This month Melbourne recorded its highest July recorded maximum temperature of 23.3°C. Then just two days later the maximum was just 9.7°C. This temperature difference seemed highly unusual to me and indeed it was. The difference of 13.6°C is easily the highest recorded for the month of July in over 150 years of records (the previous record was 12° in 1975 when it was 11.1°C on the 3rd and 23.1°C on the 30th of the month). The graph below shows the distribution of the difference between highest maximum and lowest maximum temperatures for each month (data from the Australian Bureau of Meteorology website). In a box and whisker plot the thick horizontal lines represent the medians and the box boundaries are the 25th and 75th percentiles. Outliers are shown for data more than 1.5 times the inter-quartile range away from the 25th or 75th percentiles. We see that in Melbourne much larger maximum temperature fluctuations are expected in the warmer months. July has a median range of just 7.7°C between its highest and lowest maximum temperatures.
Here are a few other outliers indicated on the graph when there were large extremes in maximum temperatures during the month.
- Dec 1867: 10.4°C (12th – coldest Dec maximum on record), 40.3°C (19th)
- May 1905: 28.7°C (9th – warmest May day on record), 11.4°C (11th)
- Nov 1911: 12.2°C (1st), 40.7°C (30th)
- Oct 1922: 35.8°C (22nd), 9.0°C (29th – coldest Oct maximum on record)
- Dec 1924: 40.1°C (12th), 11.5°C (26th)
It’s interesting that it has been so long since we have had such an anomaly in maximum temperatures within a month!