I checked out the Sydney weather this morning on the SMH Website (as you do on a Monday morning…). The site gives a nice snapshot of the forecast as well as historical statistics. As I cast my eye over the week’s forecast maximum temperatures (24°C, 26°C, 23°C, 21°C, 21°C, 22°C, 22°C) I was pleased by the mild weather. I then noticed the “Average Max” statistic for May: 19.4°C. A quick calc produced an average maximum temperature of 23.7°C for the next seven days! Hardly rigorous scientitific analysis, I admit, but still…

Needing any credible excuse to avoid starting the work week, I went off to Google to find long term temperature statistics for Sydney, which, as you would expect these days, are available on the WWW. Even more impressive, you can get it off the BOM’s “Climate Data Online” website. The oldest official data was recorded at Sydney’s Observatory Hill weather station and dates back to 1859 – only 150 years of data unfortunately. The first land-based observations were made by William Dawes, who arrived with the First Fleet in 1788. He built an observatory at Sydney Cove and for the next three years kept daily records of the wind, temperature, pressure and rainfall, but that’s another story…

Mean Maximum Daytime Temperature

I wanted to do a bit more than just look at average maximums for the month, so I downloaded and analysed:

• Monthly mean maximum temperature
• Monthly mean minimum temperature
• Monthly lowest maximum temperature
• Monthly highest maximum temperature

I calculated the mean, maximum and minimum temperatures for winter (May to August) and summer (November to February the following year) and added a trend line to each data set to show the 10-year moving average.

The daily maximum temperatures (lowest and highest for the month) aren’t all that useful, because they really are the outliers and, while the trend may be usefull, the scatter makes interpretation difficult. The monthly mean maximum temperatures shown above are very interesting though, particularly the maximums for winter.

There is no doubt it’s been getting steadily warmer over the last 150 years. The mean winter daytime maximum temperature is probably the most significant indicator and winters are definitely a lot milder now (10 year moving average of 19.2°C) than in the 19^th century (16.7°C) . While there is a noticable “kick” upwards in the graph in the last decade, the rate of increase (2.5°C in 150 years or 0.017°C/per annum) appears to have been pretty steady though and I can’t really see an acceleration in the temperature rise during the past three of four decades (when greenhouse gas emissions were meant to have become the main contributing factor). In fact, if Sydney-siders had been paying attention to the climate instead of worrying about the Great Depression and WW1, they’d have been pretty anxious about the temperature increase during the period 1901 to 1927, when 10-year moving average maximum temperatures in winter increased by 2.3°C, from 16.0°C to 18.3°C.