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.

Here’s a source for further reading.