Chance of Rain: Making Sense of Precipitation Forecasts

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What's the chance of rain today?

It's a very simple question. And while its answer seems equally as simple, most of us misunderstand it without even realizing we do.

What "Chance of Rain" Does (and Doesn't) Mean

Chance of rain — also known as chance of precipitation and probability of precipitation (PoPs) — tells you the likelihood (expressed as a percentage) that a location within your forecast area will see measurable precipitation (at least 0.01 inch) during a specified time period.

Let's say tomorrow's forecast says your city has a 30% chance of precipitation. This does not mean...

  • There's a 30% chance it will rain and a 70% chance it won't
  • Three out of 10 times when the weather is similar, it will rain 
  • Precipitation will fall 30% of the day (or night)
  • Thirty percent of the forecast area will experience rain, snow, or storms

Rather, the correct interpretation would be: there is a 30 percent chance that 0.01 inch (or more) of rain will fall somewhere (at any one or multiple locations) within the forecast area.

PoP Adjectives

Sometimes a forecast won't mention the percent chance of precipitation outright, but instead, will use descriptive words to suggest it. Whenever you see or hear them, here's how to know what percent that is:

Forecast TerminologyPoPPrecipitation's Areal Coverage
--Less than 20%Drizzle, sprinkle (flurries)
Slight chance20%Isolated
Chance30-50%Scattered
Likely60-70%Numerous

Notice that no descriptive words are listed for probabilities of precipitation of 80, 90, or 100 percent. This is because when the chance of rain is this high, it's basically a given that precipitation will occur. Instead, you'll see words like periods of, occasional, or intermittent used, each conveying that precipitation is promised.

You may also see the type of precipitation punctuated with a period — Rain. Snow. Showers and thunderstorms.

If we apply these expressions to our example of a 30% chance of rain, the forecast could read in any of the following ways (they all mean the same thing!):

A 30 percent chance of showers = A chance of showers = Scattered showers.

How Much Rain Will Accumulate?

Not only will your forecast tell you how likely your city is to see rain and how much of your city it will cover, it'll also let you know the volume of rain that will fall. This intensity is indicated by the following terms:

TerminologyRainfall Rate
Very light< 0.01 inch per hour
Light0.01 to 0.1 inch per hour
Moderate0.1 to 0.3 inches per hour
Heavy>0.3 inches per hour

How Long Will the Rain Last?

Most rain forecasts will specify a period of time when the rain can be expected (after 1 p.m., before 10 p.m., etc.). If yours doesn't, pay attention to whether the chance of rain is advertised in your daytime or nighttime forecast. If it's included in your daytime forecast (that is, ​This afternoon, Monday, etc.), look for it to occur sometime from 6 a.m. to 6 p.m. local time. If it's included in your overnight forecast (Tonight, Monday Night, etc.), then expect it between 6 p.m. to 6 a.m. local time.

DIY Chance of Rain Forecast

Meteorologists arrive at precipitation forecasts by considering two things: (1) how confident they are that precipitation will fall somewhere within the forecast area, and (2) how much of the area will get measurable (at least 0.01 inch) rain or snow. This relationship is expressed by the simple formula:

Chance of rain = Confidence x Areal coverage

Where "confidence" and "areal coverage" are both percentages in decimal form (that is 60% = 0.6).

In the U.S. and Canada, chance of precipitation values are always rounded to increments of 10%. The UK's Met Office rounds theirs to 5%.

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Means, Tiffany. "Chance of Rain: Making Sense of Precipitation Forecasts." ThoughtCo, Dec. 9, 2017, thoughtco.com/chance-of-rain-3444366. Means, Tiffany. (2017, December 9). Chance of Rain: Making Sense of Precipitation Forecasts. Retrieved from https://www.thoughtco.com/chance-of-rain-3444366 Means, Tiffany. "Chance of Rain: Making Sense of Precipitation Forecasts." ThoughtCo. https://www.thoughtco.com/chance-of-rain-3444366 (accessed January 16, 2018).