From what I can tell, they like cooler and drier air masses over warmer waters. This time of year, around here, all it takes is a below normal night temp(to some level) and the lakes or rivers will produce some steam/fog. That happens pretty often. If it we get a long period of cold rain, that can cool the lake water, and just make you need even cooler nights for this steam to form. So it's nice when it doesn't do to much of that too early. Nice now it's around 90 here again, so it'll stay primed for the next cold front and high pressure.
The trick seems to be being cold enough AND dry enough. I've seen days where they just don't seem to want to form, with plenty of steam and light winds. Others, like the other morning, they form like crazy. The difference at least appears to be tied into the relative humidity by morning. Often the cold mornings the temp will drop near the dew point, and those don't seem to be so prolific. This time though, the temp dropped a long way(mid-40s...not far off records) yet stayed 3-5 above the dew point. Maybe it's something else, but it has seemed to like it more dry while cold.
They aren't really rare I wouldn't think. It is a little surprising to me to see so few videos of them on youtube though.
Here is another day with them, earlier this year.
The water this time though was a touch cold, lol. That is the Missouri River instead of a lake, and it's full of big ice chunks. The air temp to get that steam off there was around -20F. So they can form anytime the water isn't frozen solid, if the air temp is cold enough in relation to the water temp. The lake ones this last time were a lot more energetic than these icy river ones. They also seemed to like the tree shadow line as the sun rose. I noticed that once before on that lake when seeing them.
Light winds also are required, for the most part. No winds seem best. You aren't going to find them in general fog.
