This sounds just like something I’ve always wondered about: the percentages they give in weather reports, for likelihood of rain. Does ’30% chance of rain’ mean that they estimate a 30% chance of getting any rain, or that they think it’ll rain for 30% of the day, or what?
Yes, it means 30% chance of any rain today, and as Cosmos points out it’s based primarily on historical data: 30% of historical situations significantly like this have head rain. And since the estimates update based on new data, they’re practically tautologous.
It depends on where you live. According to The Straight Dope it is typically used to mean probability it will rain compared to historical data. But in some places it is used differently, especially where the weather conditions permit more definite rain forecasts. I live in Florida and there is rarely a question of whether or not it will rain but rather a question of how much geographical area will receive rain. During the wet months the probability of any rain is very high, so it’s only a matter of how much area is going to get hit. During the dry months, the only rain we receive typically comes in the form of fronts moving down from the north, so again chances of any rain tend towards 1 or 0 while the coverage can vary.
This sounds just like something I’ve always wondered about: the percentages they give in weather reports, for likelihood of rain. Does ’30% chance of rain’ mean that they estimate a 30% chance of getting any rain, or that they think it’ll rain for 30% of the day, or what?
Yes, it means 30% chance of any rain today, and as Cosmos points out it’s based primarily on historical data: 30% of historical situations significantly like this have head rain. And since the estimates update based on new data, they’re practically tautologous.
I don’t have a reference for that though.
I thought this was based off of historical data, although I don’t remember the source and could easily be wrong.
If I am not wrong, it should be interpreted as: “It has rained 30% of the time we have had similar weather conditions in the past.”
My memory is telling me that it should be translated, “We expect 30% of the geographic area to get rain today.” I have no good reference for this.
It depends on where you live. According to The Straight Dope it is typically used to mean probability it will rain compared to historical data. But in some places it is used differently, especially where the weather conditions permit more definite rain forecasts. I live in Florida and there is rarely a question of whether or not it will rain but rather a question of how much geographical area will receive rain. During the wet months the probability of any rain is very high, so it’s only a matter of how much area is going to get hit. During the dry months, the only rain we receive typically comes in the form of fronts moving down from the north, so again chances of any rain tend towards 1 or 0 while the coverage can vary.