Is It Going to Rain Today?
Understanding The Weather Forecast
(Courtesy: National Weather Service/NOAA, Department of Commerce)

Every day people check the weather forecast to plan their daily activities, but do they really understand what the meteorologist is telling them? This document is intended to develop a better understanding of those forecasts. It is designed to help people  make informed decisions about travel plans, sports practices and events, campouts, or other outdoor activities before hazardous weather develops.

The National Weather Service (NWS) is the federal agency charged with providing weather services to the nation. It is the official source of watches, warnings and advisories for hazardous weather. Weather forecasts and warnings can be received directly from the NWS through its nationwide network of NOAA Weather Radio.

This document will use terminology as defined by the NWS; forecast products issued by countries other than the US have different criteria. Forecasters outside of the NWS (such as on radio and television) may use different terms or place a different meaning on weather terms.


The basic weather forecast includes the following weather elements: precipitation, probability of precipitation, sky condition, temperature and wind. Forecasts describe the weather in 12-hour increments such as "today", "tonight" and "tomorrow." Forecasters often use descriptive terms to convey the forecast message. While these terms may be subjective, the NWS attempts to standardize them. Details on precipitation probabilities, sky condition and wind are provided in subsequent sections.


The probability of precipitation forecast is one of the most least understood elements of the weather forecast. The probability of precipitation has the following features:
                    ..... The likelihood of occurrence of precipitation is stated as a percentage
                    ..... A measurable amount is defined as 0.01" (one hundredth of an inch) or more
                                (usually produces enough runoff for puddles to form)
                    ..... The measurement is of liquid precipitation or the water equivalent of frozen precipitation
                    ..... The probability is for a specified time period (i.e., today, this afternoon, tonight, Thursday)
                    ..... The probability forecast is for any given point in the forecast area

To summarize, the probability of precipitation is simply a statistical probability of 0.01" inch of more of precipitation at a given area in the given forecast area in the time period specified. Using a 40% probability of rain as an example, it does not mean (1) that 40% of the area will be covered by precipitation at given time in the given forecast area or (2) that you will be seeing precipitation 40% of the time in the given forecast area  for the given forecast time period.

Let's look at an example of what the probability does mean. If a forecast for a given county says that there is a 40% chance of rain this afternoon, then there is a 40% chance of rain at any point in the county from noon to 6 p.m. local time.

This point probability of precipitation is predetermined and arrived at by the forecaster by multiplying two factors:

                    Forecaster certainty that precipitation will form or move into the area
                                    Areal coverage of precipitation that is expected
                              (and then moving the decimal point two places to the left)

Using this, here are two examples giving the same statistical result:
(1) If the forecaster was 80% certain that rain would develop but only expected to cover 50% of the forecast area, then the forecast would read "a 40% chance of rain" for any given location.
(2) If the forecaster expected a widespread area of precipitation with 100% coverage to approach, but he/she was only 40% certain that it would reach the forecast area, this would, as well, result in a "40% chance of rain" at any given location in the forecast area.

The following terms of duration imply a high probability (80-100%) of occurrence: brief, periods of, occasional, intermittent, frequent.

Terms of uncertainty are listed:                            Probability of                        Descriptive Terms
                                                                          Precipitation                         Used
                                                                          0%                                        None
                                                                          10%                                      Slight Chance, Isolated
                                                                          20%                                      Slight Chance
                                                                          30-50%                                Chance, Scattered
                                                                          60-70%                                Likely, Numerous
                                                                          80-100%                              Categorical  ("Rain this afternoon")


The predominant or average sky cover for the forecast period is also presented in the forecast. This percentage of the sky is the amount expected to be covered by opaque clouds, the type that do not allow other clouds, or blue sky to be visible through or above them. The following table lists descriptive terms for sky conditions:

                        TERMINOLOGY                AVERAGE SKY COVER
                        Cloudy                                    90-100%
                        Mostly cloudy                         70-80%
                        Partly Cloudy/Partly Sunny      30-60%
                        Mostly Clear/Mostly Sunny     10-30%
                        Clear/Sunny                            0-10%
                        Fair                                        Less than 40% cloud cover, no
                                                                        precipitation and no extreme weather


There are several descriptive terms often used to better define wind conditions in weather forecasts:

                        SUSTAINED WIND
                                    SPEED                            TERMINOLOGY
                        0-5 miles per hour                        Light/Light and Variable
                        5-15 miles per hour                      ---No terminology used---
                        15-20 miles per hour (mild)          Breezy
                        15-25 miles per hour (cold)          Brisk
                        15-30 miles per hour                    Windy
                        25-40 miles per hour                    Very Windy
                        40 + miles per hour                       Strong, High, Damaging

(Editors Note: This information is taken from the National Weather Service Brochure, "Is it Going to Rain Today?  Understanding the Weather Forecast.")