What do rain probability percentages really tell us?

Have you ever looked at your weather app and wondered how they calculate the percentage chance of rain? One app might show a 75% chance of rain for your area tomorrow, yet when you delve into the hour-by-hour forecast, that percentage never surpasses 50.

Publication: 02.03.2024 - 12:22
What do rain probability percentages really tell us?
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The figures provided by different sources often conflict. For instance, the Met Office and BBC might each predict an 80% chance of rain at various times throughout the day. So, what should you do with this information?

In the UK, weather app rain probabilities are derived from how often it rained in 100 simulations with similar weather conditions. Thus, these percentages represent the likelihood of rain at any given location during a specified time.

However, in the United States, the scenario differs slightly.

There, the percentage is calculated by considering both the chance of rain and the extent of the area expected to be covered by rain. For example, if there's a 50% chance of rain covering 80% of an area, the overall probability would be 40%.

But what do these percentages actually convey?

Unfortunately, they don’t indicate how long the rain will last or its intensity. Furthermore, weather prediction is inherently challenging, and rain is particularly difficult to forecast accurately. Even slight deviations in the timing or location of precipitation can significantly disrupt forecasts.

Remember, when encountering percentages in weather apps, they indicate the chance of rain in a specific area at a certain time but don’t provide information on the duration or intensity of the rain.

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