Do your eyes glaze over when you read statistics? Some of the statistics that we are confronted with regularly are difficult to use. When you read the weather forecast, and it says there is a 40% probability of precipitation – how do you interpret that information? It’ll rain for 40% of the day? Over 40% of the area? A small chance of rain somewhere in the area? (And how come sometimes a 40% PoP gets a rain icon and sometimes not? But that’s another grey area.)
Here’s the definition from Environment Canada:
“Probability forecasts are a subjective numerical estimate that any point in your forecast area will get measurable precipitation during the forecast period. For example, a 40% probability of rain today means there are 4 chances in 10 of you getting wet today.”
In other words, if you looked at 10 days with similar weather conditions, on 4 of those days there would be some precipitation somewhere in that area. Or, 40% of the time when we have weather conditions like these – it rains. The PoP is based on the occurrence of precipitation on days with similar weather conditions. This means that Environment Canada collects weather statistics and then uses them to predict the probability of precipitation on a given day with a given set of conditions based on the records. Hmm – still not sure about bringing your umbrella?
Back to Environment Canada:
“Probability forecasts cannot be used to predict when, where or how much precipitation will occur. For example, a 60% probability of snow today does not mean that it will snow during 60% of the day. However, the probability figure does mean that there is a 60% chance of a measurable amount of snow falling at that location.”
So the probability of precipitation is not going to tell you what’s happening where you are – it covers a broad geographic are. If you want to know the probability of getting soaked in North Van, your weather area is Vancouver – observations are made at YVR. Anyone on the North Shore knows how reliable that’s going to be. It’s also not going to tell you how much rain to expect.
Back to Environment Canada for the last word:
“Probability forecasts are more than a novelty or an interesting variation on existing forecasts. They have considerable practical value and should allow many people to make better decisions about weather sensitive activities.
To see how probability of precipitation may be used in making decisions, consider a contractor who has to decide whether or not to pour some concrete for a job. Suppose the probability of precipitation is 40%. The contractor has to calculate the costs of re-doing the job if it rains and, if he/she goes ahead, the expectation is that the risk is 40% of that cost. Compare this to the costs of delaying the job until the next day. If the costs of delay is less than the risk of going ahead, the best decision would be to delay. If, however, the risk is less than the cost of delaying, then it is worthwhile to go ahead.”
OK. Say the job costs $500. So the risk of doing the job with a PoP of 40% is .40 X $500 = $200. The cost of delaying could be zero if the contractor has other work, or more if not.
Don’t forget your umbrella today!
