Climate of Geraldton |
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Introduction
Local and Broadscale Influences The climate of the region is strongly influenced by a band of high pressure known as the sub-tropical ridge, and in the warmer months by the 'West Coast Trough', an area of low pressure that extends from the semi-permanent heat low in the tropics. For much of the year the ridge is located to the south of Geraldton, allowing winds with an easterly component to prevail, although south to southwesterly afternoon sea breezes are a common spring, summer and autumn phenomenon. Occasionally during the cooler months the ridge moves north allowing cold fronts to pass over the west coast and deliver much of Geraldton's annual rainfall. Rainfall The mean annual rainfall at Geraldton Airport, since records commenced in 1941, is almost 460 millimetres, though the annual total decreases with decreasing elevation near the coast. The city records rainfall on an average of approximately 80 days annually. As over most of southwest WA, annual rainfall has decreased in Geraldton during the latter half of the twentieth century. Evidence suggests that both natural variability and the enhanced greenhouse effect have most likely contributed to this decrease. June is the wettest month, with a long-term average over 100 mm at the airport, whilst rain occurs on a little over two days out of every five during an average winter. Approximately three quarters of the annual total falls between May and October, on average, mostly as a result of the passage of cold fronts. Sometimes these fronts are associated with cloud bands from the northwest, which enhance the totals. The driest months are December and January with means of just over 5 mm. In summer it rains on about four days over the three months, on average. Rainfall in summer is often associated with thunderstorms, which can occasionally produce heavy localised falls in short periods. Although rare, tropical lows or weakening tropical cyclones, that usually originate off the northwest coast of Western Australia, can bring widespread rain to the region. Temperature Average maximum temperatures peak in February in Geraldton, with a monthly mean of about 30°C on the coast and just over 32°C at the airport. Regular sea breezes moderate the climate in the warmer months, however hot dry northeasterly winds from the interior of WA can sometimes dominate. The temperature at Geraldton Airport reaches 40°C about nine times in the average summer, however the frequency decreases closer to the coast. Such hot days are usually followed by a cool change with fresh to strong southerly sea breezes as the West Coast trough moves inland and a new ridge moves south of WA. Overnight minima also peak in February at just over 19°C on average. In contrast, winters are mild with the July average maximum temperature being the lowest of any month at just under 20°C. August minima are the lowest on average at just over 10°C near the coast and decreasing to about 9°C at the airport. Due to the position of the sub-tropical ridge during winter, winds with an onshore component are common and along with increased cloudiness help to moderate temperatures. Overnight temperatures fall to 5°C at the airport approximately seven times during an average year, though this frequency decreases closer to the coast. Temperatures below 2°C are rare. Wind The wind climatology at Geraldton is strongly dominated by the effects of the land-sea interface where offshore land breezes are common in the morning, whilst afternoon sea breezes are common in the warmer months. Winter tends to be the period of most variability in winds due to the latitude and mobility of the sub-tropical ridge, and a weak land-sea temperature contrast. It also tends to be the season with the lightest winds, however cold fronts can occasionally bring strong winds and gales to the area. During the remainder of the year, the sub-tropical ridge is generally south of Geraldton and thus winds with an easterly component prevail overnight and in the morning, however afternoon sea breezes are regularly experienced, most commonly from the south to southwest sector. Summer sea breezes are frequently quite fresh and often reach 25 knots (46 km/h) or more near the coast. Sunshine Geraldton is one of the sunniest locations in Australia in January with over 11 hours of sunshine per day on average. This value reduces to a minimum in June and July of between 5 and 6 hours per day. The annual mean is between 8 and 9 hours per day. Evaporation The average annual pan evaporation at Geraldton Airport is just under 2460 mm and the mean daily figure ranges from just under 3 mm in July to almost 11 mm in January and February. Daily pan evaporation can range from zero on a cold wet winter's day to over 20 mm on a very hot, blustery summer's day. |