Californians caught the sight of a lifetime when rare clouds recently illuminated the sky.
Photographer Rain Hayes tweeted a photo on Dec. 16 of cobweb-looking wisps of clouds in the night sky in Oakland.
“Strange bright cloud over Lake Merritt during the tail end of astronomical twilight this morning,” she wrote in her tweet.
The beautiful landscape caught the attention of climate scientist Daniel Swain, who offered an explanation as to what the internet was looking at.
“Great shot of what appears to be a noctilucent cloud over the SF Bay Area this morning!” he wrote. “Such clouds are very rare at this latitude and also in winter, and are the Earth’s highest and driest clouds, forming in the mesosphere about 50 miles.”
Swain also explained that the photo was singular — especially so close to Christmas.
“It’s a pretty unique shot, given the very rare confluence of location, seasonal timing, and other unique conditions (let alone visible from Oakland). Congrats!” he wrote.
As Swain mentioned, noctilucent clouds are usually visible in the summer months and only happen when multiple elements align.
Most clouds form in the closest layer of the Earth’s atmosphere, which is called the troposphere. But the clouds in the image formed in the mesosphere, which is the third layer of Earth’s atmosphere. That makes them Earth’s highest clouds as they usually form between 47 to 53 miles above the surface, according to Space.com.
They also require water vapor, dust and very low temperatures to come together, according to the UK Met Office.
The sun then needs to illuminate the dust particles from below. Ice crystals reflect the sunlight and appear as electric blue wisps in the night sky, reaching out toward the edge of space.
Earlier this year, a Minnesota woman captured a cloud formation that appeared as an ocean in the sky. In August, “rainbow clouds” were spotted in China and puzzled the internet.