THINGS THAT MAKE YOU GO, “HMMMM……”
Many folks, by definition of “1,000 Year Flood,” think, “OK, it’s 2015 and that just happened in Columbia, SC. We’re good for 1,000 years here.” Well, not really. Kinda reminds me of how they predict rain and snowfall. See below. FJ
Does a 1,000-year rain really happen every 1,000 years?
In short: Not really. The 1,000-year – or 100-year or 500-year – terminology is really a shorthand way of talking about statistical probability and isn’t related to how often a certain event is expected to happen.
“We’re trying to convey the probability of that happening in any given year,” said Jim Kolva, hydrologist with the U.S. Geological Survey’s Office of Surface Water.
In my mind, it’s kinda like a local rain forecast. Some of you may already know this, but if you check your iPhone tomorrow, and it tells you there’s a 40% chance of rain, that does not mean that there is a 4 out of 10 chance it will rain on your head during the day where you are standing.
This is why so many people get so furious at their local news weather person. Truth is, we’re not speaking the same language, and the reason the local weather personality gets lit up on social media, etc. Conversely, this may be why he/she can have such thin skin about what we think. Again, they know we don’t know what they are actually saying.
Think of it. The weatherman is on the hook for being “wrong,” but also free because the public doesn’t know what he or she actually reported. In all, we’re just pissed because it did or didn’t rain or snow on us when he told us it would/might…er, whatever. 🙂
20% Chance Of Rain, Means 100% of Confusion.
by wxbrad+
It’s one of the most misunderstood terms in all of weather, the chance of rain. You hear it and see it in just about every weather forecast nowadays, but few people understand exactly what it means. Whenever people hear the chance of rain, I find that every person interprets it slightly differently. So when we say there’s a 40% chance of rain some people think that’s means 40% of the area will see rain. Some people think it means it will rain 40% of the time. Some people just think it’s the odds of seeing rain. The 3rd option is closest to the truth, but it’s not the whole story.
What it is supposed to mean:
You have to start with the real terminology which is Probability Of Precipitation or POPS for short. That’s what us weather geeks call it. In the purest meaning of the term “chance of rain” used by most forecasters and The National Weather Service. It is a mathematical calculation. It’s an equation using the forecasted coverage of the rain multiplied by the confidence in the forecast.
Its CHANCE OF RAIN = COVERAGE x CONFIDENCE
So I say tonight in Charlotte there is a 50% chance of rain. I am 100% confidence that 50% of our area will see measurable rain of 0.01” or more. The amount isn’t factored in, but I’ll get to that later. So the equation looks like this.
50% (Coverage) x 100% (Confidence) = 50%
What if I think that 50% of our area will have rain, but I’m only 50% confident in that forecast?
50% (Coverage) x 50% (Confidence) = 25%
You can see here even though I think the same area will be covered I’m not as confident in the forecast.
There are times I am 100% confident in my forecast, but just 20% of the coverage area is expected to see rain. So even in this case the chance of rain is 20%. So only when I am 100% confident in my forecast does the rain chance equal the coverage of rain.
There ya go. 🙂
Franklin
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