I can only speak, based on what happened to me, with one particular strain I was growing, a few grows back... Crystal. If I had my fan placed so that it was constantly blowing on the plants, they would curl like that, perhaps not able to respire fast enough, in the 40% humidity, and 83* temps. My other strains weren't effected, just the Crystal. Also, keep on mind that I was going heavy on the nitrogen, at the time, which may, or may not, have contributed. I wish I could say for sure, but after adjusting things, and easing off the N, they did great.