Climate

Tropical and sub-tropical climate

The North East Queensland region of the tropical savannas has high rainfall summers and drier winters and lies in a tropical climatic zone with its southern extent adjacent to the sub-tropics (Bureau of Meteorology 1989). It extends inland from the coast but does not include the higher rainfall sectors of the coastline where savanna gives way to rainforest.

Floods are an occupational hazard of northern summers Photo: Greg Calvert

Average temperature

Hot, summer days in January reach an average temperature of 33^oC to 36^oC. Pockets of cooler conditions are experienced in elevated inland areas as well as along the coast. Relative humidity decreases away from the coast ranging from 30-70 per cent, which varies little from winter values. The combination of on-shore winds, humidity and lower temperatures along the coast create an average of only 50 climatic discomfort days each year but inland this increases to around 150 (Colls & Whitaker 1990).

Intertropical Convergence Zone

The Intertropical Convergence Zone (ITCZ) stretches across north-east Queensland in summer so this region's warm and moist conditions originate from both the north-west monsoons and the easterly quasi-monsoons (Tapper & Hurry 1993). Unstable conditions, with heavy cloud and intense rainfall, are associated with the ITCZ but the average cloud cover for this region in January is considerably less (Colls & Whitaker 1990) than for each of the other tropical savanna regions. Thunderstorms are more prevalent inland with an average of 60 thunder days a year with coastal areas only experiencing as few as 10 (Bureau of Meteorology 1989).

Rainfall

Rainfall is more intense in summer than winter and extensive falls can occur in association with the passage of tropical cyclones across the coast (Gentilli 1972). Annual average totals diminish from north to south and with distance from the coast and range from 1200 mm to as low as 400 mm in the south-west. Drought conditions occur more frequently inland but the whole area has a moderate to high variability in rainfall (Bureau of Meteorology 1989). During La Nina events rainfall increases across the region in an east-west direction (Partridge 1991).

Winter

The long, drier winters are dominated by warm, moist south-east trade winds, which maintain the region's relative humidity. The days are clear and sunny with temperatures decreasing from north to south and towards the coast. In July the average overnight minimum temperatures are between 9^oC and 18^oC (Bureau of Meteorology 1989) and in elevated areas frosts can occur (Colls & Whitaker 1990). Winter rains are sporadic in the sub-tropical region and are most likely to occur in the east (Gentilli 1972).