How to interpret reliability
October 30, 2019
I am almost sure that you are familiar with this image. It is taken from my favourite weather forecast portal, nut there are many others. I highlighted the reliability of the forecast, that is 50%. You may think: “Oh, not bad, after all”. Indeed, it is not the case: 50% means absolute uncertainty; it is pure case, like playing heads or tails with a coin.
In information theory this is called “entropy”, so the situation of maximum uncertainty occurring when two events have the same frequency; there is no information.
It is not that meteorological models do not work, but they simply return the probability of a forecast on the basis of all the data they have inside, and the calculation algorithm they use.
Consider, that if they stated 30%, this would be a more reliable forecast that 50%: it would in fact mean that, at 70%, the weather would not be what forecasted. But indeed, you will probably never read 30% or 40% reliability. Or, in case this happens, it would be a nonsense. Maximum uncertainty is indeed 50%.
So ? We must still wait a while to know how will be the weather on Sunday: so that, the model could better orient itself. It is evident that the tendency is of bad weather, but it not for sure that it could rain.
Be aware: the same considerations hold also when the forecast says: sun.
Weather forecasts. Filippo D’Antuono. piudimille.com. All rights reserved