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.
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.
Putting t=o, 1, &c., in succession, we get the percentages of the total number of auroras which occur in January, February, and so on.
These show how the frequency of visible auroras diminished as cloud increased from o (sky quite clear) to 10 (sky wholly overcast).
loo 82 57 46 8 Out of a total of 1714 hours during which the sky was wholly overcast the Swedish expedition saw auroras on 17, occurring on 14 separate days, whereas 226 hours of aurora would have occurred out of an equal number of hours with the sky quite clear.