The presentation of wind data creates one of the major challenges of this presentation, because in some way or another seasonal aspects need be considered. The information on three bands of most frequent speeds, most frequent directions and periods with speeds in excess of 1 knot (~ 50 cm/sec) in terms of percentages of total numbers of observations during January, April, July and October has been extracted from speed-direction histograms of observations taken over periods ranging from 2 - 17 years.
The basic histograms subdivided speeds into the seven groups of 1-2, 3-6, 7-10, 11-15, 16-21, 22-26, 27 and more knots and 8 direction sectors. Each entry on each band comprises two rays, corresponding to observations at 0900 and 1500 hrs, respectively, the morning readings following the afternoon readings in the clockwise direction. The ray lengths indicate the percentages of days during the month in question over which directions of speeds fell into the sectors of the speed bands indicated by letters or numbers. In this context, speed bands have been indicated by their upper limits unless neighbouring bands contained almost equal percentages, when the rays represent combined percentages and the lower bound of the upper speed band is given. The entries in the third bands were computed by subtraction of percentages corresponding to calm periods from 100%.
Apart from geostrophic winds associated with weather systems, two basic processes influence Australia's wind patterns: diurnal land and sea breezes and seasonal continental and ocean winds. The former, acting at right angles to the coastline, are very pronounced for large portions of the year and account largely for the differences in the percentages corresponding to morning and afternoon observations and for the changes in speed and direction which take place from the morning to the afternoon. They are due to local temperature differences between land and sea.
Less pronounced, but equally important, are the seasonal variations in speed and direction which arise from the temperature differential between Australia as a whole and the surrounding ocean. In Summer, air masses over the continent rise and are replaced by air overlying the ocean. In Winter, air masses rise over the ocean and are replaced by air moving from the interior of the continent towards the coast. Such large-scale motions of air are, of course, also subject to the direction-changing mechanism of the rotation of the Earth, just as the winds associated with moving weather systems. Therefore the air, as it moves across the coast, does not necessarily appear to originate from Australia's centre.On the Southern hemisphere, the air tends to veer to the left of the direction in which it is flowing.
The following table presents the most frequent directions, extracted from the tables for six stations around Australia:
It shows that local breezes have a more important role in the South, where the seasons are more pronounced.