When the weather man/woman says a 30% chance of rain, does that mean 30% of the viewing area or the chance?

Question

I have heard both?

Answer 1

It is about probability. It has nothing to do with the area of coverage. In other words, if the IDENTICAL weather conditions occured 100 times, it would rain 30 of those times. This is just a hypothetical scenario but it is an easy way to grasp statistical concepts like probability. A layman would think "it either will or won't rain, so that's 50% probability!" but that is incorrect.

Although completely identical weather conditions never repeat, there are certain measurable parameters that do (humidity, temperature, pressure, etc) and computer models can use these in their statistical analysis to predict the chance of rain. Obviously, the accuracy is not foolproof because you cannot account for every variable in the atmosphere.

Answer 2

I'd like to know as well.  I thought the definition or usage was regional.  

When I lived in Seattle, a 20% chance of rain meant that it would rain for 20% of the day.  It ALWAYS rains in Seattle when there is any percent chance of rain.   But living in California and Kentucky, the 20% chance of rain seemed to mean that there is only a 20% chance that a given location would receive rain that day.  So in Kentucky, I take a 20% chance of rain to mean a good possibility that it will not rain at all.  Whereas in Seattle with a 20% chance, it definitely will rain but its a question as to when it will rain.

...that may be why the answers are going both ways!  Maybe they are both right.

Answer 3

The percentage describes the chance of rain at any one point within the viewing area.  It has nothing to do with coverage.  In other words, it is possible for there to be a 30% of showers, but for it to rain everywhere within the viewing area.

Answer 4

That means there is a 30% chance of precipitation within the viewing area.  So it could rain only in a small contained area (an area less than 30% of the viewing area).

Answer 5

That is just the probability of rain in the general region, it has nothing to do with percent of viewing area.