When it comes to forecasting the chance of rain or snow, Environment Canada's Guide to Public Forecasts is pretty clear: "The use of 50 per cent is not permitted."

Wait. Why not?

To understand, it's a good idea to start at the beginning: What exactly is probability of precipitation or POP?

Environment Canada defines it as "the chance that measurable precipitation' (0.2 mm of rain or 0.2 cm of snow) will fall on any random point of the forecast region during the forecast period."

If the amount is under that you likely won't see a chance of rain in the forecast.

The National Weather Service in the U.S. has a formula for this: the probability of precipitation is C x A, where C is the confidence that precipitation will occur somewhere in the forecast area and A is the per cent of the area that will receive the precipitation, if any.

So for instance, if there is a 100 per cent probability of rain somewhere in the area, but the forecaster expects it to only produce measurable rain over 50 per cent of the area, then the probability of precipitation would be 50 per cent.

Cloudy Winnipeg

Forecasters look at both the chance of precipitation and the area likely to be effected when making their forecast. (Darren Bernhardt/CBC)

'Carved in stone'

But there's that darned 50 per cent chance that Environment Canada refuses to use.

Since we seem to use the same methodology, why is it OK in America but not Canada?

To get the answer, I talked to Geoff Coulson, a meteorologist for Environment Canada.

Was it disastrous records from a time when it caused an outdoor picnic panic? Mass hysteria of people torn between packing sunscreen or an umbrella?

"There isn't any legal edict of 'thou shall not', but this has always been the case — and it was one of the first things I learned when training with Environment Canada," said Coulson.

"I'm not sure when or why that decision was made, but it does appear to be officially carved in stone."

Commit one way or the other, says meteorologist

What it comes down to, said Coulson, is about providing information that is useful to the public.

"We have to assume that a 50 per cent probability of precipitation is really just sitting on the fence and doesn't help people know whether to bring their umbrella with them or not," he said.

"A forecaster needs to make an informed decision and a 40 per cent or a 60 cent call gives a far better idea of how the day is going to go. More chance or less chance. Commit."

Here in Canada, Environment Canada rounds its probabilities to the nearest 10 per cent, while forecasters in the United Kingdom round to the nearest five per cent.

So everyone does things a little different. 

Coulson acknowledges other forecasters use models that allow for a 50 per cent chance.

"But in Canada?" he said. "It's just bad form."


Teri Loretto has been the on-air back-up to climatologist Ian Black for almost a decade. She worked in flight training at the Ottawa Airport for 12 years, learning to decipher weather reports and read the winds. She also spent a year on a tall ship, where accurate weather forecasting became a priority.