Wet and Dry Seasons
The dry season begins towards the end of October, just when the butterflies arrive. If you look at a graph the precipitation just drops, drops, and drops down--almost to zero--for November, December, January, February, and March.
It begins to get a little bit wetter in April. Just as it's beginning to get wetter--when it would be very cloudy in these areas--the monarchs leave. It's really important for them to leave before the wet season, because the wet season can trap them. So they’re out of there at the end of the dry season!
Thus, you have 5 months of very dry weather (November, December, January, February, and March) followed by 7 months of wet weather (April, May, June, July, August, September, and October).