Understanding the meaning of precipitation "probablities"

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Anonymous's picture

A quote from today's NY Times Weather Report, page B7:

A 30 percent chance of rain is a familiar part of weather forecasts, especially in the summer. But low probabilities of precipitation at any time of year do not preclude rain or snow. A 30 percent chance of precipitation at a given spot means that in past, similar weather patterns, measureable rain or snow fell on 30 of 100 occasions. Surprisingly, even on a rainy day, a probability forecast of a 30 percent chance of rain could be part of a perfect prediction providing that it rains on only to 30 percent of the days when such a prediction is made.

Anonymous's picture
An anonymous cow! (Christian Edstrom) (not verified)

"I believe the only possible reply to this post is:

""Yes, people do not typically understand statistics."""

Anonymous's picture
<a href="http://www.OhReallyOreilly.com">Peter O'Reilly</a> (not verified)
True words

"For those of us including myself whom have a good grasp of probability theory, could someone please explain to me the difference between: ""partly cloudy"" and ""partly sunny""? My take is it's the former if the weather man is in a depressed mood and the latter for a cheerful mood."

Anonymous's picture
Kate (not verified)
i always thought it was day vs night.....

"QUESTION: You guys use the terms partly sunny, partly cloudy, mostly sunny, mostly cloudy. Isn't mostly sunny and partly cloudy the same thing? The same with mostly cloudy and partly sunny? Is there a specific definition for each of these four terms? Thank you.

ANSWER: Yes there is a specific National Weather Service definition for these terms, but I also try to include the public perception into my terminology. The specific definition for ""partly cloudy"" is 30-70 percent sky covered by clouds. Partly sunny is similar to ""partly cloudy"" but is used more frequently during the day to emphasize daytime sunshine. I find that when people hear ""partly cloudy"" they think more clouds than sun and when they hear ""partly sunny"" more sun than clouds. ""Mostly sunny"" is less than 30 percent covered by clouds and ""mostly cloudy"" more than 70 percent covered by clouds. When the sky is ""overcast"" more than 90 percent of the sky is covered by clouds."

Anonymous's picture
jmf (not verified)

Even with 30% probability, the expected amount (at the top of the 4th horizontal panel at http://tinyurl.com/hj3sr ) can sometimes be pretty low, such as the 0.08 over the first period or 0.01 inches over the second period for today Friday; but I don't know whether these expectation values need to be divided by the fractional probability (e.g. 0.08/frxprob & 0.01/frxprob inches if it does rain) :

Anonymous's picture
Hank Schiffman (not verified)
Thank you Troll for that comment...

... which reminds me of the statistician who drowned in a river of which he calculated the average depth to be 6 inches....

Anonymous's picture
Evan Marks (not verified)

And when you posted the original quote you were expecting ... what?


Anonymous's picture
Hank Schiffman (not verified)
30 years in NYC and I am still a hick!

The subject is germaine to cycling. But it is spinach to some.

That bit on roadbikerider.com is pure anti-science. Let a smile be your umbrella and you will have wet feet.

Out of doors sports hinge on weather. Playing the hand you have been dealt correctly is the name of the game.

Anonymous's picture
Mordecai Silver (not verified)
Wise words from Laurent Chambard

"This was in yesterday's roadbikerider.com e-mail newsletter:

Don't check the weather!

That's the strategy used by veteran randonneur Laurent Chambard of New Jersey. Why? Simple: Forecasts that indicate less than ideal weather have caused more no-shows at cycling events than any other factor.

Chambard organizes brevets (rides of 200, 300, 400 or 600 km) for Randonneurs USA, the outfit that qualifies American cyclists for the quadrennial Paris-Brest-Paris randonnee, a 1,200-km timed ride. Chambard's first three brevets (say brev-ays) in New Jersey this spring all happened to fall on rainy weekends.

Some hearty randonneurs rode for hours in the wet, upholding the spirit of long-distance, self-sufficient cycling. And they all lived to tell about it. But some who'd signed up saw the predictions for precipitation and stayed home.

Says Chambard, ""Trying to avoid rain tends to be a hopeless tactic on brevets. Sooner or later the wet stuff catches us.

""I have simply ceased to even consult the weather forecast prior to riding a scheduled event. If I did look at it I would probably spend all these weekends in bed.

""It's a bit like support vehicles,"" he continues . ""People think they make the rider's life easier, but I am sure if I had one I would never be able to ride long brevets. It wouldn't be 50 miles before I would end up sleeping in the back of the car.""

When you train for a big ride, putting in time and effort towards a date circled on the calendar, don't let the weatherman change your plans. Have the clothing you need in case of inclement conditions and go for it. You just might find that after riding through a day in the rain, you have an even deeper feeling of accomplishment -- and a more impressive story to tell."

Anonymous's picture
April (not verified)
Some people have something to prove

"""You just might find that after riding through a day in the rain, you have an even deeper feeling of accomplishment -- and a more impressive story to tell.""

Others simply wants to enjoy life"

Anonymous's picture
An anonymous cow! (Christian Edstrom) (not verified)

What could be more enjoyable than a ride on a bike with proper fenders? With fenders and a rain cape, you can stay dry and comfortable, even on a wet day.

And I've met Laurent. He seems a pretty amiable chap, fairly content with life.

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