Southerly (also known as Southerly Buster) is the name of a storm or front of air coming from the south.
Contents
In the southern hemisphere these can be cold and have bad weather.
Pacific region
In Melbourne and Sydney, Australia these events are known as southerly busters.
In Wellington, New Zealand these storms are normally short and frequently have winds gusting between 120 km/h and 160 km/h though higher speeds are known.
History
On hot afternoons in the late nineteenth century, before the days of radio, the Sydney Observatory would fly a flag bearing the letters "JB" to indicate that the southerly buster had reached Jervis Bay, about 150 km to the south as the crow flies, and Henry Lawson even wrote a poem on the subject.
Cause
On sunny days in the Sydney summer, the land heats up rapidly during the morning, while sea temperatures remain cool, typically 22 to 25 °C (72 to 77 °F). The prevailing early morning wind is a light south-west offshore breeze (a katabatic wind) that blows from land to sea, but as the land heats up a north-east convection wind develops. This is a typical sea breeze. It starts shortly after sunrise on the coast and gradually pushes inland as the day proceeds, typically reaching the City by mid to late morning and the Western Suburbs by early to mid afternoon. Frequently, a strong offsea gale develops from the south, bringing a rapid fall in temperature, and sometimes a short, violent rain/hail storm.
Other fronts
Sometimes, a strong cold front approaches from the south or south-west, marking the boundary between hot and cool air masses. The temperature is dramatically dropped by a violent storm-laden southerly wind, and it is this that is known as a "southerly buster".
Black nor'easter
A more persistent and potentially violent Sydney north-easterly storm is known as a "black nor'easter". This is not a convection wind, but a storm system that develops offshore which can last several days. This is heralded by the rapid build-up of dense black cloud that can convert to a gale in well under one hour.