Given my career, here’s one I couldn’t resist sharing.
Historical weather information for Canada, the UK and USA is available from http://www.wolframalpha.com/. Enter the word weather followed by a date and a location. For instance:
weather 25 December 1966 Ottawa
See what Wolfram Alpha returned here.
Be careful to ensure it got the location correct as the same name is used for different places. This is via Dick Eastman’s Online Genealogy Newsletter.
The period covered by Wolfram Alpha is limited. For longer including earlier periods, as posted before, try the direct-from-the-source historical weather data for Canada to be found starting at https://climate.weather.gc.ca/historical_data/search_historic_data_e.html /. For the UK check the Daily Weather Report /Daily Weather Summary (METDLA/6/1/1) back to 1860 along with other resources at the Complete Archive from the Digital Library and Archives of the British Meteorological Office.