Another Lively Season of Night-Shining Clouds


Extremely cold and wet conditions in the mesosphere have led to abundant noctilucent clouds.

Every summer in the Northern Hemisphere, electric blue streaks form high in the atmosphere. These seasonal clouds typically lurk about 80 kilometers (50 miles) overhead in the mesosphere around the Arctic, but every once in a while they form at lower latitudes. In 2019, the clouds showed up in places where they were only rarely seen in the previous decade, including California, Colorado, and France. This year, the clouds are equally impressive.

“It‘s another incredible year,” said Lynn Harvey, an atmospheric scientist at the Laboratory for Atmospheric and Space Physics at the University of Colorado. “When noctilucent clouds extend to mid-latitudes—where people live and notice them on a daily basis—we consider that a noteworthy season.” This year’s clouds have been seen as far south as Joshua Tree, California.

Noctilucent clouds form when water vapor aggregates and freezes around specks of meteor dust floating in the mesosphere. These thin, wavy ice clouds reflect sunlight and usually shine bright blue and white. Known as “night-shining” clouds, they typically appear around dusk or dawn when the Sun is below the horizon at an angle that lights the clouds from below.

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Image via NASA Earth Observatory