In April the coastal winds are practically all S.S.E. or E.S.E. over this immense stretch of coastline, the only marked deviation being the occurrence of some S.W. winds (20 per cent. only) off Kimberley. In May, the belt has moved north to Geraldton, but is otherwise unchanged. In June it does not extend south of North-west Cape, but is stronger off Kimberley. In July, August, September, and October, it is practically continuous from North-west Cape to Rockhampton.
In November, as the sun moves south, the belt extends down to Geraldton.
In December the Australian monsoon causes a break from North-west Cape to the Gulf of Carpentaria (dominated by west winds), and the South-east Trade forms a fringe on each side. This hiatus lasts through January and February, and in the last month the South-east Trade is hardly distinguishable on the West Coast. In March, the intervening region of S.W. winds is confined to the north-west coast, and the trades are again in evidence south to Perth.
In April the whole north-west, north, and north-east coasts are controlled by the trade winds.
Land and Sea Breezes.
Since so large a portion of the population of Australia lives on the seaboard, these winds are of great importance. Data are lacking except at the capitals, but type examples from Perth and Adelaide will serve to indicate the effect of the diurnal change.
In Fig. 36 the winds throughout the day are averaged for each month for Perth. (Data for all years to the end of 1911.)
In the first column the average for the month is given. (Here we tend to get the diurnal extremes neutralizing each other so that the normal wind of the region is shown.)
Regional Winds at Perth—
| South wind | In October, November, December, and January | Summer |
| South-east | In February, March, and April | Autumn |
| North-east | In May, June, and July | Winter |
| South-west | In August, September | Spring |
In this locality the invigorating sea-breeze—called the "Doctor"—blows from the W.S.W. during the afternoon, while a land wind (E.N.E.) sets in when the land has cooled below sea temperatures in the early morning.
The sea-breeze is most active when the land is much warmer than the sea, i.e., in the summer. The land breeze is most active for similar reasons in winter.
Thus we see that in winter the two factors (the normal north-east wind and the land breeze) are assisting each other. Hence the constant N.E. winds every morning during this season. For the rest of the year the morning winds are from the south-east, the resultant of the southerly regional wind and the N.E. land breeze.
The sea breeze absolutely controls the wind direction at Perth in the afternoon, even in winter, and in the summer not much deflection of the normal south wind is necessary to bring it into line.
