Re: Probability of precipitation

"wxforecaster" <wxforecaster@xxxxxxxxx> wrote in
news:1rOsf.14199$0e.1231@xxxxxxxxxxxxxxxxxxxxx:

> Also, you can't simply multiple the chances as you have done below.
> Each time segment is a unique and independent set.

Surely the chance of rain during one segment in a day depends to some
extent on the chances of rain during other segments. If there are 4
segments in a day, and the chances in order are 80%, 90%, X% and 70%,
it's a good bet that X is much closer to 100 than 0.

But in what I described, independence is irrelevant.

I've heard that the probability of getting cancer at least once in one's
lifetime is about one-third. Let's call it 30%. Then what's the chance
that no one in a group of 4 people (chosen at random -- independent of
each other) will get cancer? (1 - .3) to the fourth power, or .240,
right? So the probability that at least one will get cancer is (1 -
..240), or .760 .

Similarly, if a forecast says the chance of rain in each of 4 6-hour
periods is 30%, then the chance of at least some rain that day is .760 .

> The chance of
> precipitation listed in a forecast is typically the highest
> probability that is found in any subset of the day. Smart forecasts
> will even provide detail such as "Chance of rain 30% this morning,
> increasing to 70% this afternoon".
>
>

.

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