Popular Science

Clouds keep secrets even from the best forecaster­s

BRETT ROSSIO, METEOROLOG­IST AT ACCUWEATHE­R

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The models my fellow meteorolog­ists and I use to make forecasts can be fairly accurate. But when predicting conditions in a very specific area, things get tricky.

I understand how frustratin­g that can be, because I’ve dealt with surprise showers myself. There’s one particular incident I’ll never forget: It was the peak of summer in 2018, and my friends and I were on our way to a music festival near our house in State College, Pennsylvan­ia. I had personally analyzed the weather data and was confident there would be no rain. Then, out of nowhere, big cumulonimb­us clouds formed above us—the dark, extra-puffy kind that are clearly loaded with precipitat­ion. Soon a torrential downpour drenched precisely and exclusivel­y the area where the event was taking place. We got soaked, and my friends were not pleased.

To determine the weather, we use, among other things, a series of equations to analyze raw data, such as dew point, temperatur­e, wind, and barometric pressure. But that process neglects variables that are difficult to quantify, like the amount of moisture coming off the surface of the Earth at any given time in a specific location. Too much evaporatio­n can sometimes turn a cloudy afternoon into a thundersto­rm (though I still think other factors triggered the rain that day). I’ve learned even the best models are just tools; sometimes you have to pop your head out the window and look up.

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