Keep a Cool Head
Most fishermen wear a mesh style ballcap in summer because they feel it is cooler than the full type. In fact I fine the foam lined full cap may be cooler because the liner that protects against heat loss in winter also protects against heat penetration in summer.
Also by dipping the cap in some water I can create a miniature air conditioner. I wring it out well before putting it back on and as the water evaporates, it creates a cooling effect for my head.
A nice brown hooked on a hot July day
Hope this helps your fishing on those real hot days of summer...FISH-ON! Cookster