Rare clouds that glow in the dark are visible over the top of the US, Canada and Europe.

The rarest clouds on Earth were spotted by sky watchers in parts of the western US, Europe and Canada over the weekend. This spectacular sight has not been observed for about 15 years.

Known as noctilucent, these clouds glowed a stunning blue light in the sky just after the sun had disappeared below the horizon.

Eerie-looking clouds have been reported from Oregon, Washington, Alberta, the UK and Denmark.

Noctilucent clouds (NLCs) form in the mesosphere, which is about 50 miles high, making them the highest in Earth’s atmosphere.

Clouds are made up of ice crystals that become visible at dusk when the sun is shining over the horizon.

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“There really is nothing like them,” the Seattle office of the National Weather Service wrote on social media, noting that these are “the brightest displays of noctilucent clouds” seen in the area in decades.

Clouds usually form in late spring and early summer when the lower atmosphere becomes warmer.

Atmospheric circulation pushes air up, which then expands and cools.

Water vapor enters the clouds, freezes into ice crystals and forms meteoritic dust.

Known as noctilucent, these clouds glowed a stunning blue light in the sky just after the sun had disappeared below the horizon. Eerie-looking clouds have been reported from Oregon, Washington, Alberta, the UK and Denmark (pictured).

Noctilucent clouds (NLCs) form in the mesosphere, which is about 50 miles high, making them the highest in Earth’s atmosphere. Pictured is Seattle, Washington.

Clouds appear with bright blue and silver bands and are usually visible at latitudes of 45 and 80 degrees in the northern and southern hemispheres.

And this stunning spectacle can even be seen from space, as astronauts aboard the International Space Station have shared photos of the phenomenon.

There are some beliefs that climate change is also contributing to their development and even that they can be seen at latitudes that were not there before.

For example, in 2019 they were seen as far south as Joshua Tree, California, which suggests that the more greenhouse gases in the atmosphere, the more water vapor is available to form glowing clouds.

Cora Randall, a professor at the University of Colorado Boulder, told The Washington Post that the increase in clouds could be due to excess water vapor in the atmosphere from rocket launches.

Another study suggests that the appearance of EFAs fluctuates from year to year and even decade to decade, but overall they have become “significantly” more visible.

In 2020, a photographer shared a stunning image of an apparition in the early morning hours that gave the 12th-century church a ghostly glow.

Ollie Taylor, an astrophotographer, photographed “night-shining” clouds that lit up the night sky in southwest England in spectacular streaks of blue and silver.

Clouds appear with bright blue and silver bands and are usually visible at latitudes of 45 and 80 degrees in the northern and southern hemispheres. Pictured is Alberta, Canada.

On June 22, Taylor went on a mission to capture night-glow clouds in Dorset, on the south coast of England.

He arrived at Knowlton Church in the middle of the Neolithic site and began filming the scene from 2:00 to 2:50.

“It was a great shooting night when I arrived on site in the evening, already met by noctilucent clouds better than I had seen before in the south of England,” Taylor said.

“The bright blue complemented the foggy landscape and eerie structure.”

According to the European Space Agency, Taylor tracked the clouds using a combination of different sources, including space weather updates, webcam observations and a Facebook group.

In 2020, the photographer shared a stunning image of the apparition in the early morning hours, which gave the 12th-century church a ghostly glow (pictured).

Noctilucent clouds were first described in the mid-19th century after the eruption of the Krakatau volcano.

Volcanic ash spread through the atmosphere, creating spectacular sunsets around the world and provoking the first known NLC sightings.

At first, people thought it was a side effect of the volcano, but long after Krakatoa’s ashes settled, thin glowing clouds remained.