Why did traditional barns need to be large and drafty for winnowing grain?

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A large grain elevator and several metal silos stand on a farm surrounded by green fields, with hills and a partly cloudy sky in the background. — Farm building Grain elevator and silos on a small farm.

Traditional barns were designed to be large and drafty primarily to facilitate the process of winnowing.1

Before the invention of threshing machines, farmers separated grain from its stalks by hand, often using a flail or animal trampling. Following this manual threshing, the grain remained mixed with straw dust and chaff. The drafty nature of the barn allowed farmers to toss the mixture into the air; the breeze would then blow away the lighter straw dust while the heavier, valuable grain fell to the floor.12

These structures were also built to be large because they served as the primary storage space for the entire harvest until it could be processed during the winter months.1