The Canadian Atmospheric Environment Service (AES) examined the temporal variation of rain-fall using almost 2000 extreme events in Canada. Hogg (1980) conducted the analysis of the time distribution of rainfall in short duration events. Rainfall duration of 1 and 12 hours were selected for the analysis and to develop Time-Probability curves. The events were chosen to have samples from both thunderstorms and large-scale cyclonic origin. The selected events did not have to be individual storm entities, they could also be part of a larger storm sequence. Fixed rainfall durations were selected so that the same definition would be used to identify rainfall that is used in obtaining depth-duration- frequency curves.
In analyzing the rainfall, the one-hour events were divided into twelve 5 min. increments and the 12-hour events into 1-hour increments. Rainfall for each event was expressed as a cumulative percentage of, total event rainfall for the twelve equal increments through the storm. All the events for a particular duration were analyzed and the cumulative rainfall distributions for different probability levels were computed.
AES RAINFALL DISTRIBUTION
The temporal distribution patterns were found to vary in the different regions of Canada. The temporal distributions for the coastal regions were quite different from the distributions for the continental regions. A comparison was made between Huffs 50% distribution for the second and third quartile storms and those Hogg developed for southern Ontario. The second quartile distribution was up to 25% different in the cumulative rainfall. The third quartile distribution does not resemble any of the distributions Hogg computed.