Outdoor FunTools & Forecasts

What A Chance Of Rain Really Means… In Terms Of Probabilities

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By Joshua

chance-of-rain-photo-by-joshua.JPG When you watch your local weather forecast and hear the weather person say ‘there’s a 40 percent chance of rain,’ you may find yourself wondering what, precisely, that chance of rain is referring to.

Does a chance of rain refer to…

  • How much rain will fall?
  • How many clouds are dark?
  • How much of your local area will see rain?
  • The chance that it’ll rain versus the chance it won’t rain?

Let’s look at exactly what it means when the forecaster says there’s a chance of rain near you.


What A Chance Of Rain DOES Mean

So, when you hear about a chance of rain, what does that mean?

As the National Weather Service defines it, a chance of rain refers to the probability that it will rain on a given day at any point in a given area.

Therefore, if the forecast says there’s a 20% chance of rain in, oh, say New York City on August 10, that means that any area of New York City has a 20% chance of seeing at least a trace of rain on August 10.


What A Chance Of Rain Does NOT Mean

So what does a chance of rain not mean?

When you hear about a chance of rain, it doesn’t necessarily mean that

  • 20% of the area will see rain
  • Or that it’ll rain 20% of the day
  • Or that 20% of the clouds you see will be dark and rainy
  • Or anything else like that

A chance of rain also has nothing to do with how heavy or how long it will rain.


So Is A Chance Of Rain A Guaranteed Thing?

Always remember that meteorology is an imperfect science. While great strides have been made in forecasting technology in recent decades, there are still too many variables — and Mother Nature has a mind of her own.

It takes a pretty solid forecast to predict something like a 90% or 100% chance of rain. Likewise, it’s pretty infrequent in many areas of the country that a forecaster can suggest a zero percent chance of rain.

However, there are indeed many days — and many areas of the country — where a 0% chance of rain or 100% chance of rain can be reasonably forecast.

Even still, it never hurts to carry along an umbrella, just in case…


Here’s something interesting!…

They studied the Weather Channel’s ability to predict the probability of rain within a 12-hour time slot. As it turned out, the Weather Channel’s forecasts turned out to be pretty good for short-term predictions. When the Weather Channel predicted precipitation probabilities between 40% and 90% within the next 12 hours, they were almost always correct.

See if it’s going to rain or not today!