Changes in the frequency of record temperatures in Australia, 1957-2009

The temporal distribution of record high and low temperatures in Australia during the 1957-2009 period has been investigated. Use has been made of two different data-sets: station data of daily maximum and minimum temperatures at stations drawn from the Australian high-quality daily temperature data-set, and area-averaged anomalies of monthly maximum and minimum temperature for the six Australian States and the Northern Territory. For both data-sets, there is a marked tendency for low temperature records to outnumber high temperature records in the early part of the period, and for high temperature records to outnumber low temperature records in the later part of the period, with the ratio of high to low temperature records mostly being below 0.5 to 1 in the 1957-66 decade and above 2 to 1 in the 1997-2009 period. Time series of the difference between the numbers of high and low temperature records show positive trends significant at the five per cent level for all cases examined. An examination of the frequency of record-setting in area-averaged data, detrended using the observed trends in mean annual Australian maximum and minimum temperatures, suggest that the frequency of record-setting is broadly consistent with the mean annual temperature trend for maximum temperatures, but for minimum temperatures changes in the frequency of record-setting are somewhat smaller than would be expected given the mean annual temperature trend.