How this is worked out
Four things spoil a trip: it is too hot, it is too cold, it rains all day, or it is dark by four. All four are measurable anywhere on earth, so that is what the verdict is made of — no opinions, and the numbers stay on the screen next to the answer.
A red month is not a ban. Costa Rica in the green season is half the price and twice as empty, and plenty of people take that trade happily. Red means: this is the thing that will go wrong, so decide about it on purpose rather than on arrival.
Where we have checked a place ourselves, a written note rides on top of the calculation — because no climate record knows that the Inca Trail shuts every February, or that half of Rome closes for two weeks in August.