dewfall (n.)

"water vapor deposited from the atmosphere by condensation, especially during the night," Middle English deaw, deu, from Old English deaw, from Proto-Germanic *dawwaz (source also of Old Saxon dau, Old Frisian daw, Middle Dutch dau, Old High German tau, German Tau, Old Norse dögg "dew"), perhaps from PIE root *dheu- "to flow" (source also of Sanskrit dhavate "flows, runs").

Used figuratively of something refreshing (late Old English), or suggestive of morning and youthful freshness (1530s). As a verb, "to wet with or as with dew," Old English deawian.

The formation of dew is explained by the loss of heat by bodies on the earth's surface through radiation at night, by which means they and the air immediately about them are cooled below the dew-point ....Dew is thus deposited chiefly on bodies which are good radiators and poor conductors of heat, like grass; hence also it appears chiefly on calm and clear nights--that is, when the conditions are most favorable for radiation. It never appears on nights both cloudy and windy. In winter dew becomes hoar frost. [Century Dictionary]

c. 1200, "a falling to the ground; a dropping from a height, a descent from a higher to a lower position (as by gravity); a collapsing of a building," from Proto-Germanic *falliz, from the source of fall (v.). Old English noun fealle meant "snare, trap."

Of the coming of night from 1650s. Meaning "downward direction of a surface" is from 1560s, of a value from 1550s. Theological sense, "a succumbing to sin or temptation" (especially of Adam and Eve) is from early 13c.

The sense of "autumn" (now only in U.S. but formerly common in England) is by 1660s, short for fall of the leaf (1540s). Meaning "cascade, waterfall" is from 1570s (often plural, falls, when the descent is in stages; fall of water is attested from mid-15c.). The wrestling sense is from 1550s. Of a city under siege, etc., 1580s. Fall guy is attested by 1906.