I. Introduction to Meteorology

5. What is the probability of precipitation forecast?

Weather is often compared with human moods for its unpredictable nature. Sometimes it rains when the weather report tells you the day is going to be clear and fine. Why is weather forecast so inaccurate at times? This question has almost certainly run through most of our minds during an unexpected weather change. However, it is not always the Central Weather Bureau who is to be blamed. Even though modern meteorology is an advanced science, the subject is simply too complex and fickle to be completely mastered. Especially when rainfall is concerned; there are too many sudden factors that can result in precipitation making precise forecasting difficult. Therefore the Bureau provides the probability of precipitation in its forecast as a reference for us to decide whether or not to take an umbrella.

The probability of precipitation forecast is made by collecting, analyzing, researching and discussing different meteorological data by forecasters. Forecasters will make a prediction in percentage of the probability of precipitation within a specific time and region. Remember though, it is a prediction of the "probability" of precipitation; it is not 100% accurate!

The probability of precipitation is the estimate of the chance of a measurable amount of rainfall (0.1 mm or above) over the 12-hr forecast period in the coming 36 hours in the given forecast region. The forecast is not concerned with the precise time or the measure of area of rainfall. For example, if the forecast states that the probability of precipitation in Taipei City is 70%, it means that there is a 70% chance of rain in the Taipei City area. Currently the Bureau issues the forecast twice daily, at 4:30 a.m. and 4:30 p.m.

Understanding the definition of probability of precipitation will help you be well prepared in a likely raining day. If you don't mind a little rainfall and actually get annoyed more by the need to carry an umbrella, then even when the forecast reads a probability of 70% or 80% you may decide not to take one. However, for elderly people or those who really don't like getting wet, a 30% or 40% forecast is enough for them to take their umbrellas with them.