Newsday Tuesday

Rainy Weather Forecasts Misunderstood by Many

To bring an umbrella or not to bring an umbrella? That's the perennial question on those days where the chance of rain is less than 100 percent. But only half the population understands what a precipitation forecast means well enough to make a fully informed answer, a new study finds.

If, for example, a forecast calls for a 20 percent chance of rain, many people think it means that it will rain over 20 percent of the area covered by the forecast. Others think it will rain for 20 percent of the time, said Susan Joslyn, a cognitive psychologist at the University of Washington.

A statistically insignificant amount of students tested thought that only 20 percent of the rain would hit the ground because of "evaporization and stuff".

To test people's understanding of these precipitation forecasts (known as probability of precipitation and used in public forecasts since the late 1960s), Joslyn and her colleagues tested more than 450 Pacific Northwest college students in a series of experiments.

The use of Pacific Northwesterners may have skewed the results, since they all presumed that it would always be raining every day of the week. Therefore, the numeric percentage would obviously have to represent something other than the chance of rain. Attempts to normalize results by polling villagers from Wadt Halfa, Sudan failed because they didn't understand what rain was.

The first experiment evaluated forecasts of either a low or a high percentage chance of precipitation accompanied by a series of icons, or "precipicons," that were visual representations of the chance of rain. The precipicons included the familiar cloud symbols used by many forecasting outlets, as well as pie charts and bar graphs.

According to Wikipedia, the Precipicons were a third faction of Transformers which could transform into various stages of the water cycle, led by Hailotron. Susan Joslyn noted that this faction was relatively unknown, which might have influenced the results. Students performed more strongly in a second experiment which used familiar Mega Man villains to represent weather patterns, but unfortunately those results could not be published because of a rights dispute with Dustin Hoffman.

In another experiment, the participants saw one of three forecasts: One had the typical chance of rain; the second had the chance of rain and the chance of no rain; and the third had a pie chart below the chance of rain.

In this experiment, only 22% of students correctly interpreted the forecast. 54% were utterly confused by the format of the test and expressed that they were angry at the researcher but didn't know why. The other 24% predicted that it was going to rain at the bakery.

Joslyn said that the research [...] shows the difficulty of making decisions where uncertainty is involved. People find it easier, she said, to simplify the situation to a single outcome: that it will definitely rain, but not for the whole day or the whole area.

Somewhere during the course of the study, the conclusion that "people are failing to interpret a symbol with a commonly accepted definition" morphed into "people are intentionally changing the meaning of a symbol because they don't like uncertainty". This is a patently false observation. If an escalator consumes your child because you thought the safety sign told him to play Dance Dance Revolution near the edges or the top, it's not the escalator's problem -- you are most likely a moron.

And understanding of how forecasts are interpreted could be useful to government officials who have to decide on school closings, road closures and other potentially expensive measures.

It is actually highly useful for the government to know that 48% of their citizens aren't so bright, because it makes the decisions quite easy. Close schools? Morons needs schooling, so no. Close roads? Morons can't drive, so yes. The possibilities are limitless.

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