Frogs and Thunderstorms

By Kent R. Kroeger (July 8, 2019)

Growing up in America’s farm belt, weather proverbs were commonly heard and taken seriously. The one I always remember I first heard from my grandmother: “Frogs croaking on the lake, means an umbrella one must take.” Or something like that.

She didn’t live near a lake, so I’m not sure how useful that piece of folk wisdom was for her, but it stuck with me. And, as it turns out, the proverb has some basis in fact. Frogs do croak more on hot, humid days — which is a good predictor of stormy weather.

But the way my grandmother used the proverb, or at least how my child’s mind interpreted it, I believed for years that croaking frogs caused thunderstorms. Croaking frogs, of course, do not have such power.

Years later, I would realize my grandmother offered me my first lesson in spurious correlations, and I’ve used the croaking frogs proverb in statistics classes many times since.

The lesson is one most people hear many times during their education: Correlation is not causation. Two events (x and y in the graphic below) can be statistically correlated but not be causally related, as they are both impacted by the true causal factor (z).