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Comment by jrmg | Hacker News Reader

I remember reading in The Signal and The Noise* that people _think_ that forecasts are bad if it rains, but the chance of rain was reported as below 50%. Getting rain when the forecast told you there would probably not be rain is annoying; getting a sunny day when the forecast predicted likely rain is a pleasant surprise.

To get what people judge to be a ‘good forecast’, the chance of rain has to be adjusted to be wildly too high - so that’s what consumer-focused forecasters do.

* https://en.wikipedia.org/wiki/The_Signal_and_the_Noise

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Here in the Netherlands everyone mostly uses short-term live radar tracking of rain clouds and precipitation over actual predictive weather forecasting.

In an urbanized area most "is it going to rain?" questions are short-term, e.g. is now or 30 minutes later a good time to bike home?

Perhaps this wouldn't be as useful in other areas. The Netherlands gets very spotty rain. So even if you've got a 100% chance today it's probably 1-2 hours spread throughout the day, and sometimes very heavy rain followed by a dry spell.

The only time I've seen it to be incorrect is if a moving rain cloud just barely misses you due to changes in wind patterns.

Same here from Germany.

Never really thought about it, but I've opened the "Rain radar" more frequently than any weather app including the native one during the last couple of years, too.

Same in UK

Though I don't know anyone else who does this that doesn't cycle.

Same in Portland, and when cycling. So checks out here :)

Unfortunately the current Apple weather is terrible with their blurry blobs of clouds. Maybe they're trying to hedge when there is any chance of showers.

I sometimes wonder if people think forecasts are bad because they think of it in terms of: there are two possibilities, the forecast will be wrong, or it won’t. Therefore, the weatherman should be right at least half the time.

Of course, there are countless ways for the for the forecast to be wrong, and only a couple ways for the forecast to be right!

I had a high school teacher (not math!) who insisted that if you guessed on a multiple choice question, your chance of getting it right was 50%, regardless of how many choices there were.

> 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.

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

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 :)

> getting a sunny day when the forecast predicted likely rain is a pleasant surprise.

Less so in FL where drought begins 4 minutes after the last rainfall. The 13th month of summer can have us begging for days w/o the migraine-making cancer ball.

Amusingly he was unwittingly writing about his own future. People still make fun of Silver for Trump's win in 2016 because 538's final prediction of about 30% likelihood for Trump was 'wrong'.

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