It is shown, for instance, in Loomis's auroral data, which are based on observations at a variety of European and American stations (Ency.
During some displays, auroral light appears in irregular areas or patches, which sometimes bear a very close resemblance to illuminated detached clouds.
Sunlight is not the only disturbing cause in estimates of auroral frequency.
The mean sun-spot frequency for the group of years of few sun-spots is almost exactly the same for the two subperiods, but the auroral frequency for the later group is nearly 40% in excess of that for the earlier, and even exceeds the auroral frequency in the years of many sun-spots in the earlier sub-period.
Rubenson (14), from whom Tromholt derives his data for Sweden, seems to accept this view, assigning the apparent increase in auroral frequency since 1860 to the institution by the state of meteorological stations in 1859, and to the increased interest taken in the subject since 1865 by the university of Upsala.