Grouse hunting in hot weather is awful. The clothing that is required to protect a hunter in typical grouse cover has to be hot to work. So it’s either roast, or let the forest make mincemeat out of your arms and legs.
Several years ago I started using a strap vest, back when they were hard to find in the catalogs. Now there are a lot more to choose from and some are pretty fancy. A strap vest is much cooler than a traditional bird hunting vest and certainly cooler than any jacket ever devised. A cotton shirt heavy enough to protect your arms is all that is needed under the vest on warm days.
Brush pants are available in light weights. Find a florescent orange hat with mesh panels and that’s about the best you can do. You will probably want to carry water to stay hydrated, and don’t forget the dog’s water too.
And then where do you find the birds? Try looking around stream bottoms, particularly if they are lined with softwood trees and a food source isn’t too far away. Maybe the moving water makes it cooler, along with the shade, and probably the birds like a drink on a hot day just like we do.
It has worked for me.