Various Websites try to provide historical precipitation amounts. They find the data where they can. Weather Stats is just one of those sites, drawing reports from a number of places. If data is not available from one source, they move on to another potential source.
Weather Stats has a priority list for sources; The Ottawa Airport is number one. But if that is not available, it can move onto source two, which is at the Central Experimental Farm.
I looked up another source, looking specifically at July 1, 2026. According to what Weather Underground read from the Ottawa Airport there was NO PRECIPITATION on that date. Obviously, that is false, but if the Airport’s feed is not working, then that is what automated sites report – ZERO.
Look for your self:
https://www.wunderground.com/history/daily/ca/ottawa/CYOW/date/2026-7-1
So, when Weather Stats scraped its data, I suspect that the Ottawa Airport precipitation data was not available. Thus, the app went down its list of possible sources. It finally found one that gave it a value of 82.4 mm of rain. That might have been at the Central Experimental Farm, or another location within Ottawa. The rain did not fall evenly everywhere.
Another site that gives the date and amount of greatest precipitation per year (1890-2023) is
Current Results. I didn’t use that site because I did not want to limit the search to one day of the year. However, for September 9, 2004, it gives a rainfall of 108.7 mm. I have seen sources (including Weather Stats) that say 135.4 mm fell that day. Again, the amount of rain that falls in any one location is not necessarily the same as in another location.
My point above was not to provide absolute numbers for precipitation. It was to point out that Ottawa sometimes gets lots of rain. Infrastructure should be designed to handle more than it is currently designed for.
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