The threat of severe weather last night had lots of us keeping an eye on the sky — including WFPL listener Eric Cooke, in Underwood, Indiana. What he saw prompted him to call us.

The sun set at 8:10 p.m. tonight,” Cooke said. “Then my wife and I walked outside at 8:50 and we saw the most vibrant blue sky! It was sunshine. And I have literally no explanation for what that was.”

We decided to find out, as part of our Curious Louisville project.

The sun setting at 8:10 p.m. is only part of the story. As WAVE 3’s Chief Meteorologist Kevin Harned explains, there are actually three stages of twilight: civil twilight, nautical twilight, then astronomical twilight.

And after that? Nighttime.

“Once it hits night, even though the sun has set, you can still oftentimes see,” Harned says. “So night really began for Louisville at 9:41.”

So it makes sense that he could still see some light. But why was it so bright? Harned says it has to do with light from the sun being scattered by particles in the atmosphere:

“We often think of beautiful sunsets as being red and orange because that wavelength of light gets scattered out a little easier through the atmosphere. It’s a little more rare to see the bluer hues on the other end of the spectrum, which are the whites and the bluer and the violet hues. And in this instance that’s what that would have been. Sunlight going very high into the atmosphere — that’s what you’re seeing.”

Surely this spectrum of sunset light has been visible from Underwood before. Why did last night’s sky look different? 

Cooke sent us a picture he’d taken of the sky, looking toward the west. Along the bottom of the picture, right on the horizon, is a solid mass of dark black clouds. Drew Foster, technical coordinator for the Gheens Science Hall and Rauch Planetarium, says those clouds are the key to answering this question.

“It’s like when we look up at the sky but we want to subtract a bright street light,” Foster says. “We put our hand up in front of the street light, so we still see a bright glow around our hand, but our hand’s dark and the sky’s bluer.”

The contrast between those thick dark clouds and that white and blue light is what made the sky seem so very bright last night from Eric’s front porch.

Or, in Foster’s words, “Mother Nature plays tricks on us.”

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Laura is the producer of Strange Fruit, a weekly talk show focusing on race and gender, and oversees WFPL's Curious Louisville project.