"3/4 or 1/2 the maximum day-night temperature difference" says eclipse2017.nasa.gov[0], space.com has more details on that[1]. If this drop means the air cools below its current dew point you get fog. Seen and felt it twice with partial eclipses (both of which just happened to be where I was, so no loss).
[0] https://eclipse2017.nasa.gov/typically-how-big-temperature-d... [1] https://www.space.com/37201-solar-eclipse-temperature-drop.h...