Issue 57
September 15, 2005

Weather Trivia

 You Asked Us

Environment Canada's knowledgeable specialists answer questions about wildlife, air pollution, water, weather, climate change and other aspects of the environment.

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What is probability of precipitation (POP)?  Ottawa, Ontario

The probability of precipitation (POP) is the chance that measurable precipitation — at least 0.2 mm of rain or 0.2 cm of snow — will fall on any point of the forecast region (such as the city of Ottawa) during the forecast period. For example, a 70 per cent probability of precipitation in Ottawa means that the chance of you getting rained on (or snowed on in winter) is 7 in 10. In other words, there is a 70 per cent chance that rain or snow will fall on you, and therefore, a 30 per cent chance that it won't.

Probability forecasts cannot be used to predict when, where or how much precipitation will occur. For example, a 60 per cent probability of snow today does not mean that it will snow during 60 per cent of the day. However, the probability figure does mean that there is a 60 per cent chance of a measurable amount of snow falling at that location.

A person who did not encounter precipitation during the time period would be tempted to say the forecast probability should have been zero; while someone else who did see precipitation during the same period would say the probability should have been 100 per cent. Statistically, one cannot determine the reliability of a single probability forecast. The reliability can only be verified after a number of forecasts. A 30 per cent probability of precipitation forecast is reliable if the same forecast was made on one-hundred occasions and if it rained on 30 of those occasions.

The weather office forecasts a probability of precipitation for your region based on the information available at the time the forecast is issued. As the time of the event nears, and more information becomes available, the prediction becomes more accurate.

Weather is an incredibly complex phenomenon. Despite the use of computers, radars, satellites and skilled forecasters, it is still difficult to forecast future precipitation. So unfortunately, rain or snow cannot always be predicted with a simple yes or no.