I like a bright sunny day the best for hopper fishing as I think they can just see the bugs better, especially in deep and fast water. The old wives tale abut cloudy days is true in the spring and fall for BWO hatches, but I'd take a gentle breeze and bright sun all summer long. Maybe my fondness for the bright light is based on my experiences in NZ where bright days usually bring the best fishing (because the fish are easier to spot). Anyway, a cloudy day is better than a rainy day for hoppers but the glare can make it tough to see even the biggest hopper.
I'm fishing down low again tomorrow and then head to the Horn for a couple days and then back to the lower stone. I'm going to milk this hopper hing as long as it lasts as this is the best I've seen in many moons.