- Education and Science
Noctilucent Clouds are also known as "Space Clouds."
The Description of Noctilucent Clouds
Noctilucent clouds are clouds that are illuminated after the sun has set because they are so high above the ground that they are in the sunlight as the light passes overhead. While the ground is in the Earth’s shadow, light passing high enough can be reflected by clouds at great altitudes. This makes noctilucent clouds visible in the night sky.
Noctilucent clouds are thin, silvery white clouds. They can show some structure, such as a wave nature. These unusual beautiful clouds are composed of ice crystal clouds.
This is the book featured in the introduction image.
The Atmospheric Layers
So, where exactly is the mesosphere, and how does it fit in the Earth’s atmosphere as a whole?
The lower part of the Earth’s atmosphere is the troposphere, which is where the weather occurs. The next layer is the stratosphere, which is normally above the weather, although severe thunderstorms can have cloud tops that penetrate the stratosphere. Included in the stratosphere is the ozone layer. Higher still comes the mesosphere. Above the mesosphere are the thermosphere, then finally the exosphere that slowly weakens to outer space.
Noctilucent clouds are mesospheric clouds, which is normally too high for water vapor from the ground to rise. This caused the mystery of where could the water come from to cause the ice, and what condensation nuclei could be present at that altitude in great enough abundance to promote cloud growth. The air normally gets cleaner with height.
These mysteries are now believed to have been solved, and are treated below in the section titled “The Current Theory.”
The Erroneous Association with Volcanoes
In the 1880s noctilucent clouds began making frequent appearances. The search for moisture being introduced into the mesosphere, and dust as well so the moisture had something to condense upon, naturally led to the erroneous conclusion that these are the result of a violent volcanic explosion. Krakatoa had exploded in the early eighteen eighties, and did propel large quantities of dust and water upwards. The explosion was so massive that sunrises and sunsets were reddened more than usual due to light being more efficiently scattered, and temperatures dropped around the world.
There are frequent volcanic explosions around the world, but not many are as violent as Krakatoa. Another reason Krakatoa injected dust high in the atmosphere was the explosion was directed upwards. Some volcanoes, like Mount Saint Helens, explode laterally. The size and direction of the explosion made Krakatoa a likely suspect.
Indeed, as late as the middle of the twentieth century noctilucent clouds were still believed to have originated with Krakatoa.
The Problem with the Old Theory
If indeed Krakatoa had caused the necessary dust and moisture to form noctilucent clouds, the dust and ice should have dissipated, either downward due to gravity or to out space by random movement. This means that noctilucent clouds should be diminishing in frequency and coverage. The reverse is true. Not only are noctilucent clouds still occurring, they are occurring at lower latitudes, making them more extensive than ever.
The Current Theory
According to a NASA site devoted to this phenomenon, noctilucent clouds probably are acquiring material from outer space. In outer space the Earth acquires dust and water from small bits of asteroids, and from comet material. The micrometeorites, small remnants of meteors that are left over after the meteor has burned in the atmosphere due to friction, serve as condensation nuclei. The water probably is water that rises from the Earth, but some may also be from old comets, possibly as the Earth passes through old comet orbits. Comets shed debris and water which then orbit the sun in the same orbit as the comet, but spread over the orbit.
Also according to the NASA site the water vapor condenses as ice on the dust. This processes produces the clouds.
The reason the clouds do not have the problem of dissipating, which had proved a problem with the volcano theory, is that both micrometeorites and moisture are being constantly resupplied from outer space.
Meteors are often rocky or metallic, and many of the micrometeorites are made of dust or small pieces of metal. Stray electrons, possibly once part of the solar wind, can charge small rocky and metallic debris, and because water is a dipole, meaning it has a positive end and a negative end, it is more easily attracted to a charged nucleus.
Why They Occur at Lower Latitudes in the Southern Hemisphere
Noctilucent clouds are associated with latitudes far from the equator where the mesosphere is colder. However, the clouds get about six degrees of latitude closer to the equator in the southern hemisphere than they do in the northern hemisphere.
Noctilucent clouds are associated with a colder mesosphere on a seasonal basis, and the phenomenon is a summer event, which seems less than intuitive. Summer is caused by the tilt of the Earth’s axis, and occurs when the sunlight is more direct. What is rarely mentioned is there is some effect from the Earth’s orbit. When the Earth is closer to the sun, the incoming sunlight is more intense. So, as the Earth moves in its elliptical orbit, it happens that it is closer to the sun in the northern hemisphere winter, and farther from the sun in the northern hemisphere summer. This makes the tilt of the axis and its position in its orbit act in opposition in the northern hemisphere, but in conjunction with each other in the southern hemisphere. Hence, the southern hemispheric mesosphere gets colder at a given latitude than the northern hemisphere does at the same latitude. Of course the comparison must be made for like seasons, in particular summer. The important season for noctilucent clouds is summer. The surface temperature of the Earth tends to be warmer when the mesosphere is colder, and colder when the mesosphere is warmer.
Unusually Early Start
The Recent Considerations as a Possible Connection with Global Warming
NASA has eluded to the possible connection between how close noctilucent clouds occur relative to the equator to global warming. They have been occurring farther south in the northern hemisphere, which is taken to indicate the Earth is warming up, so the mesosphere is cooling farther from the poles to the point of triggering noctilucent clouds.
NASA continues to release information. Now, studies are indicating there is a connection between the mesosphere in both hemispheres of the Earth. Mesospheric clouds forming at extreme cold offer a view of air movement in the mesosphere. As cold currents of air move, they are shown by noctilucent clouds. As studies continue even more may be discovered.
How Noctilucent Clouds Differ from the Aurora
The aurora is a glow, not a physical cloud. Charged particles from the sun get trapped in the Earth’s magnetic field, and these particles spiral towards the Earth’s magnetic poles. As they spiral along they can encounter the atmosphere. The stream of particles, which are often negatively charged electrons, pull on the nuclei of atmospheric molecules while pushing on the molecule’s electrons. This can give extra energy to the molecular electrons. But, electrons do not stay energized. They give the energy off as electromagnetic waves, and depending on the amount of energy involved a color of light (the electromagnet wave) is produced. Different molecules emit different colors, so as the charged particles get lower the color changes due to different kinds of molecules being given energy, then emitting the energy away.
This is a completely different mechanism from the process involved in forming noctilucent clouds. Both are beautiful, but quite different.
In the video below the aurora display begins a few seconds after over a minute twenty seconds have elapsed. It appears faint at first, and is difficult to determint its exact beginning, but then explodes into a fantastic display.
Noctilucent Clouds and the Aurora together
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