Weather forecasts help you plan your activities. Typically, rainfall forecasts do not specify “yes” or “no,” but rather provide a percentage. So what does this percentage mean?
When you see a high percentage chance of rain in the forecast, you should be prepared to avoid getting wet (Image: skynesher/ Getty Images).
The percentage chance of rain or snow is referred to as Probability of Precipitation (POP). This represents the probability of receiving at least 0.25 mm of rain, snow, or sleet at that location.
For example, if the weather forecast on Tuesday in Hanoi is “30% chance of rain,” it means there is a 30% chance that at least 0.25 mm of rain will fall in Hanoi on Tuesday. This figure does not imply that it will rain for 30% of the day or over 30% of the area of Hanoi. It also does not indicate how much rain will fall.
For instance, a brief afternoon downpour could result in a total rainfall that exceeds the light drizzle throughout the day.
Understanding POP Predictions
Weather agencies combine various models to generate forecasts. These models are akin to “parallel universes” that start with the same conditions but evolve differently. Some models may indicate rain, while others may predict no rain at all.
In the example of a 30% POP, this means that 3 out of 10 models predict that there will be rain, snow, or sleet.
These models are fundamentally calculations based on equations, using data on temperature, humidity, and wind speed provided by satellites, radar, ground observation stations, and weather balloons.
Meteorologists utilize different models to create forecasts. There is no global standard, so they must apply their scientific reasoning to determine which model is more accurate.
Often, models are refined, such as by using statistical data and sometimes even artificial intelligence, to ensure the most accurate probability forecasts possible.
Even with maximum effort, forecasts often change due to the dynamic nature of the atmosphere. However, since the 1970s, models have significantly advanced and are very useful in long-term weather predictions.
