What is a skyglow? Lets Find Out!
A glowing cloud lit up Northern California?s skies on December 19, 2018. Thousands of people in San Francisco, Calif., gawked at the eerie neon-blue spiral that loomed for an hour after sunset. Even the National Weather Service was baffled by what could have caused it.
Then dashcam video emerged. It showed the instigator was from out of this world. A meteor left a trail of dust that created the noctilucent (Nok-tih-LU-sint) cloud. The cloud?s name comes from the Latin words for ?night-lit.?
Smoke from the burning space rock ?seeded? Earth?s upper atmosphere with dust. Water vapor can condense around those dust bits to form clouds. Meteors burn up high in the atmosphere. So these noctilucent clouds also form high up.
Given the curvature of the Earth, objects high in the sky can still catch some sunlight well after the sun sets closer to the ground. Noctilucent clouds? extreme height is what keeps them shining in the dark. And they appear blue because all of the other wavelengths of light have scattered.
Noctilucent clouds typically emerge at high latitudes, meaning near or over the poles. They almost never appear above the lower 48 U.S. states ? not unless the atmosphere there gets some help, as it did that December night.
Reports of the glowing cloud had begun pouring in around 5:40 p.m. Onlookers flooded the local National Weather Service office with pictures. Many also began guessing at the cloud?s cause. A rocket launch, for instance, might explain it.
The next day, the American Meteor Society (AMS) described 180 eyewitness accounts of what did: a meteor. A so-called fireball, it appeared brighter than Venus as it burned up in Earth?s atmosphere. AMS estimated that the space rock broke apart over open water some 56 kilometers (35 miles) west of the Golden Gate Bridge.
Although space rocks commonly enter Earth?s atmosphere, they have seldom spawned clouds. The reason: Those rocks tend to break apart too high. The mesosphere, where the breakups typically happen, is some 81 kilometers (50 miles) above the ground. It hosts very little water.
But that could change. More water is entering the upper atmosphere as Earth?s climate warms.
A pivotal role for space rocks
For a noctilucent cloud to form, the mesosphere must be super cold ? below ?40