What the 'Probability of Precipitation' on Your Weather App Really Means

Marco VDM/iStock via Getty Images
Marco VDM/iStock via Getty Images / Marco VDM/iStock via Getty Images

After temperature, the most important number to look for when checking the weather is the Probability of Precipitation, or PoP. The day's PoP usually appears as a percentage near a raincloud on your weather app, and without context, it's hard to determine what the figure means for your weekend plans. Does it measure the amount of rain forecast for the day, the area of land it's supposed to cover, or the chances of rain in general? As Refinery29 reports, even experts don't always agree on the true meaning of Probability of Precipitation.

In meteorology, there are at least five accepted ways to measure PoP. NOAA's definition looks at the chance that a forecast zone will receive at least 0.01 inches of rain in a certain time span. So, technically, the number says nothing about how much rain a given area will receive. This is the definition most widely used by meteorologists.

Another method for calculating the percentage is PoP = C x A—or Probability of Precipitation equals the confidence that it will rain in the forecast zone multiplied by the percentage of the forecast area that will receive rain. This formula is less popular with people who study weather, but it has given rise to some misconceptions.

A recent viral video on TikTok claimed that PoP only measures the percentage of the forecast zone the rain will cover, not the chances of rain that day. Another popular misconception is that PoP tells you what percentage of the day will be taken up by rainy weather. Neither interpretation is true—nor are they accurate reflections of the formula above.

Probability of Precipitation is meant to be used as a tool to prepare for the weather in your area. Generally speaking, the higher the percentage is, the more prepared you should be for rain. It's important to remember that the number is just an estimation, and it leaves out information like amount of rain that's forecast and how long it will last. So if you see a 20 percent PoP forecast for the weekend, don't take it as an invitation to plan a beach trip.

[h/t Refinery29]