The following table summarizes trends in short duration rainfall intensities for long term Southern Ontario climate stations (below latitude of 44 degrees). Stations have at least 30 years of record. The change in 2-year 5 minute rainfall intensity and 5-year 10 minute rainfall intensity have been calculated using a starting point of then Environment Canada's 1990 IDF tables (obtained from Environment and Climate Change Canada in 2017), and an ending point of the Version 2.3 Engineering Climate Datasets.
|Change in average and frequent rainfall intensities in southern Ontario.|
The Insurance Bureau of Canada and the Institute for Catastrophic Loss Reduction have reported that average rainfall intensities have shifted by an entire standard deviation (thus making extreme 40 year storms become more frequent 6 year storms) - this has been refuted by Environment and Climate Change Canada (see Canadian Underwriter editor's note). The data in the above table indicate no such shift.
It is a commonly held belief that rainfall intensities have increased dramatically as a result of climate changes effects. Recently the Globe and Mail reported "It is hard to ignore the growing relationship between climate change and the resulting impact of severe flooding events." .. actually its hard to explain the role of changing climate given rainfall intensity data in some regions. It may be best to ignore rainfall and focus on other flood risk drivers like urbanization and intensification.
Datasets from Environment and Climate Change Canada refute the belief that rainfall is becoming more extreme.
The following tables show the 2 to 5 year IDF trends for 5 to 10 minutes (first table), and 5 to 10 year trends for 1 hour and 2 hours (second table)
|1990 (pre-version 1) IDF Dataset Worksheets have been prepared for Ontario stations:|
1 Volume Tables :
2 Volume Tables :
3 Volume Tables :
4 Volume Tables :