Interesting question that I'm not qualified to answer, but I did a little digging and found something that might be of use. When I first started fly fishing I turned to books to learn as much as I could (outside of the expert advice I received from a friend who has been my fishing mentor from the very start). Among the books I got early on was Caucci and Nastasi's Hatches II. My friend told me I didn't need that book yet, but I was drawn by entomology, the Italian surnames of the authors, and the thickness of the book so I got it. Since it arrived and I flipped through it once in a state of complete mystification (having only seen some caddis at that point), it's made a pretty good stand for my tying materials on a far corner of my desk.
So, I finally remembered I had the book and I dug into it searching for an answer to your question. Surprisingly, even Caucci and Nastasi don't seem to provide a straight answer. They write of the genus Ephemerella, "When the 'magic hour' arrives, the nymphs swim laboriously toward the surface with a peculiar wiggle" (p. 61). I think it's even more peculiar that in such a thorough treatment, even they refer to the emergence as the "magic hour." Surely they have an idea of what makes that particular hour the magic one, so maybe there's more to be found on the subject.
In the sections on subgenera, there is a bit more information that might be useful to you. Now that my interest is piqued and I have more experience actually fishing hatches, I'm going to give this book a more thorough read. So, thanks for that. I'll report anything I find.
In my limited experience, I've learned from time on the water and from my friend and fishing mentor (who you met fishing in Homer City one day last early spring) that hatches are definitely dependent on weather conditions. Light rain and clouds make for a longer more sustained hatch, while bright sun suppresses a hatch. Why that is, I couldn't say...and whether that's the same for all hatches I don't know either. I'm guessing you already have observed that yourself. But I'm looking forward to people weighing in on this because I have an analytical mind as well and am curious to know.
