'Tech' fibers such as capilene for shirts do a great job without making you absolutely miserable like cotton would in the heat. For multi-hour mid-day hikes in

'Tech' fibers such as capilene for shirts do a great job without making you absolutely miserable like cotton would in the heat. For multi-hour mid-day hikes in the middle of summer (>100F daily temperatures) I wear my 50 SPF 92% polyester/8% spandex long sleeve hoodie. It's white so it reflects heat, and the synthetic mix allows it to breathe and be flexible. As it gets soaked through with sweat, it does tend to retain more heat, so it takes careful planning to find shade and dry off.