(WKBN) – Let’s start off with a trivia question.
What does a 50% chance of rain mean? Here are your choices:
A) It will rain 50% of the time.
B) Fifty percent of the area will get rain.
C) Half the area will get rain, and it will rain half the time.
If you picked any of these answers, I am sorry, but you would be incorrect. I apologize for the trick question, but it proves my point that the chance of rain is the most misunderstood forecast tool that meteorologists use.
The good news is that this article will explain exactly what it means and provide some history to back it up.
What does a 50% chance of rain actually mean?
Now, it is time to talk about the actual answer. Did you know that the chance of rain actually goes by a more formal definition? The scientific definition is called the “probability of precipitation,” or PoP for short, and it is actually a mathematical equation:
Probability of Precipitation (PoP) = C x A
“C” is the percentage confidence that rain will form across the area, and “A” is the percentage area that is expected to get rainfall.
PoP is expressed in a 12-hour window (6 a.m. to 6 p.m. and 6 p.m. to 6 a.m.).
Here are two examples where there would be a 50% probability of precipitation.
Example 1: I am 100% confident that rain will form, but only 50% of the area will be affected. Plug these numbers into the equation
PoP = 1.0 x 0.5 = 0.5 = 50% Probability of Precipitation
Example 2: I am 50% confident that rain will form, but if it does, I am 100% confident that the entire area will be affected. Once again, plug these numbers into the equation:
PoP = 0.5 x 1.0 = 0.5 = 50% Probability of Precipitation
What about a 30% chance of rain?
Example 3: I am 30% sure that rain will form, but if it does I am 100% confident that the entire area will be affected
Pop = 0.3 X 1.0 = 0.3 = 30% Probability of Precipitation
Thus, the chance of rain is actually a statistical probability that evaluates the likelihood of rain, the coverage of rain, and the timing of rain.
What is the history of PoPs?
The idea of probabilistic forecasts is not a new idea. In fact, it was first discussed by some scientists in the late 1700s due to the difficult nature of forecasting. Probabilistic forecasting gained major traction in the early 1900s. The Probability of Precipitation was officially adopted by the Weather Bureau (now the National Weather Service) in 1966 and is still used in forecasts today.
Now, the next time it is going to rain, you can think about how the chance of rain is calculated and be more informed about your probability of seeing rainfall.