Adam Clarke on Ruth 3:2

He winnoweth barley tonight - It is very likely that the winnowing of grain was effected by taking up, in a broad thin vessel or sieve, a portion of the corn, and letting it down slowly in the wind; thus the grain would, by its own weight, fall in one place, while the chaff, etc., would be carried to a distance by the wind. It is said here that this was done at night; probably what was threshed out in the day was winnowed in the evening, when the sea breeze set in, which was common in Palestine; and as this took place in the evening only, that was the time in which they would naturally winnow their corn.