Bass Strait and Port Phillip Bay lie on the boundary between the westerly-wind belt of the southern hemisphere and the subtropical trade-wind belt. In summer, the trade-wind belt moves southward with the thermal equator and the belt of variable winds is also moved further south. Hence, the path of the pressure systems across Australia is further south in summer than in winter and, as the land areas heat and become low-pressure areas, winds near the earth’s surface blow mainly from sea to land. Conversely, in winter, the land is covered by high pressure, and the winds blow mainly outward from land to sea. The impact of these systems on the study area is that in winter, June to August, the most prevalent winds are northerlies; while in summer, December to February, southerly winds, south east to south westerly are prevalent. Spring and autumn represent a transition between the two dominant seasons and the wind patterns are a mixture of the summer and winter patterns. Historical wind data has been obtained from the anemometer at Point Wilson Jetty, from 1991 to 2010. Annual and seasonal-average wind roses have been derived from the data, Figure 3-2. The same data are presented as frequency of occurrence in Table 3-3. The strongest winds are from the north and west. The most frequently occurring wind directions are the north, west and south.

Wind is a particularly important component within the Port Phillip Bay area as waves are fetch-limited, meaning they are generated by local winds and the wave height is limited by the distance over water that the wind can blow, i.e. the fetch.

Figure 3-2     Point Wilson wind speeds and directions - overall and seasonally (1991-2010, BOM)

Table 3-3      Wind matrix showing percentage occurrence of speed and direction (1991-2010, BOM)