Since I have lived most of my life in the SW, I've visited the GC in most every month of the year. I love going there in July. Yes, the daytime highs might be in the 80s, but almost every afternoon, you'll get monsoon rains that bring fantastic photographic opportunities and cool breezes. The nights are cool to even crisp. You can usually sleep very well with just a couple of windows open. I've even had to turn on the furnace a couple mornings in July and August. Just make sure you seek shelter if a monsoon storm is right on top of you. They can contain strong lightning, winds, rain and hail. But most of the time they are pretty spotty and make terrific viewing at a distance.