You see it in almost every weather app and forecast nowadays - a percentage that tells you the chance it will rain for each day. Almost everyone seems to have a different interpretation of what it means, and often leads to a lot of confusion.

If there is a 50% chance of rain, some people think it means 50% of the area will receive rain. Others may interpret it as it will rain 50% of the time during the day. Some may think it is just the chance of actually seeing rain. The last one is close to being right, but there is more to it.

The technical term meteorologists use for rain chances is Probability of Precipitation, or PoP. There is actually a math equation to it (don't worry, it is very simple to solve). There are two things we take into account: 1)The CONFIDENCE it will rain somewhere in the area, and 2)The amount of the AREA that will be covered by rain. Multiplying the percent confidence with the percent area will give you the rainfall probability. (PoP=CONFIDENCE x AREA)

Recently, a viral TikTok video stated that a 30% chance of rain means there is a 100% chance it will rain in 30% of the area. That could be true, but not necessarily. What if I am only 70% confident that 40% of the area will receive rain? Doing the math, that roughly comes out to 30% also. 

If I am 100% confident that roughly 50% of the area will receive rain, there is a 50% chance of rain. What if I still think 50% of the area will receive rain (if it were to occur) but my confidence in rain actually occurring is only 50%? That takes the rain chance down to 25%.

Here is where rain probabilities have their biggest shortcoming. The percent actually has NOTHING to do with the intensity, duration, or amount of precipitation that will occur (as long as at least 0.01" falls). What if there is a situation where we are 100% confident that 90% of the area will get rain, but it is only for a quick moving squall line? It may only rain for 5 minutes of the day, yet the 90% rain chance could still verify! 

To further add to the confusion, some meteorologists have their own method of determining rain chances which may not line up with NOAA. When we do show percentages on air, we will normally show it with a chart to discuss whether the rain is isolated or more widespread. Our weather app does show daily percentages, but they are updated daily by a human meteorologist to make the forecast practical and reliable. Other free apps are only computer-driven and can be very misleading. Hopefully this blog helped you better understand what rain chances mean the next time you see them in a forecast!