Wicking

Synthetic fibres like polyester do not absorb water into the fibre itself. Instead, capillary action moves moisture along the surface of the fibres and out to the face of the fabric, where a large surface area lets it evaporate quickly. Evaporation is what cools you.

Cotton does the opposite. It soaks water into the fibre, holds it, and sits heavy and wet against your skin. A cotton t-shirt therefore feels hotter in the same conditions and colder once the sun goes down.

Wicking is why a synthetic long sleeve is genuinely cooler than bare arms in direct sun, which sounds backwards and is the single most useful thing to know about fishing clothing.

It has limits. In very high humidity there is nowhere for the moisture to evaporate to, so even a good wicking fabric ends up damp. Airflow becomes the deciding factor at that point, which is what vents and looser cuts are for.