What Does a 30% Chance of Rain Really Mean?

It's one of the most common things seen and heard during a weather forecast..."We have a 30% Chance of rain in Central Illinois today?", but do you really know what that means?

It may seem like a simple phrase, but believe it or not, it's a simple phrase that means something different to different people. For instance, some people believe that a 30% chance of rain means that 30% of the area will see rain while others think that it will rain for 30% of the time. Others simply believe it's just the odds of seeing rain. None of these are bad guesses and the last one is actually the closest, but there's a lot more that goes in to that chance of rain.

What Does it Mean?

A chance of rain, also known as the Probability of Precipitation, or PoPs for short, is comprised of two factors...the coverage of precipitation and the confidence that measurable rain (or snow) will fall. Mathematically the equation looks like this:

PoP = (Coverage) X (Confidence)

Here is an example....

Let's say that I am 100% Confident that it will rain over 30% of Central Illinois. In this example:

Coverage = 30% (0.3)

Confidence = 100% (1.0)

So.....Chance of Rain = 0.3 X 1.0 = 0.3 or 30%

That's a simple example of how to get a 30% chance of rain. However, meteorologists are not always 100% confident it will rain.

So, let's now say that I am 60% Confident that it will rain over 50% of Central Illinois. In this example:

Coverage = 50% (0.5)

Confidence = 60% (0.6)

So.....Chance of Rain = 0.5 X 0.6 = 0.3 or 30%

The confidence of the meteorologist plays a significant role in what the chance of rain will ultimately be. Even though more of Central Illinois could potentially see rain, my confidence in rain falling is a little less resulting in only a 30% chance of rain for the region.

A Higher Chance Doesn't Mean More Rain

One thing I often hear is "Your forecast is wrong! You only had a 30% chance of rain and it is pouring at my house!"  

Many people think that the higher the chance of rain the heavier that rain will be...that is incorrect. The chance of rain does not factor in precipitation intensity. For Instance, it is possible for some areas to see over an inch of rain on a day with only a 30% chance of rain while only seeing a tenth of an inch of rain on a day where the chance of rain is 100%. 

How Does Your Local Weather Authority Use Chance of Rain?

We like to use words such as Isolated, Scattered and Likely to describe the chance of rain for Central Illinois. You'll often see these words in our extended forecast, so what do they mean?

10% to 30% Chance of Rain = Isolated 

- When we say "isolated showers" or "isolated thunderstorms", it means that most locations will get through the day without receiving a drop of rain or a seeing a flake of snow. One shouldn't expect rain in these instances but don't be surprised if your recently washed car ends up with a few water spots. Anytime the chance of rain is at 10% or lower we typically leave it out of the forecast.

31% to 60% Chance of Rain = Scattered

- When we say "scattered showers" or "scattered thunderstorms", it means that a good portion of the region could see rain. In these scenarios showers and storms are often more hit or miss in nature.

- We can often break this category down in to two sub-categories, "Widely Scattered" and "Numerous or Widespread".

Widely Scattered - In this scenario, scattered showers tend to cover a large portion of the region, however, there is often a lot of dry area between each shower or storm. Showers and storms tend to be more "miss" than "hit" when they are widely scattered.

Numerous or Widespread - In this scenario, the scattered showers cover much of the region, but there are more wet areas than dry. Showers and storms tend to be more "hit" than "miss" when we forecast showers that will be widespread or numerous. 

61% to 100% Chance of Rain = Likely 

- When we say that showers or thunderstorms are "likely", it means that most of not all of the region will see precipitation at some point during the day. These are days you should expect rain and you'll want to have the rain gear on hand, but don't forget....just because rain is likely to fall doesn't mean you'll see a lot of it! 


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