Amazing Planet

Noctilucent Clouds Illuminate The Night Sky Near Earth's Poles

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According to NASA, 1885 was the first time people noticed them: silvery blue clouds shining brightly high up in the night sky. These noctilucent, or "night shining," clouds, only appear near the poles, and their existence has led to more than a century of research into their origins.

Looking down from above, AIM captured this composite image of the noctilucent cloud cover above the Southern Pole on December 31, 2009.

Twinkle Twinkle, Little Cloud

The noctilucent clouds are formed above the North and South Poles in the mesosphere, about 50 miles (80 kilometers) up. They're not only the planet's highest cloud formations, they're also the coldest—their chilly vapor gets down to -210 degrees Fahrenheit (-134 degrees Celsius). Scientists have determined that the likeliest explanation for their unearthly glow is that they're made up of ice crystals that form around meteoric dust. The sky-high location of those crystals helps them reflect sunlight from beyond the horizon—sunlight that doesn't reach people on the ground, who therefore see a dark sky and an illuminated cloud.

Noctilucent clouds are formed during the warmest months of the year because the warming air in the lower layers of the atmosphere rises, then expands as the atmospheric pressure drops. That expansion cools it down. NASA compares it to the way liquid from an aerosol can feels cold—that rapid expansion lowers its temperature, even if the can itself is at room temperature.

Noctilucent clouds at night in Finland.

The "Why Does It Matter?"

Scientists aren't studying noctilucent clouds just because they're beautiful. Knowing more about them has been helpful in determining what exactly is happening in our atmosphere. For example, researchers have discovered that the clouds above the South Pole respond to what's known as the southern stratospheric vortex, a winter wind pattern that circles above the pole. That tells us why the formations are so inconsistently timed, with the differences often varying by more than a month from year to year.

In fact, further analyses will better understand the significance of how connected the South Pole's noctilucent clouds are to the North Pole's. Despite the fact that they're more than 13,000 miles away, the air circulation of the winds seems to correspond with both. Since patterns in the lower layers of the atmosphere don't seem to affect what's happening in the mesosphere as heavily as previously thought, the clouds could unlock new understanding of how air circulates through our atmosphere. Not only will that help us better understand the atmosphere as a natural terrestrial force, but also how human activity can affect it.

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