All Weather News

# What Does a Chance of Rain Really Mean?

19 Aug 2016, 8:31 pm

As meteorologists we often catch a lot of flack for getting the forecast wrong. And sometimes the margin of error we are held accountable for is incredibly small.

If we go on camera and say, “There is a 50% chance of rain.” We don’t necessarily mean it comes down to the flip of a coin. There are many factors that go into predicting precipitation. Moisture, something to lift that moisture high into the atmosphere, and then something to get that moisture to fall out of the sky. All of these factors make it much more difficult to predict rain in local areas.

A simple way to think about it is that if there is a 50% chance of rain, 5 out of 10 days with those exact same weather conditions would produce rain in a given area.

The actually equation to find the percent chance of rain is a bit more complicated and takes two main factors into consideration. First, the chance that it will actually rain on a given day. Then second, the percentage of forecast area that will see the rain.

Take, for example, Denver, Colorado during monsoon season. If we know it will rain in the area, that is a 100% chance. But if only half the forecast area will see the rain, we have to multiply that by 50% since only half the area will see rain. This is an incredibly simplified example, but 1 x 0.5 = 0.5; hence the 50% chance of rain.

So, we aren’t just hanging out in the weather studio flipping coins trying to decide whether or not it will rain. There’s actually a lot more that goes into producing an accurate forecast. But don’t get us wrong— sometimes a coin toss would be much easier.

For WeatherNation: Meteorologist Jeremy Lagoo