Project PoSSUM | About Noctilucent Clouds
Noctilucent clouds — the most unusual, extreme clouds on Earth. Stay alert for noctilucent clouds in the next several weeks. Watch the northern skies in evening and morning twilight. Any noctilucent clouds will be bright white or bluish, while other clouds lower down in the troposphere will be already dark.
Noctilucent clouds come in many different shapes and forms, from tenuous veils, to long streaking, parallel bands, closely spaced waves and ripples and sometimes even whirls. In the evening watch the sky starting about minutes after sunset and keep watching for the next hour or so.
In the morning, go out about an hour and a half before sunrise and watch the northern sky.
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Steenderen, the Netherland. Poznan, Poland. Noctilucent clouds are generally made of ice crystals that formed around tiny dust particles in fairly cold temperatures.
Unlike most clouds that float fairly close to the ground, they exist at heights up to 85 kilometers above the surface of our planet, high in the atmosphere that sustains life on Earth. They may look like thin cirrus that we can see throughout the day or night but are generally only visible when the Sun is no more than 16 degrees below the horizon.
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The term "noctilucent" means "night-shining" and it describes these clouds perfectly. They can't be seen during the day due to the brightness of the Sun. However, once the Sun sets, it illuminates these high-flying clouds from below. This explains why they can be seen in deep twilight. They typically have a bluish-white color and look very wispy. Noctilucent clouds were first reported in and are sometimes linked with the eruption of the famous volcano, Krakatoa in However, it's not clear that the eruption caused them — there's no scientific evidence to prove it one way or another.
Their appearance may simply be coincidental. The idea that volcanic eruptions cause these clouds was heavily researched and eventually disproved in the s. Since then, atmospheric scientists have studied noctilucent clouds using balloons, sounding rockets, and satellites.
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They seem to occur pretty frequently and are quite beautiful to observe. The ice particles that make up these shimmering clouds are quite small, only about nm across.
That many times smaller than the width of a human hair. They form when tiny particles of dust—possibly from bits of micro-meteors in the upper atmosphere—are coated with water vapor and frozen high in the atmosphere, in a region called the mesosphere. Noctilucent cloud formation seems to vary as the solar cycle does.
In particular, as the Sun emits more ultraviolet radiation , it interacts with water molecules in the upper atmosphere and breaks them apart. That leaves less water to form the clouds during times of increased activity. Solar physicists and atmospheric scientists are tracking solar activity and noctilucent cloud formation to better understand the connection between the two phenomena.
In particular, they are interested in learning why changes in these peculiar clouds don't show up until about a year after UV levels change. Interestingly, when NASA's space shuttles were flying, their exhaust plumes which were nearly all water vapor froze high in the atmosphere and created very short-lived "mini" noctilucent clouds.
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