First of all, July is not the best time to backpack in the Grand Canyon. It can be done; I have done it, but the one time was because I was involved in an archaeological project near Phantom Ranch. I started my hike down into the canyon at 2:30AM. I really enjoyed the starlight walk. Working at that time of year, we were moving dirt at 4:30 AM, stopping at noon for a siesta, and returning after dinner for about three hours in the evening. That is a pretty good schedule for walking as well - avoid the mid-day heat and take advantage of the coolest part of the day.

In mid-July, you will be likely encountering the summer "monsoons" (thunderstorms), so there is a chance you will get drenched and this will raise the humidity, temporarily. Depending upon your altitude and the local conditions, you might find it surprisingly cool, at least for a time. Remember the North Rim is right around 9000 feet high; it will be very pleasant. Pay particular attention to the weather report wwhen you arrive at the Canyon.

What will work best is lightweight, versatile clothing that you can arrange to meet existing situations. I have always liked long sleeve shirts that can be rolled up, with, with a lightweight t-shirt underneath if I want to strip down. This is also a great environment for zip-off pants; my preference was usually to wear them as shorts.

Pay attention to your headgear. You want shade for your noggin and face. I recommend a full brim, floppy hat with dark underbrim. A bandanna or two is also handy. These are two items that might best be cotton.

If I were going with you today, my wardrobe would be light weight synthetics almost exclusively. On most of my trips in the past, I wore cotton and cotton will do OK. Whatever you wear, wet it down at every opportunity (if it is hot).

You are concerned about the humidity - there are real advantages to low humidity. Evaporation of your perspiration will cool you off very effectively, much more so that you have experienced in midwest summers! This is good! I predict that you will be surprised at your increased water consumption. The classic rule (and it is true) is "Ration your sweat, not your water." That is the prime reason for the early schedule I recommend.

By all means take, and consume, plenty of water, along with electrolytes. Be sure you know where your next water source is. My last trip was a run, from the North Rim to the South Rim, done in about six hours (in May, incidentally), and I remember running with one water bottle in my hand, at least another in my waist pack, and drinking constantly. It worked - my urine was clear as a mountain stream. Incidentally, we dodged snow drifts at the North Rim, and encountered 100 degrees at Phantom Ranch, just to give you an idea of the temperature range involved in the Canyon.

Pay attention to what the rangers tell you about local conditions when you check in at the canyon. They know their stuff and will give you good information (particularly about the situation concerning Clear Creek). This is not true at every park, but it is true at Grand Canyon. I am retired NPS (as you might suspect from my handle) and perhaps you might suspect that I would be a cheerleader for the team - this is definitely not the case.

Honestly, if you can go two or three months later, conditions will be significantly better, but if you don't have that flexibility, go and enjoy the Canyon - it is a very neat place. Just be sure to stay hydrated.