You’ve seen it in weather reports: “Today there is a 60% chance of rain”. Does this mean you have a 6 out of 10 chance of getting wet if you are outside? Will only 60% of your area definitely get rain? Will it rain 60% of the day? Just what does percent chance of rain mean to a weather forecaster?
- The Probability of Precipitation or POP is the chance that anywhere in the forecast area sees rain (or sleet or snow).
- It does not tell how long it will rain.
- Similarly, it does not tell how much it will rain.
PoP and Chance of Rain
Weather forecasters use the term PoP, or Probability of Precipitation, when describing the percent chance of rain. PoP is defined as the chance of measurable precipitation occurring at any point in a selected area. This definition means that at any given point in the forecast area, there is a PoP percent chance of measurable precipitation during the time period of the forecast. But where do weather forecasters arrive at this value?
The probability of precipitation is the product of two variables. The first variable is “confidence”. How confident is the forecaster that precipitation will occur somewhere in their forecast area? This value ranges between 0 (no precipitation possible) to 1 (definitely going to rain) and comes down to the forecaster’s best guess from other data and models.
The second variable is “area affected” or A. This is the percent of the forecast area that will receive the precipitation if there is any precipitation. Again, this variable comes down to the forecaster’s estimate based on models and historical data.
PoP Examples
Since both variables have a certain slush factor, PoP has different meanings depending on which variable the forecaster focuses on.
Example #1
If the forecaster thinks it definitely will rain (C = 1) over 60% of their forecast area (A = 0.60), then
PoP = C x A
PoP = 1 x 0.6
PoP = 0.6 or 60% chance of precipitation.
Example #2
If the forecaster is only 75% sure it will rain (C = 0.75) and if it does, only 80% of the forecast area will see that rain (A = 0.80), then
PoP = C x A
PoP = 0.75 x 0.80
PoP = 0.60 or 60% chance of precipitation.
Chance of Rain Analysis
Both of these examples give the same value for the chance of precipitation, but they have different meanings. The first example shows that 60% of the area definitely will see some measurable rain while 40% will not. The second example shows a smaller chance of rain that affects a larger area.
Typically, weather forecasters offer additional information where you can make your own decisions about possible rainfall. Some forecasters emphasize the chance you will see rain at some time during the forecast period. Others emphasize the area affected by the forecast. If they say 60% chance of rain, 60% of their area will experience rain at some point during the forecast period. The forecaster often mention which interpretation they adhere to.
Keep in mind, PoP only refers to the chance of precipitation, not how much rain you will get. A 100% chance of rain applies to buckets of rain all day or just a light sprinkle in the afternoon. All you know is that some sort of precipitation is probable. Likewise, 20% chance of rain means you’re unlikely to see any rain at all today or 20% of the area is going to be drenched.
Alternate Terms for PoP Values
Some forecasters offer terms like ‘isolated showers’, ‘scattered showers’, or ‘showers likely’ when they wish to avoid presenting a numeric value for PoP. ‘Isolated’ generally means ‘low PoP’, ‘scattered’ means roughly 50-50, and ‘showers likely’ corresponds to high PoP values.
Your best interpretation is knowing your forecaster. An individual uses the same interpretation each time and offer explanations if there are other discussion points to their predictions.
References
- Enyedi, Angie (2010). “What are POPs?“. National Weather Service Weather Forecast Office, Jacksonville, FL.
- “Precipitation Probability.” weather.gov.