In a conversation yesterday, it was brought up that there might be rain in the forecast, the number bantered about was 30% chance of rain. Since, it seems that most people either didn't take Earth Sciences in high school, or slept through it, an informal survey was done to determine what was meant by a 30% chance of rain.
The answers were admittedly amusing: rain will fall on 30% of the area, rain will fall, but only with 30% as many raindrops as possible, or, for every hundred drops that fall, only thirty will hit the ground. What these answers lack in accuracy, they certainly have in originality.
They are also, all, wrong. Yes, in Tucson at least, where rain is hit or miss, the idea that the percentage of rain must be the percentage of the area that will actually see rain seems logical, but, in most cases, it is not.
So, for those who were passing notes during Earth Science, a 30% chance of rain is arrived at by establishing what the likelihood is that rain will fall anywhere in the area multiplied by the percentage of the area that will receive rain (or for the math nerds out there 30% = C (chance of rain) x A (area that will receive rain))
The chance of rain is arrived at through the simple expedient of looking at history; under similar circumstaces in the past, how often was the rain the result?
So, as an example, during the monsoon season, when the chance that the storms will dump rain somewhere is a given (100% chance of rain) but only the outlying mountain ranges will probably be seeing any of it (10%) then the chance of rain is 10%.
But for winter storms which tend to cover the whole city and either rain, or not, as the whim strikes them, the chance of rain may be 50%, but the likelihood that if it does rain that any given place will see it is 100% then the chance of rain is 50%.
In most cases, neither number is a 100%. If, in the past a certain weather system only produces rain 50% of the time, and when it did, it only fell on half the area, that comes out as a 25% chance of rain.
But, let's face it, all these numbers are tiny, in actual fact, especially in Arizona, they are calling for a 75% chance that it will not rain where you are. You would pay a lot of money if the odds were 75% in your favor, so certainly, a 25% percent chance of rain is negligible, and hardly worth the effort it took to figure out what it means.
All of this information was double checked, not through a high school Earth Sciences book, but through the Natinal Oceanic and Atmospheric Administration.
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