What makes it rain?

Most significant rainfall events in New South Wales involve air that has been brought in over the state from at least one of six major influences:

• monsoons and cyclones in the tropical regions;
• north-west cloud bands from the northern Indian Ocean;
• 30-50 day oscillation cloud bands;
• cold fronts pushing up from the Antarctic;
• trade winds coming across the Pacific onto north-eastern Australia;
• on the east coastal strip, high pressure systems in the Tasman sea bringing moist easterlies onto the coast.

A good source of moisture and a mechanism to force the air to cool are necessary for precipitation to occur.