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> In everyday life, one conflates probability with severity of outcome. My everyday example for that is the weather forecast and the question 'will it rain' of

> In everyday life, one conflates probability with severity of outcome. My everyday example for that is the weather forecast and the question 'will it rain' often answered with a precipitation probability? - Probability: How likely is it that I will be hit by at least one rain drop - Severity: How many rain drops will hit me It sounds a little abstract, but whenever I see some everyday weather forecast I wonder what they are trying to tell me. At least 0% seems unambiguous to me :-)

Infernal 8y ago
From http://www.wesh.com/article/what-does-a-chance-of-rain-really-mean/4440726 http://www.wesh.com/article/what-does-a-chance-of-rain-reall... 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 measurable 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%. )
caf 8y ago
Some forecasts show both the "chance of any rain" as well as various percentiles of the expected rainfall amount, eg this one: http://www.bom.gov.au/places/vic/melbourne/forecast/detailed/ http://www.bom.gov.au/places/vic/melbourne/forecast/detailed...
dionidium 8y ago
If you ask Alexa, "will it rain today?" she'll respond, "it probably won't rain today" for any probability below 50%. I always thought this was an interesting and illustrative example. If there's a 40% chance of rain, I wouldn't in casual conversation say that, "it probably won't rain today."
- other_herbert 8y ago
  At least where I am in the summer 40% or higher is just about a guarantee that there will be rain... That seems like something that might need to change based on location and season
- threeagles 8y ago
  She shoulda just answered that it never rains in southern california. And followed up with what kind of croissant-bakery you'd like to stuff your mouth from.

Infernal 8y ago

From http://www.wesh.com/article/what-does-a-chance-of-rain-really-mean/4440726 http://www.wesh.com/article/what-does-a-chance-of-rain-reall... 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 measurable 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%. )

caf 8y ago

Some forecasts show both the "chance of any rain" as well as various percentiles of the expected rainfall amount, eg this one: http://www.bom.gov.au/places/vic/melbourne/forecast/detailed/ http://www.bom.gov.au/places/vic/melbourne/forecast/detailed...

dionidium 8y ago

If you ask Alexa, "will it rain today?" she'll respond, "it probably won't rain today" for any probability below 50%. I always thought this was an interesting and illustrative example. If there's a 40% chance of rain, I wouldn't in casual conversation say that, "it probably won't rain today."

other_herbert 8y ago
At least where I am in the summer 40% or higher is just about a guarantee that there will be rain... That seems like something that might need to change based on location and season
threeagles 8y ago
She shoulda just answered that it never rains in southern california. And followed up with what kind of croissant-bakery you'd like to stuff your mouth from.