WHAT EXACTLY DO “RAIN CHANCES” ACTUALLY MEAN? I have been asked this question more during the last six months, presumably because of a viral TikTok video that incorrectly explains what a “chance of rain” actually is. Before we answer the question at hand as to what exactly “rain chances” actually mean, there is a dirty little secret you need to know:
Meteorologists across the nation and the world do not agree on how rain chances are calculated each day.
You read that correctly. You may be watching our RedZone Weather coverage and hear me say “70 percent chance of rain,” yet you may watch a local TV station and hear another meteorologist say “a 50 percent chance of rain” for a given day. The truth is: We both can be right, even though the numbers are different. How so?
The way I calculate rain chances each day for south Alabama and northwest Florida is a fairly simple formula: Probability of Precipitation (POP) = confidence of rain happening somewhere in the forecast area times area, or expected area to receive measurable rain. The formula looks like this: PoP = CxA.
Let’s take tomorrow as an example: a 60% chance of rain. How did I devise that forecast? My confidence that rain will happen somewhere inside our 12 county coverage area is 90%, aka very high confidence. I expect approximately 65-70% of the land areas in our region to have rain at some point during the daytime hours. Expressed as a decimal, 0.9 (for the 90% confidence) times 0.7 (for the 70% of the area) = 0.63 or a 60% chance of rain on Tuesday.
One more example, and this is one that we see on many summer days around here: a 30% chance of rain. Most summer days around here feature at least one shower across the area, so let’s say confidence is at 70%. On this particular day, I expect 40 percent of the area to have rain before the end of the day. 0.7 times 0.4 equals 0.28 or rounded up to a nice, even 30% chance of rain.
Now, again, the meteorologist down the road may use a different formula to arrive at a slightly different result. He or she may have a forecast area that only includes one or two counties. The rain chances in those two counties may be different than across our 12 county RedZone Weather coverage area. This is one of many reasons why you may hear different “chances of rain” on the same day coming from different credible sources of weather information.
Another twist in all of this is that some meteorologists do not use the PoP formula and instead may be forecasting for one point and totally rely on the confidence factor for if rain is going to happen.
What rain chances do NOT mean is that if there is a 30% chance of rain that 30% of people in a given area will see rain on a given day. I have personally never seen a meteorologist use a rain chance number exclusively based on population density, however, that does not mean there isn’t someone out there who uses this method. The viral TikTok promoted this theory, but they didn’t get their facts straight. Most of the meteorological world uses the PoP formula.
If you or someone you know had a conversation about that viral TikTok, please join me in helping get the truth out about what rain chances actually mean. Lots of confusion around the topic, but we’ll do our best to make sure you understand exactly what to expect each day. This is a reason we show the projected radar in just about EVERY, single forecast video I produce.
APP… If you haven’t already downloaded the RedZone Weather app, now is a great time to do that. redzoneweather.com/app is the link to the free download. Once you have the RZW app installed on your iOS or Android device, be sure to visit the Alerts tab to turn on the specific notifications you’d like to receive. All notifications are handcrafted by me. No automation and we promise not to bug you!
Have a nice Monday evening!