When we check the weather on our electronic devices or listen to a meteorologist on television or radio, we are presented with a percentage of precipitation or rain (for example, 70%). This simple figure tells us the probability of rain throughout the day. Many people don't know **what does the percentage of rain mean**.

Therefore, we are going to dedicate this article to telling you what the percentage of rain means in weather forecasts and its importance.

## What does the percentage of rain mean?

The interpretation of this numerical value is often misunderstood and you may have been confused about the definition of percentage of precipitation in a weather report. It is important to note that even meteorological experts, who are responsible for creating such forecasts, **They do not always reach a consensus on the precise method to calculate this percentage.**

The probability of rain in a forecast is commonly interpreted as the probability of rain at any given time during the forecast period. For example, if the probability of it raining is 30%, then the probability of it not raining is 70%, correct? However, this is not entirely accurate.

The meaning of “percent rain” may not be immediately clear. To clarify, it refers to the probability of precipitation occurring.

According to the United States National Weather Service (NWS), the probability of precipitation (PoP), or the official definition of the percentage chance of rain, denotes the statistical probability of receiving a minimum of 0,01 mm of precipitation within a specific time period at a particular location in the forecast area.

## How to calculate the percentage of rain

To determine the probability of rain, also known as "PoP," meteorologists rely on two factors. The first is your level of certainty (“C”) that it will rain in a given area. The second factor is the degree to which precipitation will be widespread ("A"). PoP is calculated using a simple equation: PoP = C x A. So what does this equation mean? Basically,** The PoP serves as a measure of the meteorologist's confidence that it will rain in the forecast area.** The “A” factor, on the other hand, represents the estimated percentage of the area where a measurable amount of rainfall is expected to occur.

Suppose we know with certainty that precipitation will occur in 30% of Paris. In that case, we can confidently say that there is a 30% chance of rain. Similarly, **If our confidence level is 50% that it will rain throughout Porto, then there is a 50% probability of precipitation.**

If the weather forecast indicates a 50% chance of rain, it is advisable to carry an umbrella with you. However, it is important to note that the probability of precipitation does not correlate with the severity or duration of the predicted rainfall.

The approach used by meteorologists to assess the probability of precipitation is not uniform across the profession.

As discussed above, even with the established formula that serves as the standard in meteorology, **Various professionals in this field rely on their own interpretation and approach to assess the probability of precipitation.** There may be some disagreement among meteorologists regarding the precise calculation of this probability, but the discrepancies are not significant enough to alter our daily routines.

## What does the percentage of rain mean in apps?

When leaving home, many people discover that it is no longer enough to look out the window to check the weather and know if it is going to rain. Now, more and more people are checking mobile apps to check weather conditions before going out. However, you may be wondering what percentage of rainfall most weather services provide and why it doesn't always match what you see outside your window or what ends up happening. **Don't worry, it's not a bug on your phone.**

One of the most basic answers to what this percentage means is that "it represents the probability that it will rain in your city, but they are not always correct." Others say that this percentage corresponds to "the time they tell you the area of the territory where it will rain."

This rain percentage can tell you more information than you think because the number means the probability of rain, the surfaces that will get wet, and how intensely they will get wet. According to the National Meteorological Service (AEMET), **This number is prepared based on previous data and uses it to show the percentage of times it has rained** in the area you are looking at in the same conditions you were in at the time.

The organization provides precipitation percentages for each location that are as close as possible to actual conditions. However, the larger the area over which we are looking for rainfall data, the less accurate this number will be.

So when we see 60% on a weather app, for example, it doesn't tell us that it's going to rain on 60% of the land, or that there's a 60% chance it will rain that day. In fact, it tells us how often it rained when similar weather conditions occurred in the past. In this case, that means that under current atmospheric conditions, six out of ten times it has rained in the past.

## Different results

To make a forecast, weather analysts multiply two factors: **the certainty that a precipitation system is forming or approaching it**l, calculated from atmospheric measurements, multiplied by the extent (physical area) that precipitation system is expected to have. Within the analysis area (the result is shifted only two decimal places and the probability of precipitation is obtained).

This shows that the same percentage of precipitation can be achieved by setting different values for each factor.

To see how this idea works, let's go back to our example where **The area has a 40% chance of precipitation: if the analyst is 80% sure that it will rain in the area** (measuring wind speed, air temperature, air humidity, etc.), but expecting the system to cover only 50% of the area, it will say there is a "40% chance of rain" during that time.

On the other hand, if another analyst estimates that the precipitation will cover 100% of the analyzed area, but is only 40% certain that the precipitation will reach the area, he will obtain the same result: "Anywhere in the area during this period the rain is 40%.

I hope that with this information you can learn more about what the percentage of rain means and its importance.