Thanks, everyone, for your comments.
Seen close-up, the droplet-covered webs are extremely beautiful but also extremely fragile. All those drops of water load the structure to its limits, so that the least movement of air can tear it. They appear suddenly, overnight, and disappear just as suddenly. My guess is that it is temperature-related so the first few nights when we are down near zero, the spiders get to work. Two nights ago it was cold but not freezing, and a lot of webs appeared. Last night it froze and this morning there were uncountable webs - on bushes which had had 50, now there were more like 500. It does happen most years, but this year it's kinda special.
