North Island (Houtman Abrolhos) - Climate

Climate

As of 2007 the Australian Bureau of Meteorology has not published climatic data for North Island, but an automatic weather station has been installed on the 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. This is the only weather station in the Houtman Abrolhos, so its data underlies climatic models of the island chain as a whole. Based on the data for North Island, the Houtman Abrolhos has been described as having a Mediterranean climate with warm, dry summers and cooler, wet winters. Mean temperatures range from 9.3 to 19.5 °C (49 to 67 °F) in July, and from 19.1 to 32.4 °C (66 to 90 °F) in February. This is 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, in particular the Leeuwin Current.

Eighty-six percent of the island's rain falls between April and September; on average there are 89 rain days, resulting in 469 mm (18 in) of rain. The wettest month is June, when over 100 mm (4 in) typically falls. In contrast, only about 70 mm (3 in) 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 kn (31 km/h) almost half the time. During autumn and winter, the ridge moves north, increasing atmospheric pressure over the islands and creating 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; it is not as strong in the Houtman Abrolhos chain as on the mainland, but is present.

Three classes of storm have been identified in 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 m/s (115 ft/s), the wind direction from the northwest initially, then gradually moving around to southerly.

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