About the Climate and Weather Extremes Graphs

Introduction

The purpose of the graphs is to highlight the occurrence of remarkably wet, cold or hot days in the Australian climate record. Weather and climate extremes can have significant social, environmental and economic costs, with heat waves and floods prime examples. One of the greatest impacts of climate variability and climate change occurs through changes in the frequency and severity of extreme events. The climate extremes maps provide a better basis for monitoring such changes, so that we will be better able to understand, prepare for and adapt to future changes in extreme events.

Interpreting the graphs

These graphs show how much of Australia, and the selected State or Territory, experienced extremely hot, cold or wet conditions on each day of the month or year that you select. In other words, the graphs show the percentage area of Australia, and the selected State or Territory, where temperature or rainfall records were either broken or nearly broken on each day of the month or year that you select. Results for New South Wales represent the combined area of New South Wales and the Australian Capital Territory.

The graphs are based on a percentile analysis. The percentiles for each day are calculated using all the daily observations of temperature or rainfall on record for each month. The Bureau's record starts in 1911 for temperature and 1900 for rainfall. For example, the rainfall percentiles for 8 June 2010 are calculated by comparing the rainfall on 8 June 2010 against the rainfall on all June days between 1 June 1900 and 8 June 2010. Similarly, the temperature percentiles for 20 August 2010 are calculated by comparing the temperature on 20 August 2010 against the temperature on all August days between 1 August 1911 and 20 August 2010. The observations for all the days on record in that month are then ranked from coolest to hottest for temperature and driest to wettest for rainfall and then broken into 100 equal groups. The first group is the 1st percentile, the second group the 2nd percentile, and so on.

The percentage area of each state or Territory that falls into the 1st, 3rd, 97th, 99th percentile and highest and lowest on record are calculated for each day and graphed by month or by year. For the current month and year, the graphs are updated daily and only show results for completed days. For previous months and years, the results for the entire month and year are shown. It is important to note that rainfall or temperature data for days after the nominated date, are not included in the calculation of percentiles for that nominated date.

Example climate extremes graph

The climate extremes graph below shows the percentage area for maximum temperature above the 99th percentile for March 2008, for South Australia and nationally.

The graph shows that the March 2008 heatwave event broadly lasted from early March to around the 17th, with two distinct pulses during the second and third weeks. The peak areal extent in South Australia affected by extreme temperatures occurred on 16 March when nearly 30% of South Australia experienced maximum temperatures above the 99th percentile, whilst on this day about 15% of Australia fell into this category.

Daily maximum temperature extremes graphs

For each day in March 2008, the graph shows the percentage area for daily March temperature being above the local 99th percentile, for South Australia (orange bars) and nationally (grey bars).

Data

These maps are generated from the Australian Water Availability Project 0.25° (25 km x 25 km) resolution daily analyses. The analyses are computer generated using a sophisticated analysis technique. Further information about the analyses can be found at http://www.bom.gov.au/jsp/awap/

As mentioned above, the Bureau's record for rainfall starts in 1900 and for temperature starts in 1911. The record for rainfall starts in 1900 because rainfall observations are insufficiently widespread before that date for a national analysis to be undertaken. The record for temperature starts in 1911 because that represents (approximately) the point at which temperature observations consistent with current observational practice (for example, with thermometers in Stevenson Screens) became sufficiently widespread for a national analysis to be undertaken.

Difference between extremes graphs and extremes maps

It should be noted that the extremes graphs are calculated by an area average process that only includes the continent and main land of Tasmania. Bass Strait islands, Kangaroo Island and other small islands are excluded from the calculation. These islands are included in the extremes maps analysis, therfore, it is possible that extreme rainfall or temperatures may be evident on the extremes maps (on Kangaroo Island for example) that are not represented in the extremes graphs.