inland part of the cold front is trapped against the mountains, the part over the sea continues to move along the coast—distorting the front into an 'S' shape. This process continues on the southern New South Wales coast, while areas ahead of the front are experiencing hot northerly or westerly winds.
The phenomenon moves north as a shallow 'density current', which means the air behind the southerly buster is cool and dense in comparison to the hotter conditions ahead.
The speed at which the southerly change moves is related to the depth of the cool layer of air, as well as the density contrast between the areas of cool and warm air. The process is further assisted by the presence of the Great Dividing Range, which creates a channelling effect as the southerly winds move along the New South Wales coast. In the right conditions winds will accelerate rapidly, surging north with increasing wind gusts as the buster approaches Sydney.
Image: Southerly buster at Turrimeta Beach, 26 December 1996. Credit: John Grainger
Size and strength
A typical southerly buster is between 20 and 60 nautical miles wide, with the strongest winds centred on the coastal strip. The depth of the buster is limited by the height of the mountains to west, which are mostly less than 1000 m high. Above this height westerly winds generally prevail.
Southerly busters have the strongest winds at their leading edge. Particularly strong busters have peak wind gusts of 40 knots (74 km/hour) or more, and usually occur after very hot days. Gusty conditions tend to ease significantly within 30 to 60 minutes of the buster arriving, and in many cases winds become quite light within a few hours.
The strongest southerly buster on record at Sydney Airport occurred at 6.40 pm on 18 December 1948, with a maximum gust of 61 knots (113 km/h). The strongest gust recorded anywhere along the NSW coast was 70 knots (130 km/h) at Port Kembla on 20 November 1973.
As busters rarely maintain a constant speed while progressing along the coast, forecasting their arrival time at Sydney can be quite challenging. Due to their strength, it is common for us to issue a wind warning for parts of the coastal waters, and on occasions we may issue a Severe Weather Warning for land areas along the coastal fringe.
"Winds WNW and light, but brief ENE winds over midday hours; reverted to fresh and gusty westerly until 18:20 whence a spectacular smoke laden southerly burster arrived with many gusts over 50 mph." Sydney Journal, 18 Dec 1948
Big busters in history
The following southerly busters brought the strongest winds to Sydney.
- 1942 Two notable southerlybBusters were recorded in 1942. The first gusted to 104 km/h at 4.00 pm on 25 January 1942 and was followed by severe thunderstorms. The second recorded a gust of 109 km/h at 4.40 pm on 4 October.
- 1943 Significant southerly busters in 1943 struck on 23 March (gust of 95 km/h at 3.40 pm) and 28 March (gust of 108 km/h at 9.50 pm).
- 1945 On 9 November 1945, a southerly buster gust of 105 km/h occurred at 6.10 pm, accompanied by thunderstorms.
- 1946 The largest temperature drop associated with a southerly buster occurred on November 6, when the temperature changed from 39.1°C at 4.00 pm to 19°C at 5.00 pm. (This buster also brought a maximum wind gust of 91 km/h.)
- 1948 The strongest southerly buster on record at Sydney Airport hit at 6.40 pm on 18 December 1948 with a maximum wind gust of 113 km/h. The maximum temperature that day was 34.8°C.
- 1957 After a maximum temperature of 40.4°C on 2 December 1957, a squally southerly buster reached Sydney at 5.30 pm, with a peak gust of 104 km/h.
- 1958 Wind gusts in a southerly buster reached 100 km/h at 5.10 pm on 10 January 1958.
- 1973 Notable southerly busters in 1973 struck on 6 February (maximum gust 98 km/h) after a maximum temperature that day of 39.7°C, and on 20 November (gust of 96 km/h) after a maximum of 38.4°C.
- 1982 A southerly buster on 25 November 1982 was accompanied by dust storms and preceded by a record November temperature of 41.8°C in Sydney. The maximum wind gust at Sydney Airport was 89 km/h at 4.10 pm.
- 1994 A very late southerly buster arrived at 2.00 am on 29 December 1994, with a peak gust of 98 km/h at 3.09 am.
- 2001 On 15 January 2001, a southerly buster with a maximum gust of 100 km/h caused blackouts, property damage and uprooted trees. Temperatures reached 46°C at Penrith. This was the hottest day in Sydney's western suburbs since 1939.
- 2003 The second strongest southerly buster on record reached a maximum gust of 109 km/h at 5.22 pm on 8 January 2003. Thunderstorms with small hail followed the change and 70 suburbs lost power after lightning strikes.
- 2004 A southerly buster with a maximum gust of 91 km/h reached Sydney at 4.30 pm on 4 December 2004, after a maximum temperature of 41.8°C.
- 2006 On 1 January 2006, a southerly buster reached Sydney at 8.17 pm with a maximum gust of 95 km/h. The maximum temperature at Observatory Hill that day was 45.2°C, the second highest on record.