Many people check the weather forecast before going out, and dress appropriately or bring rain gear accordingly. When the weather forecast says there is a 70% chance of rain tomorrow, some people think this means that the rain will cover 70% of the area, while others believe it indicates that it will rain 70% of the time during the day. However, the reality is different, let’s see what meteorologists have to say about it.
According to a report by the UK’s “Daily Express,” Grahame Madge, a spokesperson for the UK Met Office, said that the percentage value of the chance of rain is the result of complex calculations.
First, meteorologists collect weather data from around the world. From buoys in the ocean to satellites in space, all kinds of information flow into the meteorological office, which is then aggregated into what is called an “ensemble forecast.”
Madge explained that the “ensemble forecast” is essentially running weather forecasts multiple times based on slightly different initial conditions. If 70% of the forecasts show rain, then the value of “70% chance of rain” is derived from this.
He said, “Some people think it is based on coverage area or some other indicator, but in reality, if you run forecasts 10 times and 7 of them show rain, then the statement ‘70% chance of rain’ comes from this.”
However, current forecasting technology is not yet able to achieve absolute accuracy. Madge mentioned that the probability range displayed on forecasting apps is limited to between 5% and 95% because in meteorology, forecasting a 100% chance is technically impossible, and there will always be situations beyond the forecast range.
Senior meteorologist Crystal Egger also expressed a similar view. She said, “If the forecast shows a 60% chance of rain, it means that in your specific area, there is a 60% probability of measurable rainfall during that period. This does not mean that it will rain 60% of the day or that 60% of the area will experience rain.”
She likened it to saying that if there are 10 people in the same forecast area and the chance of rain is 50%, you can imagine that 5 of those 10 people will encounter rain. If the chance of rain is 60%, then it means that 6 out of the 10 people will encounter rain.
She also agreed that weather forecasting is not an exact science. She gave examples like a forecast of an 80% chance of rain resulting in just a little rain, or a 20% chance but a storm happens, leading to heavy rainfall.
Madge said that in terms of how meteorologists deal with a large amount of complex data and ultimately transform it into the small icon on the app that indicates whether it will rain or not, he hopes that the general public can have a better understanding.
He recommended that people check the hourly rainfall situation and not stay indoors all day just because the weather forecast shows rain tomorrow. He said, “Just seeing a rain icon doesn’t mean it will actually rain during the specific time period you plan to go out.”
The Weather Channel in the US pointed out that the chances of rain shown in weather forecasts do not cover some information, including the intensity, location, and timing of the rain.
– Intensity: The chance of rain is not related to the intensity. A “90% chance of rain” may only be a light drizzle lasting for 5 minutes; while a “30% chance of rain” could be a local but intense thunderstorm, enough to disrupt your plans.
– Location: Whether it rains in your neighborhood and whether it rains throughout the entire forecast area are two different things. Rainfall distribution is often uneven, especially in the summer. Even though the chance of rain is the same throughout the forecast area, the actual rainfall results may vary significantly.
– Timing: The probability of rain calculated per hour and for the entire day are not the same. You may see a 30% chance of rain each hour, but the overall chance for the day is shown as 50% or 60%. The total-day chance of rain is always higher than the probability for any single hour within that period, it is not an average of hourly values.