How to spot these electric-blue clouds that form at the edge of space

Clouds come in all shapes and sizes, but one of the rarest forms where clouds seemingly shouldn’t exist at all: nearly 50 miles above Earth’s surface, at the edge of space.

“Noctilucent clouds are the atmosphere showing off from the edge of space. Ghostly, electric-blue ripples glowing long after sunset and before sunrise,” says Jen Carfagno, meteorologist and co-host of America’s Morning Headquarters on The Weather Channel.

Noctilucent clouds can only be seen in the summer, which means peak viewing season has arrived in the Northern Hemisphere. Here’s everything you need to know.

What are noctilucent clouds?

Noctilucent clouds are unlike all other clouds on Earth. While most clouds form less than 12 miles above the surface in the troposphere, the atmosphere’s lowest layer, noctilucent clouds develop some 50 miles above the surface in the mesosphere. That makes them the highest clouds on the planet.

Up there, the air is extremely dry and cold, as low as -200 degrees Fahrenheit. “Unlike ordinary weather clouds, which are composed of water droplets or ice crystals in the lower atmosphere, noctilucent clouds consist of extremely tiny ice crystals that form around meteoric dust particles in the mesosphere,” says atmospheric scientist Matt Makens.

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Not all of that dust comes from space. Powerful volcanic eruptions can also inject particles into the upper atmosphere, potentially helping seed clouds. That connection has led scientists to a curious historical mystery. The first documented sightings of noctilucent clouds occurred in 1885—just two years after the deadly Krakatoa eruption in Indonesia.

“Some researchers believe that the eruption, which injected enormous amounts of water vapor into the upper atmosphere, may have helped trigger the initial noctilucent cloud displays that caught observers’ attention,” says Makens. “Whether Krakatoa ‘created’ them or simply drew people’s attention to the sky more often is still debated.”

When and where can you see noctilucent clouds?

If you want to spot noctilucent clouds, summer is your best chance. They typically form in the mid- to high latitudes, including northern Europe, Canada, and the northern United States.

While summer is associated with warmer surface temperatures, the mesosphere cools over the polar regions during summer due to atmospheric circulation. That chill is a crucial ingredient in the formation of noctilucent clouds.

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“Summer is when we have the best combination of very cold temperatures high in the mesosphere combined with enough atmospheric moisture and dust to create these clouds on the edge of space,” says Carfagno.

To catch them, look toward the horizon about one to two hours after sunset or before sunrise. Because noctilucent clouds float nearly 50 miles above Earth, they can remain illuminated even after darkness has fallen below.

“Think of it like being in a dark valley while a mountain peak nearby is still lit up by a setting sun,” says Makens. “Your sky is dark, but the clouds 50 miles up are still in full sunlight.”

On occasion, you might spot noctilucent clouds outside of the summer season. “If a rocket launch happens a little before sunrise or after sunset, there is a chance you will see the exotic ‘jellyfish’ cloud! These are actually noctilucent clouds,” says Carfagno.

How atmospheric changes might affect noctilucent clouds

Scientists are also investigating whether noctilucent clouds are becoming more common—and what that might reveal about changes in Earth’s upper atmosphere.

Both observations and satellite data suggest the clouds are appearing more frequently and at lower latitudes than they once did. Researchers suspect several factors may be contributing to the trend, though the exact cause remains unclear.

“It probably is a combination of factors,” says Carfagno. Greater public awareness through social media has likely increased reporting, she says. But “there is more water vapor in our atmosphere due to a warming climate, and prolific rocket launches likely transport both dust and water vapor to higher latitudes,” she says. Because they form at the edge of space, they could serve as visible indicators of changes occurring high above Earth’s surface.

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For Carfagno, however, their appeal goes beyond the science. “As a meteorologist and card-carrying member of the cloud appreciation society, noctilucent clouds feel like spotting a secret layer of the atmosphere most people never realize exists,” says Carfagno. “It’s a glimpse at the boundary between Earth’s atmosphere and space.”