Probable Percentage of Precipitation

In our never-ending quest to find a way to use our elementary school math skills, let's start today with a quote:  "40% chance of rain tonight."

What does a 40% chance of rain mean?

Will it rain 40 percent of of the time? ... will it rain over 40 percent of the area? ... will we receive 40% of what has been forecast?

RAIN: precipitation that falls to earth in drops more than 0.5 mm in diameter.

The Probability of Precipitation (PoP) as defined by the US National Weather Service, is the likelihood, expressed as a percent, of a measurable precipitation event (1/100th of an inch) at a grid point during the indicated valid period.

How do forecasters arrive at this esoteric prediction?

Mathematically, PoP is defined as follows:

PoP = C x A 

C = confidence factor that precipitation will occur somewhere in the forecast area

(If C = 1 it is certain to rain; if C=0 it will never rain, and if C is in-between, it might rain)

A = percentage of the area that will receive measurable precipitation, if it occurs at all.

A 40% forecast may mean the forecaster believes precipitation will surely occur (100%  confidence), and he/she is expressing how much of the area will receive measurable rain (more than 1/100th of an inch, falling in drops greater than .5 mm diameter).

PoP = 1 x .4 which equals .4 or 40%

Alternatively, a forecaster might express degree of confidence and area coverage. If he's 50% sure that precipitation will occur, and if it does occur, it will produce measurable rain over 80% of the area, the PoP (chance of rain) is 40%.

PoP = .5 x .8 which equals .4 or 40%

The forecast says on average, for any point within the specified area, during the 12 hour time period, the chance that measurable rain will occur is 40 percent.

In addition to the forecast, there are special "weatherman" terms triggered by PoP percentages:

• 0% - 10% – No mention of precipitation
• 10% - 30% – Isolated or slight chance
• 30% - 40% – Widely scattered or chance
• 40% - 60% – Scattered or chance
• 60% - 80% – Numerous or likely
• 80% - 100% – Rain, plus modifiers like showers or thunderstorms

Don't you hate rain when you are camping?

I hope you learned about fractions and percentages when you were in elementary school. You need them to understand the weather. We cover this subject extensively in our Excel Math curriculum.

Go to the National Weather Service's 2000-word glossary to learn more about their jargon.