How is the chance of precipitation calculated revealed by the science and health meteorological services

If you’re one of those people who never leave the house without first checking the weather forecast, you’ve surely wondered why the percentage of precipitation given by most weather services doesn’t always match what you see outside the window.

“Because it represents the chances of precipitation in your city,” someone must have replied, almost surprised at how important the answer to your question was:

“And statistics are never 100% accurate.”

Others, after inquiring a bit about this topic, may have said that what the percentage represents is the area of ​​the area that will rain over a certain period of time (eg “from 9 AM to 12 PM”).

And to add fuel to the fire, you may have seen some TikTok videos that explain that what the percentage reflects is the certainty that meteorologists have that it will rain in a particular area, based on measurements of factors like temperature, barometric pressure, and wind speed.

Faced with such varied and distinct interpretations of something that seems so simple, BBC News Mundo, the BBC’s Spanish-language news service, decided to look for a more accurate explanation of what the number represented – and realized that in some way every world is right.

Precipitation probability

In order to determine what this percentage actually means, let’s start by reviewing the definition provided by the US Weather Service:

“The probability of precipitation is simply the statistical probability that there will be 0.01 inch [0,25 mm] or more precipitation [seja chuva , neve ou granizo] in a specific area during the specified time period.

The percentage takes into account various factors to express in a statistical value the probability of precipitation occurring at a given point.

“Let’s see an example of what this possibility means,” says the Meteorological Service in its definition.

“If a given area’s forecast says there is a 40% chance of rain this afternoon, that means there is a 40% chance of rain somewhere in the area between noon and 6pm,” he adds.

Based on this definition, the more specific and accurate the atmospheric data collected in the observed area, the higher the accuracy of the probability ratio.

This explains why the data provided by different weather services is different (but not by much).

Two measurements, same result

In order to make a forecast, the meteorological analyst multiplies two factors: the certainty he has that a precipitation system will form or approximate, calculated through atmospheric measurements, by the extent—the physical area—that such precipitation is expected to have in the area that have been analysed.

For this result, it is enough to move two decimal places, and the probability of precipitation is obtained.

This indicates that it is possible to reach the same precipitation percentage with different values ​​for each factor.

To see this idea in action, let’s go back to our example of a region with a 40% probability of precipitation: if the analyst is 80% sure that it will rain in that region (measuring wind speed, air temperature, humidity, etc.), but only expects the system to cover Precipitation is 50% of the area, it will say that there is a “40% chance of it raining” over that time period.

On the other hand, if another analyst estimates that precipitation will cover 100% of the analyzed area, but is only 40% sure that this precipitation will reach the area, he will get the same result: “40% probability of rain in any area during that time period.”

Small differences between systems

Each meteorologist will have their own models of measurement and data collection to calculate the probability of precipitation in the locations they analyze – and some will be more accurate than others.

The important thing is to determine how accurate the aerial data collection methods each service has in the specific area you are in, something that can be done by comparing them and analyzing which one best fits the reality you are observing through the window. ..

And, of course, one cannot forget that meteorology, since it is based on probabilistic models, is far from infallible.

If you rely solely on the weather forecast, you’re bound to leave home one day, no matter how good of a system you’re using, based on the app’s forecast — and stumble across the street in a downpour.