What does 70% rainfall probability mean?

The 70% probability of rainfall means that the probability of rainfall in this city tonight is 70%.

The term precipitation probability actually has two concepts. One refers to the historical climate precipitation probability of a certain place, which is completely based on the statistical calculation and analysis of historical climate data; the other is in weather forecasting. After forecasters rely on various knowledge, information and skills to analyze future weather, they make judgments about the possibility of precipitation in a certain period of time, which is expressed by precipitation probability values, which is also commonly referred to as weather probability forecast.

The value range of these two probability values ??is from 0% to 100%, but their meanings are different and the production methods are also different. Climate probability is an objective statistical value calculated based on historical data. As long as the data interval is determined, the result is unique. Weather probabilistic forecast results have greater uncertainty. Different forecasters may arrive at different probabilistic forecast values. The influencing factors include data, technology, knowledge, methods, experience and other aspects.

Practical dilemma: brings difficulty in understanding

Nowadays, "precipitation probability 70%" has become more of a public memory. In weather forecasting, deterministic forecasts such as "There will be light rain tonight and during the day tomorrow." So why are forecasts no longer issued in the uncertain form of "precipitation probability"?

my country began to introduce probabilistic weather forecasting in 1995 and conducted pilot projects in developed cities such as Beijing and Shanghai. However, the results were not ideal. The public did not understand and the leaders were not satisfied. The reason is not only the understanding and acceptance of the public, but also the problem of publicity and knowledge popularization.

Before the opening of the 2008 Beijing Olympics, the China Meteorological Administration gave a set of data. Based on an analysis of 33 years of historical data, the probability of precipitation near the Bird's Nest on August 8 was 41%. The "precipitation probability" here refers to the first concept above, that is, historical data statistics, but it is still misunderstood by many public as weather probability forecast. In addition, some people think that the "70% probability of precipitation" in a certain place means that 70% of the places in the area will rain, or that the greater the probability of precipitation, the heavier the rain will be.

Some people also believe that the probability of precipitation is created by experts to "fool" the common people. For example, if the probability of precipitation is 50%, then if it rains, the prediction is half correct, and if it does not rain, the prediction is half correct. Right the other half. These misreadings lead to poor actual results in precipitation probability forecasts.