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When forecasters see rain or snow on the way, they have some decisions to make about how to describe the precipitation they expect. They need to think about how likely it is, how long it will last, and how intense it will be. What is the probability, or chance, that it will rain? Will it rain “’til the cows come home” or only briefly? When it rains, will it “come down like cats and dogs” or merely sprinkle?

Sometimes when people hear the word rain in a forecast, that’s the only word that catches their attention. They make the mistake of assuming that it is absolutely certain that rain will fall on their head sometime during the day. And if it doesn’t, well, “Wrong again, the weatherperson said it would rain.” More likely, the forecaster used a few key words that didn’t make the same impression as “rain in the forecast.”

The National Weather Service has issued guidelines for expressing the likelihood of precipitation in hopes that certain word usages would become standard among forecasters. It hasn’t worked out very well.

For one thing, the terms the agency uses are easy to misunderstand. It leans heavily on using something called probability of precipitation, which sounds fairly simple when you hear it. But it can mean one thing for one person and something else for another and might even be used to express different things under different weather circumstances.

Technically, probability of precipitation is the chance, expressed as a percentage, that a measurable amount of rain — .01 of an inch or more — will fall someplace in the forecast area in the next 12 hours. It is a combination of two figures — the likelihood of precipitation in the area and the percentage of the area that is likely to get it. This doesn’t tell you as much as you might think. If there is a 50 percent chance that rain will develop anywhere in the forecast area, and 40 percent of the area is expected to get rain, this translates into a 20 percent chance of rain.

A 70 percent chance of rain doesn’t tell you that there is a 70 percent chance of rain everywhere in the forecast area, for example, and it doesn’t tell you that rain will fall on 70 percent of the area. It’s easy to think that a 70 percent chance of rain means a pretty hefty rainfall is likely, and a 20 percent chance means that only a little rainfall is on the way. But that would be wrong, too. Probability of precipitation doesn’t tell you anything about how long it is going to rain or how much it is likely to rain.

It’s easy to see why many television meteorologists put a different spin on the forecast, finding other ways to tell viewers what to expect. Other meteorologists at private forecasting firms such as AccuWeather Inc. employ a more user-friendly “public communications meteorology” that is intended to better describe how the day’s weather is likely to go than the National Weather Service’s way of saying things.

The National Weather Service forecast might say something like this: “Partly sunny today with showers and thunderstorms likely in the afternoon. Probability of precipitation is 70 percent.”

AccuWeather’s version of such a forecast might go something like this: “Sunshine this morning, clouds will billow up during the afternoon, bringing much of the area a brief downpour between 5 and 8 p.m.”

Often, you will hear forecasters use a variety of key words to give more meaning to the probability statistics that the computer models put out. These words can describe the likelihood of rain in other ways. They might tell you how certain the chances are of rain, for example, or they could tell you how much area the rain is likely to cover.

A slight chance of rain, for instance, generally means that the probability of rain is 20 percent or less. Expressed another way, a slight chance of rain can be described as scattered showers. At least, that’s what the National Weather Service has in mind.

When a forecaster says there is a 30 percent chance of showers, it is worth keeping in mind the other side of the coin: There is a 70 percent chance that it won’t rain! When a forecaster predicts scattered showers, it doesn’t mean it is going to scatter showers on you. (So count yourself lucky and give your forecaster a break!)

Another set of four key words often is used to describe the duration of the rain that forecasters expect: brief, occasional, intermittent, frequent.

A weather forecaster also can use key words to describe the intensity of the rain or snow to expect. Nobody does it exactly the same way, but here are how some of the key words are commonly used for describing rainfall intensities:

 Very light, when a trace, or less than .01 of an inch is expected

 Light, from .01 to .10 inch per hour

 Moderate, from .10 to .30 inch per hour

 Heavy, for anything above .30 inch per hour