All who've noticed the occasional sudden bloom after a rain.. Given that the bulb is big enough (a critical issue) and that fertilizer has been adequate the year before to grow flowers instead of leaves (lots of phosphorus and potassium) and given that other necessities are in place, e.g., sufficient elapsed time since the last blooming, or since dormancy where this is a factor. I suspect that the major factor is the acidity in the rain releasing nutrients that have been bound up by local alkalinity. Around the root system. I note that this also coincides with an increase in or a shift in mycorrhizal activity, e.g., a surge of fungi of various sorts in the rest of the yard. Not just in spring which is normative, for many plants but at other times. This is a multifactorial matter that varies for each species and genus, but many genera fortunately have common needs (having evolved under similar conditions), otherwise we'd have a harder time than we do, trying to grow plants that have not been researched and standardized for commercial purposes. Some plants don't care, as it were, given that they've completed a necessary dormancy, and been cooled and primed to bloom-and their genetics unhinged by decades of breeding -- get triggered by warmth and water. The Narcissi, and Hyacinths with sufficient predictability of bloom to lead glass and pottery manufacturers to mass produce hyacinth vases for just this purpose. One can still find them (some made nearly a hundred years ago --on eBay.) I'm sure that some of our European members can comment here. Mr. De Jager?