Violet asked in Science & MathematicsWeather · 7 years ago

# If the weather says 30% chance of rain what does it mean?

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• Anonymous
7 years ago

30% chance of rain is determined by the probability of precipitation. Probability of precipitation is the probability that more than 1/100th of an inch of precipitation will fall in a single spot, averaged over the forecast area.

In your case, a 30% chance of rain means that there will be a 30% chance that rain will occur at any given point in the area. It also means that there is a 70% chance that it will not rain.

http://www.srh.noaa.gov/ffc/?n=pop

Source(s): Storm spotter/chaser
• TQ
Lv 7
7 years ago

A 30% chance of rain means every point in the forecast area has an 30% chance of observing measurable precipitation (at least 0.01") during the forecast period. It also means rain is expected over 30% of the forecast area.

Contrary to popular opinion...it does not mean...

...it will rain 30% of the time during the forecast period.

...three out of ten times when conditions were the same...it rained.

...it's the probability that more than 1/100th of an inch of precipitation will fall in a single spot... averaged over the forecast area.

50% is considered a 'good' chance b/c either outcome (rain; no rain) is equally likely.

60% is considered a 'likely' chance b/c the odds are tilted in favor of one outcome over another.

Any forecast for precipitation having at least an 80% chance of occurrence is simply stated as 'rain' without a probability.

In this case...a 30% chance of rain also means a 70% chance of no rain.

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For the TX TD troll and his swarm of flat-earthers who dispute the idea POP infers areal coverage...

http://journals.ametsoc.org/doi/pdf/10.1175/1520-0...

Of course...it's just a peer-reviewed paper in a well-respected pointed-headed scientific journal for eggheads.

Source(s): Meteorologist.
• 7 years ago

It is a guess.

Basically, it says that there is a better chance that it will _not_ rain than it will rain. It means nothing about the amount of rain or the duration of it if it does rain.

It might not rain at all, or it might rain all day.

• 7 years ago

It means that when the conditions that exist now in your area have existed in the past, rain has occurred 30% of the time.