What’s the science behind probability of precipitation?

You’ve likely heard of probability of precipitation, also known as POP.

It’s the numerical percentage you often see in a weather forecast indicating a chance of rain or snow.

But this is one of the least understood components of a forecast, often because it’s not explained well or because some weather forecasters use a different method to calculate it than others. Evan Simpson was also puzzled by the numbers included in our recent seven-day forecast board and whether its precision has any scientific meaning.

First, let’s define it. The probability of precipitation is “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,” according to Environment Canada.

You can determine the probability of precipitation by a math equation: POP = C X A, moving the decimal point two places to the left.

C = Confidence from the forecaster that there will be precipitation. A = Percentage of forecast area expecting precipitation.

This is where it often gets confusing. Let’s use a probability of precipitation of 40 per cent as an example.

In one scenario, you could have 100 per cent confidence that 40 per cent of the area will receive precipitation, resulting in a POP of 40 per cent.

On the other hand, there may be 50 per cent confidence that 80 per cent of the area will receive precipitation. That would also give a POP of 40 per cent. See how this can lead to confusion?

It’s why I don’t use it often in most of my forecasting. It’s generated on our seven-day forecast, but I tend to avoid it when possible.

Some weather agencies, including Environment Canada, also do not permit the use of a 50 per cent probability of precipitation. It would either be 40 per cent or 60 per cent.

Therefore, words like isolated for a low chance of precipitation, scattered for a moderate chance and widespread for a high chance are easier to understand.

I hope this answers your question, Evan, and anyone else who is curious.

DID YOU KNOW?

Ice pellets and freezing rain have a complex journey to the surface.

Ice pellets occur when snow begins to melt in a warmer layer of air aloft but is able to refreeze into a pellet before reaching the ground.

Freezing rain looks like ordinary rain and occurs when snow melts in a warmer layer aloft but supercools in colder air at the surface, allowing it to freeze on contact with cold surfaces. The two are often confused but very different.