The first 7 years of Peninsula Point migration census data was examined in a study by CJ Meitner (the original project coordinator), Lincoln Brower and Andy Davis. The study looked at the effect of environmental variables on the abundance of migrating monarchs at the site. The study found that the timing of the peak migration period was not consistent from year to year - in some years the migration was early, in others it was later in the fall. However, the authors concluded that the large numbers of monarchs that pass through this site make it an important stopover location for monarch migration.
For the weather variables, the study showed that the number of monarchs seen was directly related to the wind direction, with north winds resulting in the most monarchs. Certain other variables were also important.
Meitner, C.J., L.P. Brower and A.K. Davis. 2004. Migration patterns and environmental effects on stopover of monarch butterflies (Lepidoptera, Nymphalidae) at Peninsula Point, MI. Environmental Entomology 33(2): 249-257.