This is an area of interest for commercial growers, so research has been done on the most commercial orchids. A quick search in Google Scholar gives a lot of reading material.

Murph and Bulbo, thanks for the info. I should have hit refresh before I responded to Ray, then I could have caught all of you in one post. This is all good info.

Night length can be a trigger; temperature can be a trigger; water pattern can be a trigger; barometric pressure drop can be a trigger. It varies from species to species.

Most plants growing close to the Equator don't have a night length trigger, because there is so little variation.

Rain seasonality is a big trigger to a lot of plants from areas with dry winters and wet summers.

At least one orchid, Dendrobium crumenatum, is triggered to bloom by the drop in barometric pressure with thunderstorms.

Interesting stuff, ES!