Is it true that the chance of snow (or rain) percentage refers to how much of an area can expect precipitation rather than how likely it is that there will be precipitation?—Carl Luna, via email
No. But David Phillips, a senior climatologist with Environment and Climate Change Canada, gets why you’re confused. “Probability of precipitation, or POP, is not well understood.” This value didn’t exist in Canada until the early ’80s, when news directors lobbied to make changes to how forecasts were communicated, he says. “Before that, meteorologists only ever used vague terms: scattered showers; isolated flurries. This shift changed subjective words to something seemingly more precise.”
So what does POP actually mean? We can sure tell you what it doesn’t mean. “A 60 per cent chance of rain doesn’t mean 60 per cent of the day, or 60 per cent of the area, and it tells you nothing about how much precipitation to expect,” says Phillips. “It reflects how confident the meteorologist is that you’ll see precipitation in that forecast area. The higher the percentage, the more confident the meteorologist is.”
Meteorologists only count “measurable precipitation”—for snow, that’s 0.2 cm. “If it’s less than that, it’s as if it didn’t occur.” This measurable precipitation could come at any time during the forecast period. So, “70 per cent POP”? It could snow at 1 p.m. for one hour. Or 3 p.m. for three hours. Or on and off all day. And the snow could occur at any location within the forecast area.
Those early-’80s news directors had a point: the POP gives people information to translate into an action, says Phillips. “I look at those numbers, and if it’s a 30 per cent chance of rain, I don’t carry an umbrella. If it’s 40 per cent, I do.”
Forecasters most commonly use 30 and 70 per cent. “If it’s less than 30, it’s often not mentioned,” says Phillips. “And you’d never hear 50 per cent. What the hell does 50 per cent mean?” It means that you should carry half an umbrella. Obviously.
Got a question for Cottage Q&A? Send it to answers@cottagelife.com.
This article was originally published in the Winter 2023 issue of Cottage Life magazine.
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