Behold, he winnoweth barley - The simple manners of Boaz and his times are here before us. This "mighty man of wealth" assists personally in the winnowing of his barley, which lies in a great heap on the floor Ruth 3:15, and sleeps in the open threshing-floor to protect his grain from depredation.
Tonight - For the sake of the breeze which springs up at sunset, and greatly facilitates the "cleansing" (separation) of the grain tossed up across the wind.

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.

And now is not Boaz of our kindred, with whose maidens thou wast?.... He was, and her question supposes and concludes it, and which she observes, that Ruth might take notice of it, and encouragement from it; and the rather, since she had been admitted into the company and conversation of his maidens; and which was more, though not mentioned, into the company and conversation of himself, and whom Ruth knew full well; and who being, Naomi thought, the next nearest kinsman, and obliged by the law in Deuteronomy 25:5 to marry Ruth, with which view his relation is mentioned:
behold, he winnoweth barley tonight in the threshingfloor; which afforded a fit opportunity of meeting with him, being at night, and out of the city, from his own house, and alone, and after a feast for his reapers and threshers of corn, seems, from Ruth 2:7 as it was usual to have threshingfloors in an open place without the city, so to winnow at them, whereby the chaff was more easily separated from the corn, and that, in the evening, when in those countries there were the strongest breezes of wind to carry it off; hence the Targum here has it,"behold, he is winnowing the barley floor with the wind, which is in the night.''For before the invention and use of fans in winnowing, it was only done by the wind carrying off the chaff, as the oxen trod the corn, for it was done in the threshingfloor, as here: hence Hesiod (m) advises that the threshingfloors should be , in a place exposed to wind; and so Varro (n) observes, the floor should be in the higher part of the field, that the wind might blow through it; to this manner of winnowing Virgil (o) has respect. Nor was it unusual for great personages, owners of farms and fields, to attend and overlook such service. Pliny (p) reports, that Sextus Pomponius, father of the praetor and prince of the hither Spain, presided over the winnowing of his reapers; so Gideon, another judge Israel, was found threshing wheat, Judges 6:11.
(m) Opera & Dies, l. 2. ver. 221. (n) De re Rustica, l. 1. c. 41. (o) "Cum graviter tunsis", &c. Georgic. l. 3. Vid. Homer. Iliad 5. ver. 499. & Iliad, 13. ver. 588, &c. (p) Nat. Hist. l. 22. c. 25.

BY NAOMI'S INSTRUCTIONS, RUTH LIES AT BOAZ'S FEET, WHO ACKNOWLEDGES THE DUTY OF A KINSMAN. (Ruth 3:1-13)
he winnoweth barley to-night in the threshing-floor--The winnowing process is performed by throwing up the grain, after being trodden down, against the wind with a shovel. The threshing-floor, which was commonly on the harvest-field, was carefully leveled with a large cylindric roller and consolidated with chalk, that weeds might not spring up, and that it might not chop with drought. The farmer usually remained all night in harvest-time on the threshing-floor, not only for the protection of his valuable grain, but for the winnowing. That operation was performed in the evening to catch the breezes which blow after the close of a hot day, and which continue for the most part of the night. This duty at so important a season the master undertakes himself; and, accordingly, in the simplicity of ancient manners, Boaz, a person of considerable wealth and high rank, laid himself down to sleep on the barn floor, at the end of the heap of barley he had been winnowing.

Threshing - floor - Which was in a place covered at the top, but open elsewhere, whither Ruth might easily come. And this work of winnowing corn was usually ended with a feast.