Climate
As of 2012, the Australian Bureau of Meteorology has not published climatic data for the Houtman Abrolhos. However, an automatic weather station has been installed on North Island since 1990, and hourly measures of precipitation, air temperature, wind speed, wind direction, relative humidity and atmospheric pressure have been publicly available since then. Based on this and other data, researchers have put together a picture of the Abrolhos' climate.
The islands have a Mediterranean climate, with warm dry summers and cooler, wet winters. Mean temperatures range from 9.3 °C to 19.5 °C in July, and from 19.1 °C to 32.4 °C in February. These temperatures have a substantially smaller range than on the mainland: the summer temperature is typically a degree cooler, while winter temperatures are a good deal warmer. This is due to the influence of the ocean, and to the Leeuwin Current.
86% of the rain falls between April and September; on average there are 89 raindays, resulting in 469 millimetres of rain. The wettest month is June, when over 100 millimetres typically falls. In contrast, only about 70 millimetres can be expected to fall between October and March.
It is nearly always windy. During summer a high pressure ridge lies to the south, causing persistent winds from the southeast or southwest, at speeds exceeding 17 knots (31 km/h) almost half the time. During autumn and winter, the ridge moves northwards, increasing the atmospheric pressure over the islands, resulting in highly variable winds. Winter tends to produce both the strongest gales and the most frequent periods of calm.
In addition to these winds, there is daily pattern of land breezes in the morning, followed by the onset of south-westerly sea breezes in the afternoon. This pattern is caused by temperature differences between the land and the ocean, and is not as strong in the Houtman Abrolhos as on the mainland, but is present nonetheless.
Three classes of storm have been identified for the region. Brief squalls may occur between December and April. A tropical cyclone occurs in the area about once in three years, between January and April; these may generate extremely high wind speeds that are potentially destructive. During winter, extra-tropical cyclones sometimes pass south of Geraldton, generating winter gales with gusts of up to 35 metres per second, the wind direction from the northwest initially, then gradually moving around to southerly.
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