Points on the same curve are supposed to have the same average number of auroras in the year, and this average number is shown adjacent to the curve.
In other words, auroras are much more numerous in the southern parts of Canada and in the United States than in the same latitudes of Europe.
Starting at the extreme north, we have a simple period with a well-marked maximum at midwinter, and no auroras during several months at midsummer.
In southern Europe - where, however, auroras are too few to give smooth results in a limited number of years - in southern Canada, and in the United States, the difference between the winter and summer months is much reduced.
The total number of auroras in the year is taken as 100, and t denotes the time, in months, that has elapsed since the middle of January.