Overview

One of the biggest factors in determining weather impacts is probability precipitation.  It turns out that its also the most easily misunderstood metric in our weather suite.  In this article we examine the nature of the metric "Precipitation %" and discuss how best to use it and how to avoid making incorrect conclusions based on the data provided.  Read below to view the definition directly from the source. Once you understand the technicals of this information, we can discuss the practical impact of the information and how ultimately it affects consumer behavior.




Boy is Clint right.  In the context of forecasting precipitation's impact on sales and consumer behavior, its more about whether the consumer thinks it will rain than how much it actually did rain, when and where.  This is really important, so let me explain by example.  If you're a golfer - when you look to book a tee time you check the weather forecast.  If you're like me and you live in a hot humid summer area where it rains frequently, if you see anything less than 60-65% chance of rain on Saturday, you're gonna go golfing and take the risk.  but how about at 75% chance?  Again, if it were me, probably not - I'm going to plan something indoors instead.  These sort of consumer behavior risk calculations happen constantly as consumers evaluate near-future events.  It turns out if it doesn't rain on Saturday, I may do something outside, but it has definitely altered my planning in a big way.  If it does rain, I also don't much care whether it was 0.1in or 1.1in of rainfall - either way I'm not golfing or mowing the lawn or planting flowers or anything else outside.

So forecasts matter - they affect behavior.  This is basis for studying the level of impact on behavior, not actual rainfall. But before we proceed, lets talk about one other snarly thing about rain - did it or didn't it rain?  We've had people ask - but I need to know whether or not it actually rained.  This type of question brings up the highly localized nature of rain, geographically speaking, along with the fact that its locality moves - usually at between 4-15mph.  What I mean is - where it rains during a given day depends highly on where you are and what time of day you to which you refer!  The nature of rain data collected is based on an area, usually a zip code.  Depending on where you are in the country, a zip code can be a rather small or quite large geographical area.  If a rain storm is moving through the area at 10mph and a zip code is 10 miles wide, if the rain passes over the zip code, its only going to be raining for 1 hour out of a 24 hour day!!  Then we have to know whether 100% of the area in the zip code was rained on or only a portion of it.  Rainfall measurements by their nature are good for determining that it rained, but not when, and they're only able to say with 100% certainty that it rained at the point of measurement, not 5-10 miles North, South, East, or West, unless there is another reporting measuring device.

I hope all these details don't bore you - I'm mentioning them only to state that - it doesn't matter, the biggest impacting factor is the % chance of precipitation.  Determining whether or not it rained can be calculated in surrogate by saying something similar to "if the % chance of rain was above 80% for this zip code, its rainy".  The percentage you use will be based more on the region of the country - 50% chance in the southeast during the summer happens nearly every day, whereas 50% chance in AZ in the desert is extreme where 0-10% is the normal likelihood.

So, to sum it all up - precipitation % is a good surrogate and representative metric for rain and determining behavior. With these few key points kept in mind, you'll be well armed in not overstating or understating the impact of these values when combining them with sales.  It turns out this is more than enough, thankfully.





  • Using Excel & Web Queries To Analyze Rain vs. Sales
    • Link to Article





Technical Definition of Precipitation % - Directly from the National Weather Service


Forecasts issued by the National Weather Service routinely include a "PoP" (probability of precipitation) statement, which is often expressed as the "chance of rain" or "chance of precipitation".

EXAMPLE
ZONE FORECASTS FOR NORTH AND CENTRAL GEORGIA
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
119 PM EDT THU MAY 8 2008

GAZ021-022-032034-044046-055-057-090815-
CHEROKEE-CLAYTON-COBB-DEKALB-FORSYTH-GWINNETT-HENRY-NORTH FULTON-
ROCKDALE-SOUTH FULTON-
INCLUDING THE CITIES OF...ATLANTA...CONYERS...DECATUR...
EAST POINT...LAWRENCEVILLE...MARIETTA
119 PM EDT THU MAY x 2008

.THIS AFTERNOON...MOSTLY CLOUDY WITH A 40 PERCENT CHANCE OF
SHOWERS AND THUNDERSTORMS. WINDY. HIGHS IN THE LOWER 80S. NEAR
STEADY TEMPERATURE IN THE LOWER 80S. SOUTH WINDS 15 TO 25 MPH.
.TONIGHT...MOSTLY CLOUDY WITH A CHANCE OF SHOWERS AND
THUNDERSTORMS IN THE EVENING...THEN A SLIGHT CHANCE OF SHOWERS
AND THUNDERSTORMS AFTER MIDNIGHT. LOWS IN THE MID 60S. SOUTHWEST
WINDS 5 TO 15 MPH. CHANCE OF RAIN 40 PERCENT.

What does this "40 percent" mean? ...will it rain 40 percent of of the time? ...will it rain over 40 percent of the area?

The "Probability of Precipitation" (PoP) describes the chance of precipitation occurring at any point you select in the area.

How do forecasters arrive at this value?

Mathematically, PoP is defined as follows:
PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measureable precipitation, if it occurs at all.

So... in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = "C" x "A" or "1" times ".4" which equals .4 or 40%.)

But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )

In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.