By 11:45am, Sydney was sweating in 40-degree heat for the second time this month, a feat only seen in January four times during the last 107 years. Richmond had already reached 40 degrees by 11:00am and further south, Wollongong sweltered to 43. Penrith registered 44 degrees before midday, making it the hottest day since 2006. The hottest temperature ever recorded at Sydney Observatory Hill was 45.3 degrees in January 1939. Sydneysiders may find this type of heat unusual. Last summer only saw the mercury reach a top of 33.4 degrees and recorded just two days over 30. On average, the city typically reaches 40 degrees once every three summers, although this year has seen an unusually hot start. Reduced cloud cover over central and western Australia during the past few weeks has allowed a very hot air mass to build. This heat has periodically made its way towards the nation's coast due to the passage of low pressure troughs across the nation's south. Those braving the heat at Big Day Out in Homebush will need plenty of water. The temperature just after midday was 42 degrees, although hot northwesterly winds were making it feel more like 45, even hotter in the mosh pit. Thankfully for most, a cool change will arrive in the city between 7:30 and 9:30 tonight. This change will be gusty and is likely to bring a rapid and noticeable drop in temperatures. Isolated showers and storms are also possible. Looking ahead, the heat over central Australia is showing signs of dissipating due to increased monsoonal cloud. This is likely to lessen the risk of days over 40 degrees for Sydney until at least early February. The weekend is looking cooler, reaching the mid-20s with the chance of a few showers on both days. The next warm day will be Tuesday, although a repeat of today's blistering heat is unlikely.