Did you look outside anytime after 4 p.m. on Sunday? No? Did you look at social media?

In either case, you probably saw an incredible sight, or photos of an incredible sight: Clouds stacked up in three-dimensional waves across half the sky, their edges golden and their bodies shadowed.

The sky over downtown was completely dark, but light still poured in over the mountains, beyond the edge of the clouds. Where shadow and sun met, the light traced dream shapes.

“It was absolutely spectacular. The sunsets here are just phenomenal, and last night was definitely a show,” said Russell Danielson, a meteorologist with the National Weather Service in Boulder.

Here’s why it happened, and a bunch of photos.

Essentially, strong winds blew in from the northwest. As they hit the mountains, they “will perturb the air right above the mountains, so that there’s more rising motion downstream of the mountains,” Danielson said.

In other words, the interaction of the wind and the mountains “was strong enough to push the air up and create the clouds.”

Interestingly, the variations in the hills can be reflected in the shape of the clouds, potentially making the effect particularly dramatic over the varied terrain of the foothills.

“When there’s higher terrain, perhaps the air’s forced up stronger and you get a thicker cloud,” Danielson said. “Where there’s lower terrain, the air’s not forced up so much.”

To paraphrase Michael Jordan: The ground is the clouds.

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