But what does “30% precipitation tomorrow” mean?


Opinion of Sébastien Van Bellegem, professor of statistics (1) Before leaving for a trip to a small corner of our beautiful country, there is nothing more common than checking the weather forecast on our favorite app. This now familiar tool allows us to finally avoid many disappointments. The other day, I decided to postpone a little getaway. Lalibre.be’s weather forecast indicates 30% precipitation the next day in the coveted locality. So no regrets. Except when I heard from friends returning from the region that not a single drop had fallen! How is it possible? Can I still trust weather apps? On closer inspection, I may have misunderstood what “30% precipitation tomorrow” means. On the one hand, this figure may lead one to believe that it will rain 30% of the time in the chosen location. Then my friends could count their lucky stars by avoiding going out at the wrong time. On the other hand, it could be thought that the precipitation will affect 30% of the surface of the territory considered. In this case, they are very lucky to have passed through the raindrops. Two Frequent Interpretations In fact, research in the human sciences shows that these two interpretations are the most frequently cited by those who consult weather applications. People surveyed in Europe seem to subscribe more than people in the United States. And yet, these interpretations are not accurate. The probability of precipitation combines several statistical factors that, well interpreted, can be taken advantage of. Let’s see, the forecast in my garden Predicting whether or not it will rain tomorrow in our garden is not an exact science. The most sophisticated forecasting models cannot eliminate the uncertainty inherent in the complexity of the weather. In short, the models determine a probability that it will rain tomorrow by examining, on the one hand, the current weather in our garden and, on the other hand, a set of physical parameters around it (wind speed and direction, humidity, etc. ). .). The result of this clever calculation is expressed more exactly like this: whenever we observe meteorological and physical parameters similar to those we see today, precipitation occurs the next day three times out of ten. In other words, it may or may not rain at all. Although we may not like to live in such an unexpected world, nature reminds us here that with all the intelligence in the world, one cannot always be more sure of the future. in an area larger than our garden. If we live in the town of Namur or Tournai, this must summarize the time in an area that exceeds 175 square kilometers. In this area, it can rain in the south and be very sunny in the north, or vice versa. In this particular case, our application still has to choose between the “sun” or “rain” pictogram. How do you summarize the probability of precipitation over these larger areas? Let’s take an example First scenario: the forecast model indicates that precipitation will form with a probability of 90%. However, this rain zone will only cover a third of the municipal area. In this case, the application will indicate a probability of 30%. Second scenario: The forecaster observes a very large and active area of ​​precipitation. Any surface crossed by this precipitation zone will be exposed to rain with an absolute 100% probability. However, there is a three out of ten chance that this area of ​​precipitation will affect our municipality. In this case, the application will therefore indicate a probability of 30%… as in the previous scenario, but with a very different physical situation When deciding to go hiking, with no more information than the 30% precipitation it indicates our application. , my friends had a three out of ten chance of returning drunk. Or seven out of ten to stay dry. In this case, if I want to limit the risk of encountering rain, it is best to refine my information to know if I am in the first or second scenario. This can be done by consulting the precipitation radar, also available online. A precipitation percentage of 0% provides unequivocal information since, in this case, no precipitation is expected in the entire territory considered during the set duration. And for a very high percentage, say 90%, you’ll have a hard time getting through the drops. Since 1965 in the United States about what the weather is likely to be tomorrow. The American public has been exposed to precipitation probabilities since 1965, while this trend was introduced later in European services. This is a hypothesis put forward to explain the difference in perception between the American and European public about the meaning of this information that is part of our daily life. All in all, I might have decided to do the hike anyway if I had interpreted the weather app correctly. But in any case, since we are in Belgium, I certainly wouldn’t go wrong taking a raincoat in my backpack! References: Gigerenzer, G., Hertwig, R., van den Broek, E., Fasolo, B. and Katsikopoulos, KV (2005) “A 30% Chance of Rain Tomorrow”: How Does the Public Understand Probabilistic Weather Forecasts?, Risk Analysis, 25 (3), 623-629. DOI: 10.1111/j.1539-6924.2005.00608.xMorss, RE, Demuth, JL, Lazo, JK (2008). Communicating uncertainty in weather forecasts: A survey of the US public, Weather and Forecasting, 23(5), 974-991. DOI: 10.1175/2008WAF2007088.1National Weather Service, “Precipitation Probability”, www.weather.govPrecipitation and lightning radar, IRM, https://www.meteo.be/fr/meteo/observations/precipitations-radar/foudre( 1) Original title: “Statistics in the rain”
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