🚀 go-pugleaf

In article <1e5keuz@Unify.Com>, ceb@dbrus.Unify.Com (Caroline E. Bryan) writes:
> In article <181262@ipl.jpl.nasa.gov> pdb059@ipl.jpl.nasa.gov (Paul Bartholomew) writes:
>>
>>I'm sorry, but neither of these interpretations is correct.  When weather
>>forecasters say that there is a 70% chance of rain, they mean that under
>>these same weather conditions in the past, 70% of the time it rained--i.e.,
>>out of the last 100 days that we had equivalent temperature, pressure, cloud
>>cover, fronts, etc., it rained 70 of those days.
> 
> This guy from JPL, in Pasadena, CA, tells us what DC metro weather people
> mean when they use percentages in rain?  How does *he* know?  I posted the
> above because that's the explanation the DC metro weather people gave *me*
> (and the rest of the local population) once a year, every year, for 20 years.
> DC is not Pasadena, thank God, and the ability to forecast DC weather is much
> better than the ability to forecast Pasadena weather.  What Bartholomew says
> makes sense in a theoretical way, and -- who knows? -- maybe the weather
> people in Pasadena actually use percentages in the way he describes, but I
> *am* correct about how the weather people in the DC area use rain percent-
> ages, and for all I know Mr. Price may be correct, too.
> 
> You'd think that a reader of a.f.u. would not automatically extrapolate
> theory to reality without pausing for a moment to wonder if maybe something
> else might possibly be going on outside his/her own narrow experience.

Well, I've been insulted publicly, so I guess that gives me the right to
respond publicly.  For the record, Ms. Bryan, I have lived in Missouri,
Illinois, Massachusetts, Texas, Thailand, Morocco, Delaware, California,
and spent five years living in Alexandria, Virginia--in the DC metro area
(1969-1974).  In addition, I have visited most of the states in the con-
tinental United States and spent several weeks in Hawaii as well.  In other
words, I suspect my "narrow experience" is considerably wider than yours.

The weather forecasters in DC do not have any greater ability to predict
weather than do those in Pasadena, CA, or in any other part of the country.
If you mean that the DC weather is more predictable, then say so.  Since
they all use essentially the same techniques, they all have essentially the
same results.

As to what these percentages mean, let me quote a pertinent passage from
Cecil Adams (better known as the author of the column, "The Straight Dope")
when he was asked a similar question (quoted without permission from the
book, "More of the Straight Dope"):

"The National Weather Service (and various other weather services around the
world, under the guidance of the World Meteorological Organization, a UN
agency) sends up measuring devices called radiosondes in helium balloons
twice a day.  These collect weather data continuously as they rise through
the atmosphere--wind speed and direction, temperature, barometric pressure,
and humidity.  The information is radioed to the ground and eventually ends
up at the National Meteorological Center near Washington, D.C., where it's
fed into a computer.  (There are other centers in other parts of the world
that do the same thing.)  Other data from satellites and ground reporting
stations are also fed in, and what you end up with, metaphorically speaking,
is a complete three-dimensional picture of atmospheric conditions around the
world.  The computer program then applies various laws of fluid mechanics
to predict future conditions."

"Unfortunately, there is only so far you can go with this.  While it's
possible to predict the temperature, say, with a reasonable degree of
certainty, precipitation is much chancier.  The best forecasters can do is
to give the probabilities, which they do by having the computer compare
present conditions with historical data.  When you hear there's a 10 percent
chance of rain, that means that out of the last 100 times the weather con-
ditions were just like they are now, it rained 10 times.  (More or less--I
am obliged to oversimplify a bit, you understand.)"

"The weather service has been calculating precipitation probabilities for as
long as anyone can remember, but it's only been in the last decade or two
that the nation's cadre of broadcast weather beings has deigned to convey
this to the masses."
..
"On the question of how reliable the figures are, the amazing truth is that
they are absolutely 100 percent reliable all the time--that is, presuming
the raw data were fed in properly and the calculations done correctly.
Remember, we're just talking about probabilities here.  A probability isn't
'wrong' if it tells you there's only a 10 percent chance of rain and it rains
anyway; it's only wrong if a whole series of such predictions doesn't pan
out over the long haul."

Ms. Bryan, if you care to continue this argument, I suggest you find a
reputable source to bolster your case.  The explanation above, in addition
to being correct, is the only logical interpretation of the facts.  Your
explanation, while being charming, bears no resemblance to reality and does
not reflect *my* memory of weather forecasting in D.C. during the five years
that I lived there.

You'd think that a reader of a.f.u. would not automatically extrapolate
theory to reality without pausing for a moment to wonder if maybe something
else might possibly be going on outside her own narrow experience.

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Paul Bartholomew          |   "The time has come, the walrus said, to talk of
                          |    many things..."                  Lewis Carroll
pdb059@mipl3.jpl.nasa.gov |   Disclaimer:  Opinions?  What opinions?
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