Misunderstood Probablility

Probability of precipitation is given on virtually every weather forecast, but many people don't really understand what it is and where it comes from.

Let's test your knowledge of this important weather concept.

A probability of precipitation of 30% means:

(1) One expects precipitation for 30% of the time.
(2) One expects precipitation over 30% of the area.
(3) There is a 30% chance of precipitation at some point over a particular period.
(4) That it will precipitate 30% of the time over 30% of the area over a particular period.

Time's up. Write it down. The answer is number three. Probabilities are always given for a point in space over a standard period (most frequently over 12 hr time chunks). So at that location over the specified period for similar weather conditions, we would expect it to precipitate 3 out of 10 times.

If you got it wrong don't worry about it....a lot of people do. Consider the study by some European scientists in the journal Risk Analysis (found here). They questioned folks in Amsterdam, Athens, Berlin, Milan, and New York about what a “30% chance of rain tomorrow” meant. Only in New York did the majority of the survey group supply the correct answer. You have got to respect those New Yorkers; they walk and talk fast, but boy do they know their probabilities. In the European cities, the preferred (and incorrect) interpretations were that it will rain tomorrow “30% of the time,” followed by “in 30% of the area.” So much for European sophistication.

Does being precipitation soaked give Northwesterners more insight into precipitation probabilities? University of Washington psychology professor and expert in weather information interpretation, Dr. Susan Joslyn, has completed several studies encompassing hundreds of students to answer this very question. As described in an article in the Bulletin of the American Meteorological Society (found here), she and her colleagues found that only roughly 50% of the sodden UW students got the right answer. Disappointing!

One more interesting question. When was the term probability first used in a weather forecast? The amazing answer: the very first forecast!

Following the signing by President Ulysses S. Grant of an authorization to establish a system of weather observations and warnings of approaching storms, on February 19, 1871, Cleveland Abbe issued the first “official” public Weather Synopsis and Probabilities based on observations taken at 7:35 a.m. that day:

"Synopsis for past twenty-four hours; the barometric pressure had diminished in the southern and Gulf states this morning; it has remained nearly stationary on the Lakes. A decided diminution has appeared unannounced in Missouri accompanied with a rapid rise in the thermometer which is felt as far east as Cincinnati; the barometer in Missouri is about four-tenths of an inch lower than on Erie and on the Gulf. Fresh north and west winds are prevailing in the north; southerly winds in the south. Probabilities: it is probable that the low pressure in Missouri will make itself felt decidedly tomorrow with northerly winds and clouds on the Lakes, and brisk southerly winds on the Gulf."

For his insistence in using the term probabilities, Cleveland Abbe was given the name "Old Probs." In a future blog, I will describe how meteorologists come up with probabilities. Be prepared, this is the meteorological version of sausage making.