In the 1930s, the 30-year period from 1901 to 1930 was selected as the first international standard period. In Canada in the first part of this century, there was a limited number of stations with observations over the 1901-1930 period. As a result, averages were computed based on all available data, rather than the normals period. Sets were produced after the end of each subsequent decade. The first computer for climate applications was installed in 1965, in part to facilitate normals production.
Extreme values are the highest or lowest occurrence for all years for which data are available.
Extreme values whose corresponding means are missing should be used with caution. They are often derived from less than twenty years of observations, and may not be indicative of occurrences which could be expected over longer periods. Starting and ending dates for station observing programs are given after station names. It should be noted that these dates are for the total period of observation, and hence refer to the period used to calculate the extreme values. In contrast principal stations take hourly observations for all or parts of the day.