Science, Tech, Math › Science Chance of Rain: Making Sense of Precipitation Forecasts Share Flipboard Email Print gpointstudio/Getty Images Science Weather & Climate Understanding Your Forecast Storms & Other Phenomena Chemistry Biology Physics Geology Astronomy by Tiffany Means Updated August 08, 2019 Chance of rain, aka the 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'tThree 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% 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 Terminology PoP Precipitation's Areal Coverage -- Less than 20% Drizzle, sprinkle (flurries) Slight chance 20% Isolated Chance 30-50% Scattered Likely 60-70% Numerous Notice that no descriptive words are listed for probabilities of precipitation of 80%, 90%, or 100% 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: A 30% 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: Terminology Rainfall Rate Very light < 0.01 inch per hour Light 0.01 to 0.1 inch per hour Moderate 0.1 to 0.3 inches per hour Heavy >0.3 inches per hour How Long the Rain Will 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: How confident they are that precipitation will fall somewhere within the forecast area.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, the chance of precipitation values is always rounded to increments of 10%. The UK's Met Office rounds theirs to 5%. Continue Reading How to "Speak" Weather Forecasting Why Meteorologists Struggle When Forecasting Snow and Ice How Much Is a "Trace" of Precipitation? Rain, Snow, Sleet, and Other Types of Precipitation Can You Read a Weather Map? Are Those Snow Showers or Flurries Outside Your Window? What Does Weather Smell Like? How to Find Accurate Weather Forecasts Freezing Rain: Is It Rain or Ice? Is It Weather or Climate? How to Tell the Difference Make a Home Weather Station with Your Kids How Do You Measure Weather? 7 Tips for Getting the Most out of Your Whale Watching Trip Do These Natural Signs Predict Winter Weather? What Causes a Dry Thunderstorm? How Old Is Water?