What does the weather forecast '30% chance of rain' mean?

Weather forecasts help you plan your activities. Usually, the forecast will not say "yes" or "no" rain, but will give a number %. So what does this % mean?

When you see a high % chance of rain in the forecast, you should be prepared to avoid the rain (Photo: skynesher/ Getty Images).

The percentage chance of rain or snow is called the probability of precipitation (POP). This is the probability that there will be at least 0.25 mm of rain, snow or hail at that location.

For example, the weather forecast for Tuesday in Hanoi is "30% rain" which means there is a 30% chance that it will rain at least 0.25 mm in Hanoi on Tuesday. This number does not mean that it will rain 30% of the day or over 30% of Hanoi. Nor does it indicate how much or how little rain there will be.

For example, just one afternoon shower can dump more rain than a drizzle all day long.

POP Prediction

Meteorological agencies use a combination of models to make their forecasts. These models are like 'parallel universes' that start out the same but progress differently. Some models may show rain while others show no rain.

In the 30% POP example, that means 3 out of 10 models predict rain, snow, or hail.

These models are essentially equational calculations, using data on temperature, humidity, and wind speed provided by satellites, radar, ground-based observation stations, and balloons.

Meteorologists use many different models to make forecasts. There is no universal standard, so they have to use their own scientific reasoning to determine which model is more accurate.

Often, models are calibrated, such as using statistics and sometimes artificial intelligence, to ensure that the probability predictions are as accurate as possible.

Even with best efforts, forecasts often change due to the dynamic nature of the atmosphere. However, since the 1970s, models have become much more advanced and are very useful for long-range weather prediction.