Its 11:30am and I wanted to check the forecast for this afternoon. One site says it’ll be a 45% chance of rain starting at 5pm, and another site says it’ll be clear until 8pm. How can it be so different when it’s only a few hours away?
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What i know is that all sites use the same datas and It depends on the algorithm (or whatever it is) that the sites use for calculating the probability of the weather. It depends on what the percentage means too, for examples in some sites “50% of rain in Dublin” means that it’s 100% sure that in half of the territory of Dublin will rain. In other sites it means that you have the 50% of the chance that’s raining in all the territory and this is very confusing because it could mean that maybe will rain or maybe not in every part of the city.
Maybe I’m wrong, i’m not an expert, but this is what i found when i was asking the same question.
Have a look at windy.com and go the the compare function. You can see how the actual models differ. Often they agree that rain is coming, just not exactly when or how much.
The simple answer on why they disagree is that predicting the weather is really hard, there are a near infinite number of variables that feed into what will actually happen. Hence the apocryphal butterfly effect.
I have seen torrential downpour across the road from my place and not a drop at my place.
Tornados can also destroy one house and 2 houses away could have no damage at all.
Not all weather sites have monitoring stations everywhere and pick close ones.
Also with some apps I can pick a city name and it will show the center of the city or I can use the precise location and it picks the closest one to me.
Depends where the site is getting its info, whether any of it is human generated/interpreted/processed, how old it is, what model(s) they use, etc. There’s a ton of variables that go into producing a forecast.
also at some point you just need to acknowledge that rain is not global and there is an edge somewhere
you can have 2 forecasts for the same city – rainy and sunny
both of them can be correct
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