OK so I just got caught in a driving downpour, yet the forecast said there was only 25% chance of rain through midday, how does that always happen?… and what does a 25% chance of rain really mean?
The “Chance of Rain” It’s one of the most misunderstood terms in all of weather. We hear and see the probability of rain usually written as a % chance of precipitation either hourly or for the whole day. Few people understand exactly what it means.
Whenever people hear there is 25% chance of rain some people think that’s means 25% of the area will see rain. Some people think it means it will rain 25% of the time. Some people just think it’s the odds of seeing rain for the entire day. The 3rd option is closest to the truth, but it’s not the whole story.
What it is supposed to mean:
You have to start with the real terminology which is Probability of Precipitation or POPS for short. In the purest meaning of the term “chance of rain” used by most forecasters and The National Weather Service. It is a mathematical calculation. It’s an equation using the forecasted coverage of the rain multiplied by the confidence in the forecast.
CHANCE OF RAIN = COVERAGE x CONFIDENCE
A Meteorologist in Boston might say that there is a 50% chance of rain. That forecaster has 100% confidence that 50% of that area will see measurable rain of 0.01” or more. 50% (Coverage) x 100% (Confidence) = 50%
What if they think that 50% of that area will have rain, but they are only 50% confident in that forecast? 50% (Coverage) x 50% (Confidence) = 25% You can see here even though they think the same area will be covered they are not as confident in their forecast.
There are times when a meteorologist is 100% confident in their forecast, that 25% of the coverage area is expected to see rain. So even in this case the chance of rain is 25%. So only when they are 100% confident in their forecast does the rain chance equal the coverage area of rain.
Increasing the odds & how Meteorologists use POPS:
Meteorologists often say there is a 25% chance of rain and that means for any given point on the map. So, if you stay in one spot all day your chance of rain remains 25%. The problem is people rarely stay in one spot on the map all day. So, if you travel from home to work, school, the gym, the park, the grocery store or anywhere else you will be increased your chance of seeing rain. It’s like buying more raffle tickets each one you buy increases your chances of winning.
Chance doesn’t equal intensity or amounts:
Can you have flooding if you only have a 25% chance of rain? Yes! The chance of rain is just that, the chance of seeing measurable rainfall which is 0.01” or more. There is nothing calculated into the chance of rain for how fast it falls or for how long. So, yes if you are the 25% that get rain it most certainly could be a flash flood, especially in summer. Then again, the chance of rain can be 100% and it could just be a few hours of drizzle everywhere. Just because it’s raining where you are, doesn’t mean that chance of rain should be 100%.
So how do Meteorologists use it on TV?
For the most part, they use the chance of rain as the odds of seeing rain during the forecasted times periods throughout the day at any given point on the map. For the most part, they use the chance of rain as their confidence in the possibility of rain. It is a safe bet to just use it as a scale. The higher the number, the better the chance is you will see rain on that day.
So, the next time someone says… “I can’t believe it is raining… the weather man said there was only a 25% chance of rain today!”- you can explain to them exactly what that means. : )
Whatever the chance of rain is today or tomorrow please do get out and enjoy the summer weather. If you have any questions about this blog or your health in general please feel free to contact me at: [email protected]