I have fished the sulphur hatch on the Clinch River quiet a bit in the past week. The hatch seems to start around 11:00 AM and runs steadily until the generators disperse them. I focused on throwing dry flies for two-solid days. However; I could not get consistent action with any of my dry flies.
The water was extremely clear and it seemed that most of the rising fish were taking emergent sulphurs and not adult duns.
I switched to my trusty Umpqua split case and it worked very well. My friend Mihai hooked a 20" rainbow on the third fish after changing to the nymph. We fished it on a short drop (~8-12") to take advantage of the rising fish and to keep it in their focal range. We used the weighted version of this nymph; however, the unweighted would work equally as well by allowing it to float close to the film. This technique helps to illustrate the nymph holding on to the bottom of the surface film.
From most of my observations and hours of underwater video this past week; the fish were hitting emergent nymphs and not dries or adult duns. Hence; that is why I came up thin using my dry flies. You will do well on weighted nymphs and floating nymphs. If you want to focus on top-water action; throw an unweighted nymph to a rise and let it static drift in the film. It will out fish your dry flies.
Here are some pictures to help illustrate the conditions.
Fly of choice: