The best shirt materials for hot weather will be those that are lightweight and breathable such as polyester, rayon, or bamboo. Cotton is generally heavier and not as good at wicking moisture; however, it's the softest of these options, so it can be nice to have as part of a fabric blend.
You'll see “moisture management” or “wicking” called out on many clothing items made from synthetics, like polyester and nylon, but merino wool and cotton can also be good options or as part of a comfortable blend.
In hot and dry conditions, cotton actually works pretty well. That’s because cotton soaks up sweat and lets it evaporate slowly, wicking away some of your body heat. This is due to evaporative cooling, which is why sweating or soaking yourself down with water has a cooling effect. As the water evaporates and turns into gas, it releases latent heat, and when the gas leaves the surface of your skin, it takes that heat with it. This process is accelerated in dry conditions.
But in humid conditions, you’ll want to steer toward synthetics and blends that are loose-fitting, lightweight and generally light in color. Most polyester-based knits let moisture evaporate quickly, giving you lots of cooling when you're active and keeping you dry once you're at rest.
The difference between clothing produced just a couple of decades ago and clothing made today is that textile manufacturers have learned how to turbo-boost the evaporative cooling effect. Some modern high-tech fishing shirts can actually increase the effectiveness of evaporative cooling via the fibers and weaves from which they’re crafted.
One additional factor is using mesh-lined vents to help heat and moisture escape. These are usually located around the sides or back where sweat accumulates, and are a staple for high-end fishing shirts. Vented clothing assists in the process of cooling by increasing airflow.
Whatever method you prefer, keeping cool in the outdoors has never been easier. -WR