A meteorologist puts to the test the old belief that rain is guaranteed to follow fog after a particular period of time
Most farmers have heard the common wisdom that rain follows fog.
Not right away, mind you, but a few weeks down the road.
Some way down the road, anyway. People don’t agree when the rain is supposed to come.
“I’ve heard three months, six months, five, four, or after hoarfrost,” said weather analyst Scott Kehler of Weatherlogics when he spoke to Red River valley farmers in January.
“Nobody can actually tell me what the right number is.”
Farmers offered Kehler their own numbers, with 30 days being proffered by a couple.
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“It’s some number in that range, probably,” said Kehler, to farmer laughter.
Kehler said he was unaware of the rain-following-fog belief until he started working with farmers.
“Meteorologists are obviously not taught this,” he said.
He set about sleuthing, let his fingers do the clicking and tried to hunt down the belief’s origin, basis and accuracy.
“I Googled and I Googled and I Googled and I found lots of references to it, but there’s no evidence of where it came from.”
He checked old weather data, connecting fog and subsequent rains over several periods like 30 and 90 days. He found that it indeed does rain following fog. And it doesn’t. And sometimes it rains just a little bit.
After 30 days, for the area he looked at, there is a five percent chance of a quarter-inch rain, and a two percent chance of a half-inch, after fog or hoarfrost. (Fog produces hoarfrost, so Kehler considers them interchangeable.)
After 90 days, it’s similar, with a six percent chance of a quarter-inch and a two percent chance of a half-inch.
Of course, that’s like every other day in the May-to-August growing season.
“The chance of rain on any given day is one-in-three.”
The chances of fog predicting future rain increase when observers add a day or two to the grace period in which it needs to rain. Over the course of a few summer days, it generally rains.
“It appears to work because it’s pure chance,” said Kehler.
“I suspect we only remember when it works, and it also seems like people choose the number of days when it works for them, and they give themselves more of a grace period.”
He did not, however, say farmers must give up believing that rain follows fog at some prescribed time in the future.
“If you don’t believe me, keep track yourself.”
