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Comment by HankB99 | Hacker News Reader

> the chance of rain was reported as below 50%.

I've come to think of that as "it is going to rain 50% of the time. I don't know if that's what really is meant by "50% chance of rain," but it seems to fit.

And overall I tend to believe that the forecast is astonishingly accurate. This is in the Midwest (Chicago market) where weather has to cross large portions of the country or Canada before it gets to us. I suppose there are areas on the coast where weather is more volatile and harder to predict.

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Chance of rain is defined by NWS as:

"The probability of precipitation (POP), is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Measurable precipitation is equal to or greater than 0.01 inches. Unless specified otherwise, the time period is normally 12 hours. NWS forecasts use such categorical terms as occasional, intermittent, or periods of to describe a precipitation event that has a high probability of occurrence (80%+), but is expected to be of an "on and off" nature."

Source: https://www.weather.gov/bgm/forecast_terms

One of the realisations I only really had in my early 30s is that whether or not it's raining is much more localised than I thought. It can be raining here, and not at the shops a suburb over.

The percentage chance of rain includes whether or not it might rain in your specific dot of a given forecast area, which might be a suburb or entire city, as spelt out in your quote "at any given point in the forecast area".

The first time I drove over the Nullarbor in Australia, which has an entirely flat and straight 100+ KM section of road, I got to see rain far in the distance and experience driving into it having been able to clearly see it's edge from far out. That was an experience I had never had in the costal city I live in (Perth). That also led into similar realisations as the above.

It sounds so simple in theory but was not obvious to me for a long time :)

I think it means "for any given point in the specified area, and for any given point in time, p(rain)=50%

So it of course won't rain for exactly 50% of the time on a given day, but over the long run, it will.

My experience is that about 50% is where “rain will happen somewhere nearby, it may affect me”.

"50% chance of rain" means that there's 100% chance of rain for 50% of the area

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