square kilometre, Sydney's urban area covers , comprising 35% of Sydney and is constantly growing. The south and southwest of Sydney is drained by the Georges River, flowing north from its source near Appin, towards Liverpool and then turning east towards Botany Bay. Minor waterways draining Sydney's western suburbs include South Creek and Eastern Creek, flowing into the Hawkesbury, and Prospect Creek draining into the Georges River. Cowan Creek and Berowra Creek run north from the Upper North Shore to the Hawkesbury river. Climate Western Sydney experiences a humid subtropical climate (Köppen climate classification: Cfa) with the annual temperatures having an average maximum of and a minimum of , making the region a few degrees warmer than the Sydney CBD. Maximum summer temperatures average at around and winter temperatures are mild, averaging at around , depending on the location. Autumn and spring are the transitional seasons, with spring showing a larger variation than autumn in terms of temperatures. Rainfall is almost evenly spread throughout the year, although the first few months tend to be wetter, namely February through to April. The months from July through to December tend to be drier (late winter through to early summers). Thunderstorms are common in late summer and early autumn. Winters are pleasantly cool and relatively sunny (especially August), although east coast lows can bring large amounts of rainfall, especially in June. Most suburbs in the west have an annual precipitation that averages at around , in contrast to Sydney CBD's . Sydney usually experiences a föhn effect that originates from the Great Dividing Range, where the lifting of winds on the windward side of the Blue Mountains forces the air to gradually warm up and lose moisture as the winds descend into the Sydney basin. They may exacerbate fire danger in the warm months, although they usually tend to occur between late winter and early spring when westerly cold fronts become more frequent and would therefore be blocked by the ranges – This phenomenon thereby permits the late winter and early spring period to feature the highest amount of clear days in the year. It should be worth noting that Richmond features the greatest temperature range ever recorded in Australia; to . Summer Western Sydney is much warmer than Sydney city in summer. During this time, daytime temperatures can be warmer than the city (in extreme cases the West can even be hotter).