# Why does everyday of the week show a 10% chance of rain?

My question is, “Why does everyday of the week show a 10% chance of rain”? It has rained 1 day in the last 90 days here in the Bay Area.

Where do you see this probability for rain in the Forecast Card or Forecast Page or somewhere else?

In case it isn’t being displayed on non-USA stations…

Oh I see. It is there as well in the rest of the World

Thanks for that hint.@vreihen Do you remember (as I couldn’t find that quickly), I thought we had a topic or at least a discussion on that

I think this was the post from David:

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You’re asking the person who doesn’t even remember what he ate for breakfast two hours ago???

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Exactly, Thanks @peter for helping my memory out

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A 10% round up is not acceptable in my opinion. I work for Oracle. If our software rounded up that much, we would be out of business. I think the calculations need to be adjusted. What brought this to my attention was that I gifted a Tempest unit to my father in law. He said, “It says there is a 10% of chance of rain, but there is zero clouds in the sky and it is 85 + Sunny. And it says everyday it shows 10% in the forecast.” That prompted me to look at my view and it does the same.

As far as I can tell the probabilities in the forecast are rounded to the nearest 5% (0%, 5%, 10%, 15% and so on), so the maximum rounding is likely only 2.5% (i.e. up or down to the nearest 5%).

The issue as David alluded to is that probabilities compound throughout the day. There is a higher chance of it raining at any point throughout the day than there is of it raining at a specific point in the day. That’s why the daily percentage is higher. So the hourly chances of precipitation could be in the 0 to 2.5% range (which round down to 0%), but when all these small chances are compounded together, they give a percentage in the 7.5-10% range.

There is also the issue that the WeatherFlow forecast can never be 100% certain all of the time. If it was, they’d likely be the richest company in the world. So even when it is sunny and 85+, there is always going to be a small chance of rain. For example the UK MetOffice uses “<5%” as its lowest chance, always indicating that they can never be 100% certain in their forecasting. Again those very small chances compound throughout the day to give you the 10% you see.

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It has rained 1 day in the SF Bay Area since June 1st. I think it is safe to throw up a 0% when the forecast is not calling for rain.

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Actually there is more involved than just statistics or math to “compound” those values together. As dsj mentioned some of it is even secret sauce. But even mathematically it could be that you are pretty sure that some rain is passing during the day (say 50%) and still report 0% per hour. (50% / 24 = 2%, rounded down to 0)

if it always showed 10% for daily rain during June 1st, I think it would be pretty save to say that this would be too high.
On the other hand even with 10% per day, there is still (statistically speaking) a change of 4% that you have a whole month without any rain. Unlikely, but still.

I agree - I didn’t mean to suggest the compounding was just pure maths/statistics. Simply that a small chance each hour will always give a larger chance for the entire day

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I also live in the Bay Area…there has been no rain at my house since at least early May (typical for this area)…all of September has said 10% chance of rain and occasionally 20%. It may be an algorithm and “secret sauce” but I’m not a fan of it since it is clearly inaccurate. Strange given that Weatherflow is based in Scott’s Valley, which is 20 minutes from my house.

Count your lucky stars that you have a chance! Mine shows 0% chance of rain until 11 October, then 20%, 30% and 10% for the next three days. Unfortunately we probably have a 0% chance even for those days. It is the same with our “professional” forecasts. We seldom get any rain. A few weeks (maybe months?) ago we went through a three or four day period of constant “Severe Storm Warnings” from NWS and never saw a drop while 10-20 miles either side of us got dumped on! The good news is that we are also missed by the softball sized hail stones and when everyone else gets 24-36" of snow, we get 6"! Wells are going dry here because of insufficient surface water to replenish the aquafers. Good snow pack in the high country last winter but the last two summers have been very dry and fire danger is extreme. I would pay for a 10% chance of rain!