In winter we do our best to keep warm, and in summer we try to stay cool. And where cool weather shirtings are heavier and woven more tightly to keep warm air next to the skin, summer shirtings should be just the opposite. Cloth for warm weather should be as light as possible, consistent with an appropriate degree of opacity, and use open weaves to permit air circulation.
Men who live where there are seasons should probably have a shirt wardrobe that is divided into three parts: 40% year-round broadcloths and poplins, 30% oxford cloth and heavier twills for cool weather, and 30% light-weight cloths for the heat. The mid-weight shirts should probably be acquired first, with half a dozen or more cooler wearing items following as soon as practical thereafter.
According to Esquire's Encyclopedia of 20th Century Men's Fashions, madras cotton shirts were the lightweight of choice for most of the past hundred years. They have virtually disappeared now for some unknown reason, replaced by linen and cotton blends, cotton voiles and batistes, and high twist weaves. Most if not all of these are unavailable ready to wear, where mid-weight fabrics reign supreme to minimize inventory requirements. That practice makes seasonal shirtings yet another reason why every suit-wearing man should patronize a shirtmaker.