Just what does a 60% chance of rain mean?

dgk writes:

On Wed, 5 Aug 2009 09:17:59 -0700, "Terry Neff"
wrote:

"dgk" wrote in message
. ..
Does it mean that somewhere within 100 miles of me there is a 60%
chance of rain falling, or does it mean that there is a 60% chance
that I'm going to get rained on (assumimg that I'm outside)?

I'm not a big fan of biking in the rain, although being around 90F
today I sort of am looking forward to it.

I ran into a story about this a few months back and learned something
new from it.

Apparently it is intended to indicate neither the _percentage of
different locations_ within the named area which are expected to get wet
that day, nor the _percentage of time_ any given location might get wet on
that day. Rather, it is intended to indicate the _percentage of different
days_, each having the same atmospheric conditions, when rain will fall
somewhere within the named area. And apparently then that means that on (100
minus _that percentage of days_) no rain will fall anywhere within that
area.

[...]


Ok, so all it is telling me is that there is a 60% chance of rain
somewhere in the area. Actually, it has dropped to 40%. I'm very
likely not to get wet, at least by rain.

I believe it means that there is a 60% chance of measurable rain at
any given point in the forecast area (it's assumed that the
probability is equal everywhere in the area).
From http://www.srh.noaa.gov/ffc/html/pop.shtml :

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.