Tropical Meteorology

I soon learnt that there was a diurnal rhythm in the weather which was mostly replicated from day to day. The inland ranges tended to be clear of cloud soon after sunrise and remain so until about 10 am when small cumulus cloud would appear. With the base of the cumulus clouds remaining low in the valleys the cloud tops would rapidly build in height. When we spoke to crews of higher flying aircraft in later years we were to learn that by early afternoon cloud tops could grow to well over 30,000 feet (9km). Still later, after we were able to analyse radiosonde observations, we realised that the tropopause was the limit of the height of the tops of the larger towering cumulus. While the height of the tropopause was about 30,000 feet (9km) in the higher Australian latitudes, in the tropics it was generally over 50,000 feet (15km). These towering cumulus, which produced torrential rain, moved with the upper wind and were often blown over the coast in the late afternoon. With the approach of sunset cumulus activity would subside but by then middle and high level stratiform cloud had spread from the cumulus and often remained overnight before dispersing. In the evening, the land breeze driven by the cooling air over the mountain ranges often resulted in some cumulus activity over the sea.

I began to use the term 'predominantly diurnal' in my forecasts to describe this weather sequence and received that nickname from some of the RAAF pilots. Of course, this diurnal pattern was interrupted from time to time by the passage of bands of cloud and rain, particularly in the north-west season. Any coastline athwart the flow of the prevailing wind, such as the south coast of New Britain and the coasts of Milne Bay and the Huon Gulf in the south-east season, was shrouded in a semi-permanent band of cloud and rain.

Bureau of Meteorology