About the climate extremes analyses

Extreme climate indices used

Extreme climate events such as heat waves, cold snaps, floods and dry spells have significant impacts on society. To examine whether such extremes have changed over time a variety of extreme climate indices can be defined, such as the number of days per year which exceed, or fail to exceed, fixed thresholds. However, since people tend to adapt to their local climate, a threshold considered extreme in one part of Australia could be considered quite normal in another. To overcome this problem, thresholds based on percentile values can also be defined.

The 30 extreme temperature and rainfall indices used here are based on those defined by the WMO Expert Team on Climate Change Detection Monitoring and Indices. Some of the extreme temperature indices have been modified for Australian purposes.

Extreme temperature indices Definition
Very hot days Annual count of days with maximum temperature > 40°C
Hot days Annual count of days with maximum temperature > 35°C
Very hot nights Annual count of nights with minimum temperature > 25°C
Hot nights Annual count of nights with minimum temperature > 20°C
Cold days Annual count of days with maximum temperature < 15°C
Very cold days Annual count of days with maximum temperature < 10°C
Cold nights Annual count of nights with minimum temperature < 5°C
Frost nights Annual count of nights with minimum temperature < 0°C
Warm days Percentage of days with maximum temperature > 90th percentile
Warm nights Percentage of nights with minimum temperature > 90th percentile
Cool days Percentage of days with maximum T < 10th percentile
Cool nights Percentage of nights with minimum T < 10th percentile
Highest maximum temperature Annual maximum value of daily maximum temperature
Highest minimum temperature Annual maximum value of daily minimum temperature
Lowest maximum temperature Annual minimum value of daily maximum temperature
Lowest minimum temperature Annual minimum value of daily minimum temperature
Warm spell duration Annual count of days with at least 4 consecutive days when daily maximum temperature > 90th percentile
Cold spell duration Annual count of nights with at least 4 consecutive nights when daily minimum temperature < 10th percentile
Growing season length Annual (1st July to 30th June) count between first span of 6 or more days with daily mean temperature > 15°C and first span of 6 or more days with daily mean temperature < 15°C
Extreme precipitation indices Definition
Wet days Annual count of days with daily precipitation ≥ 1 mm
Heavy precipitation days Annual count of days with daily precipitation ≥ 10 mm
Very heavy precipitation days Annual count of days with daily precipitation ≥ 30 mm
Maximum 1-day precipitation Annual maximum 1-day precipitation total
Maximum 5-day precipitation Annual maximum consecutive 5-day precipitation total
Very wet day precipitation Annual total precipitation when daily precipitation > 95th percentile
Extremely wet day precipitation Annual total precipitation when daily precipitation > 99th percentile
Annual total wet day precipitation Annual total precipitation on wet days (daily precipitation ≥ 1 mm)
Simple daily intensity Annual total precipitation divided by the number of wet days (daily precipitation ≥ 1 mm)
Consecutive dry days Maximum number of consecutive days with daily precipitation < 1 mm
Consecutive wet days Maximum number of consecutive days with daily precipitation ≥ 1 mm

Percentile calculations

Percentile-based threshold levels are calculated for 5-day windows across the annual cycle using the standard 1961 to 1990 normal period i.e. daily rainfall and temperatures values are compared relative to varying thresholds throughout the year. So it is possible to have warm spells during winter, cold spells during summer, or extreme precipitation recorded at any time of the year.

Trend maps available

Australian extreme climate trend maps are available for periods starting from the beginning of each decade from 1900 to 1970 for rainfall, and from 1910 to 1970 for temperature. Analysis periods starting after 1970 are considered too short to calculate meaningful trend values. Trend maps for the extreme temperature indices for the earlier periods (e.g. 1910-present) show fewer calculated trends than for the periods starting from 1960, since fewer locations had digitised data in the earlier period.

Interpreting the trend maps

Trends in extreme indices are presented using "bubble" plots, rather than analysed fields, due to the sparse network of locations for which some indices are relevant. Trend values are only calculated for a particular location if its series of annual extreme values are non-zero for at least 50% of years for temperature indices, and at least 25% for rainfall indices. Consequently trend values are not calculated or plotted for some indices in some regions eg. frost nights in northern Australia. Trends in extreme values also tend to be "noisier" than for the mean climate making them more difficult to analyse onto a continuous field.

The trend maps are a useful way to compare how climate extremes have changed in different regions of Australia over time. They are also interesting to compare with changes in the mean climate over the same period. However, they need to be interpreted with caution. Trend values have been determined from a linear (straight line) fit to the data, but the change may not have been gradual. For example, a calculated trend could be due to a relatively rapid "step" change, with the remainder of the series being fairly flat (see some of the timeseries graphs). Also, trend values are highly dependent on the start and end dates of the analysis. Consequently trend maps starting in different decades can look remarkably different. Users are advised to keep in mind the period over which trend values have been calculated and interpret them alongside the timeseries of averaged values.

In addition, the trend values calculated here using past observations should not be used to imply future rates of change. Due to the complex interactions between the natural and human drivers of climate change and variability, the climate of any location is always changing. Future rates of change will depend on how these drivers interact in future, which will not necessarily be the same as in the past.

Timeseries available

Spatially-averaged timeseries start in 1900 for the extreme rainfall indices and 1910 for extreme temperature indices. Stations used in the timeseries are those which have sufficient data across the full period, these are shown on the extreme trend maps from 1900 and 1910 for rainfall and temperature respectively.

Interpreting the timeseries

Timeseries of extreme climate indices are calculated using a simple average of all locations for which the index is considered relevant ie. all locations at which the index series is non-zero for more than 50% of years for temperature indices, and more than 25% of years for rainfall indices. Consequently these averages only represent those regions of the country where the index has meaning. To determine which locations have been used to calculate the spatial average timeseries for a particular index examine the corresponding trend maps.

Data used

The extreme climate analyses are calculated from homogeneous and high-quality temperature and rainfall datasets respectively. The homogeneous temperature dataset, known as the Australian Climate Observations Reference Network - Surface Air Temperature, or ACORN-SAT, dataset) was developed specifically for monitoring long-term trends and variability in the Australian climate. This dataset employs the latest analysis techniques and takes advantage of newly digitised observational data to provide a daily temperature record over the last 100 years. This data will enable climate researchers to better understand long-term changes in monthly and seasonal climate, as well as changes in day-to-day weather. The high-quality daily rainfall dataset is based on a historical rainfall dataset (Lavery et al. 1992), which involved identifying and removing problem records using statistical techniques, visual checks and station history information (metadata).

Please note that any use of these data should be acknowledged to the Bureau of Meteorology. Apart from the purposes of study, research, criticism and review, no part of these data may be reproduced, or redistributed for any commercial purposes, or distributed to a third party for such purpose, without written permission from the Director of Meteorology.

Further information

More information about the homogenised temperature dataset, including additional data, station details, the methods used and a peer review of the dataset can be found on the ACORN-SAT page.

Haylock, M. and Nicholls, N. 2000. Trends in extreme rainfall indices for an updated high quality data set for Australia. International Journal of Climatology, 20, 1533-1541.

Lavery, B., Kariko, A. and Nicholls, N. 1992. A historical rainfall data set for Australia. Australian Meteorological Magazine, 40, 33-39.