Noctilucent Clouds (NLCs), Formation, Reason:–Appearance.
Noctilucent Clouds are generally seen in the night sky. Night darkness is in short supply during the middle of the year, as sunset is late in the day and sunrise early. In addition, the Sun’s position below the northern horizon in the middle of the night means much of the UK never sees proper darkness at all. Although this is a challenging time for stargazing, there is another phenomenon that can only be seen at this time of year.
Formation of Noctilucent Clouds.
Noctilucent clouds (NLCs) are ice-sheet clouds forming at an altitude of around 80 km (50 miles) in a thin region within the mesosphere. The mesosphere is a defined layer of Earth’s atmosphere, extending from an altitude of 50-120 km (30-75 miles). During the northern hemisphere’s summer, the temperature of the mesosphere drops, and any water vapor present becomes supercooled. If small particles pass through the supercooled vapor, tiny ice crystals form, creating NLCs. If you’re wondering what the astronomical connection is here, the natural supply of seeding particles in the dust left after a meteor vaporizes in the atmosphere.
If present, NLCs can normally be seen low above the northwest horizon between 90-120 minutes after sunset, or low above the northeast horizon before being drawn. Extensive displays may appear low in the northwest, track through the north, and end low in the northeast.
Despite the Sun has set for us on the ground, from the altitude of the NLC sheet the Sun is still up. Consequently, the sheet reflects sunlight and from our perspective at least, appears to shine at night. This is where they get their name: Noctilucent means “Night Shining”.
NLCs can be very beautiful, glowing against the deep twilight sky with a vibrant electric blue color and often showing fine, net-like structures.
Places of Noctilucent Clouds.
The characteristic blue color comes from absorption by ozone in the path of the sunlight illuminating the noctilucent cloud. They can appear as featureless bands, but frequently show distinctive patterns such as streaks, wave-like undulations, and whirls. Noctilucent clouds may be confused with cirrus clouds, but appear sharper under magnification. They occur during summer, from mid-May to mid-August in the northern hemisphere, and between mid-November and mid-February in the southern hemisphere. They are very faint and tenuous and may be observed only in twilight around sunrise and sunset when the clouds of the lower atmosphere are in shadow, but the noctilucent cloud is illuminated by the Sun. They are best seen when the Sun is between 6° and 16° below the horizon. Although noctilucent clouds occur in both hemispheres, they have been observed thousands of times in the northern hemisphere, but fewer than 100 times in the southern
Paris Turkey Spain Utah
Noctilucent clouds may be seen at latitudes of 50° to 65°. They seldom occur at lower latitudes (although there have been sightings as far south as Paris, Utah, Italy, Turkey, and Spain, and closer to the poles it does not get dark enough for the clouds to become visible. These clouds may be studied from the ground, from space, and directly by sounding rocket. Also, some noctilucent clouds are made of smaller crystals, 30 nm or less, which are invisible to observers on the ground because they do not scatter enough light.
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