to the question I posted some months ago, as to the meaning of "10% chance of rain":
Precipitation Probability Forecasts
In 1965, the National Weather Service instituted the use of probabilities into precipitation forecasts. This was done since the use of words such as "chance" and "likely" are much too ambiguous for the public to utilize effectively. Today, probability forecasts ranging from 0% to 100% (issued to the nearest 10%) are used to provide the public with more concise information.
There are, however, common misinterpretations of the current precipitation probability forecasts. Consider the following statement "There is a 40 percent chance of precipitation at any location in the forecast area". Unfortunately many people interpret this statement to mean there is a 40 percent chance that measurable precipitation (>0.01") will occur somewhere in the forecast area, and a 60 percent chance that it will not occur anywhere in that area. This interpretation is wrong. In actuality, this forecast means that at any point in the forecast area (such as in the vacinity of your home or farm) there is a 40 percent chance that there will be measurable precipitation and a 60 percent chance that there will be no measurable precipitation during the forecast period.
To break this down even further, one can think of the chance of precipitation occurring at a specific location in a forecast area as the product of two quantities: 1) the probability that a precipitation-producing storm will move into the forecast area, and 2) the percent of this area which the storm is expected to cover. For instance, a forecaster can have a high degree of confidence that a storm will move through an area (say 80 percent), yet determine that only 50 percent of the area will be affected by precipitation. Taking the product yields
0.80 x 0.50 = 0.40.
The forecast in this situation will call for a 40 percent chance of precipitation. This shows that although precipitation is nearly certain (80 percent), the chance it will affect you (40 percent) is much less.