Why does the weather forecast say it will rain but it will be sunny?

For a long time, meteorologists have long used the probability method of "how likely is it" when analyzing and discussing the weather. Because it can't be accurately expressed in qualitative forecast, they can only simply use "yes" or "no" to express the possibilities of complex weather changes and precipitation. Although it is intuitive and clear, meets the psychological needs of the public and is easily accepted by the public, it masks the randomness of various weather phenomena and the nature of the weather forecast itself.

In the interview, Ding, deputy director of Beijing Meteorological Observatory, said: Any new thing has a process of acceptance and digestion. Probabilistic weather forecast is more scientific in form, more objective in forecast results and richer in forecast connotation, so it has more reference value. It is a kind of progress in the development of weather forecast, and there must be a process of acceptance and digestion.

The "precipitation" in the weather forecast is a random event, and the probability of 90% indicates the probability of this random event. We know that in an experiment, there is a 90% probability that the event may not occur, so "it didn't rain yesterday" doesn't mean that there is a 90% probability that the weather forecast for yesterday's precipitation is wrong.