Just what does a 60% chance of rain mean?

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.

"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.

In my experience it means there is a 60% chance that they will change the
forecast just before you leave.

Seriously, it seems every date this summer I penciled in for a ride the
long range forecast switched. I was almost hoping to get thunderstorm
forecasts as I knew by ride time it would have flipped. The only problem was
that the reverse worked too.

"dgk" wrote: (clip) being around 90F
today I sort of am looking forward to it.
^^^^^^^^^^^^^^^^^^
Normal body temp is 98.6F. You may be dying.

"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.

See:

http://www.sciencedaily.com/releases...0414130623.htm

------------ Begin quote from article linked to above:
....new research indicates that only about half the population knows what a
forecast means when it predicts a 20 percent chance of rain, according to
researchers at the University of Washington.

Writing in the Bulletin of the American Meteorological Society, the
researchers said the confusion comes because people don't understand what
the 20 percent chance of rain actually refers to. Many people think it means
that it will rain over 20 percent of the area covered by the forecast or for
20 percent of the time period covered by the forecast, said Susan Joslyn, a
UW cognitive psychologist and senior lecturer.

"When a forecast says there is 20 percent chance of rain tomorrow it
actually means it will rain on 20 percent of the days with exactly the same
atmospheric conditions," she said."
------------ End quote.

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.

See:

http://www.sciencedaily.com/releases...0414130623.htm

------------ Begin quote from article linked to above:
...new research indicates that only about half the population knows what a
forecast means when it predicts a 20 percent chance of rain, according to
researchers at the University of Washington.

Writing in the Bulletin of the American Meteorological Society, the
researchers said the confusion comes because people don't understand what
the 20 percent chance of rain actually refers to. Many people think it means
that it will rain over 20 percent of the area covered by the forecast or for
20 percent of the time period covered by the forecast, said Susan Joslyn, a
UW cognitive psychologist and senior lecturer.

"When a forecast says there is 20 percent chance of rain tomorrow it
actually means it will rain on 20 percent of the days with exactly the same
atmospheric conditions," she said."
------------ End quote.






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.

dgk wrote:
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.

What I remember from an explanation I heard a long time ago was that 60%
chance meant that 60% of the area in question would get rain, not that
the entire area had a 60% chance of rain and a 40% chance of no rain.

dgk wrote:

Ok, so all it is telling me is that there is a 60% chance of rain
somewhere in the area.

No, it means that there is 100% chance of rain in 60% of the area, but
they don't know which 60% that is. So you do have a 60% chance of
getting rained on.

This makes sense if you think about it. Those moving clouds are going to
dump their water somewhere over the area, it's just not clear exactly where.

On Aug 5, 1:29*pm, SMS wrote:
dgk wrote:
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.

What I remember from an explanation I heard a long time ago was that 60%
chance meant that 60% of the area in question would get rain, not that
the entire area had a 60% chance of rain and a 40% chance of no rain.

Wrong.

http://www.utexas.edu/depts/grg/kimm...forecasts.html

- Frank Krygowski

On Aug 5, 1:31*pm, SMS wrote:
dgk wrote:
Ok, so all it is telling me is that there is a 60% chance of rain
somewhere in the area.

No, it means that there is 100% chance of rain in 60% of the area, but
they don't know which 60% that is. So you do have a 60% chance of
getting rained on.

This makes sense if you think about it. Those moving clouds are going to
dump their water somewhere over the area, it's just not clear exactly where.

Wrong.

http://www.utexas.edu/depts/grg/kimm...forecasts.html

- Frank Krygowski

On Aug 5, 8:36*am, dgk wrote:
Does it mean that somewhere within 100 miles of me there is a 60%
chance of rain falling...

Yes, practically speaking, that's pretty close to it, although the 100
mile radius is too large. Actually, it means 60% of the time the
weather is exactly like this, it's going to rain _somewhere_ in "the
area." I've never found a good definition of "the area" but I'm sure
it's not as big as a 100 mile radius.

,,, or does it mean that there is a 60% chance
that I'm going to get rained on (assumimg that I'm outside)?

Unfortunately, that's not what it means. And it's too bad, because
that information would be a lot more useful!

I've never liked the way they define probability of precipitation.
For one thing, "the area" that a forecast applies to seems poorly
defined, or at least has never been explained to me. What are the
area's boundaries?

For another thing, if (say) the area consisted of one county, and
0.01" of rain fell in only one extreme corner of the county, the
definition allows the weather man to say "See? It rained, just like I
predicted."

- Frank Krygowski