I like the idea of barometric pressure and will need to test- I had been thinking it was something having to do with lightning I can tell you for a fact that several of my Zephyranthes, Habranthus, Crinum and Rhodophialas can tell the difference between natural rain and anything I can come up with. I have tried hose water, sprinklers, (both city water) and rainwater from a barrel. Depending on the bulb some of the more "easy" ones respond to quantity, a few more to quality (rainwater) but most of them stubbornly wait for the real thing- rainwater from the sky. Size, age, temperature and timing definitely come into play but I still think some bulbs have other triggers. Erin Grace Thomasville, GA, USA Zone 8b, AHS heat zone 9