Saturday, October 9, 2010

A Saturday Morning Addendum

Continuing on the subject of the continued absence of rain from our corner of Katy, I've been researching probability theory as it relates to meteorological conditions.  If the forecast calls for a 20% chance of rain on Monday, it was my understanding that those percentages  represented a comparison by computers of present conditions with the historical data: out of the last 100 times we had the same conditions as the present, it rained 20 or 30 times in the Houston area.  A friend with whom I was discussing this seemed quite certain that the prediction meant 20 percent of the greater Houston area would get rain.  

Being quite certain I was the one who was right, and stubborn enough to want to prove it to myself, I've been Googling this for an hour now. There's a lot of information out there, ranging from the academic to the simplistic.  An example of the former: a National Weather Service office in Georgia gave an explanation of "probability of precipitation" which makes my head hurt (and makes me glad I never took statistics):

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%. )

The National Oceanic and Atmospheric Administration's website offered me no help, so I returned to Googling in the hopes of finding something that would put it in simple terms.  I like how The Weather Doctor explained it (he's a PhD with over 30 years experience in meteorology and climatology, which makes him his area's equivalent of Dr. Neil Frank here in Houston, and is more of a reason for me NOT to listen to him but at least his explanation doesn't make my head hurt):

Question: My co worker has stated that when the weather person says there is a 40% chance of rain for the viewing area, that 40% of the viewing area will get rain.

I say that when they say there is a 40% chance of rain that they are saying that over the recorded history of the date that it rained 40 times out of 100 on that day.

Answer: Actually, both of you are wrong. A probability of precipitation or POP of 40% doesn't mean that it will rain 40% of the day, or that rain will fall on 40% of your area or that it rained 40 times out of 100 on that particular day in the past (that is a climatological forecast). It also says nothing about how much rain or snow will occur.

A POP of 40% means that the forecasters have calculated that in a 100 similar weather situations, rain has fallen 40 times in the forecast area. POP is for any point in your forecast area, not the whole area. So, for instance, a POP of 90% for rain means that 9 times out of 10 when this weather situation is predicted, you ought to get rain somewhere in the forecast area, e.g., at your home, playground or at the airport.

Today, however, there is zero percent POP which means one thing: time for the Head Gardener to get out there and start watering!

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