Noctilucent cloud, also known aspolar regionMesospheric cloud refers to a kind of luminous and transparent cloud that appears at high latitudes of the earth during deep twilightUndulate cloud。It generally requires three conditions: low temperature, water vapor and dust, so that water vapor can condense into tiny ice crystals.
June 2019, CanadaSaskatchewan, Alberta and other places have appeared the atmospheric phenomenon commonly known as noctilucent clouds.[1]
When noctilucent clouds are seen from outer space, they are called "night shining clouds";Seen from the earth, it is called "noctilucent clouds".It is formed in the middle layer (the atmosphere is divided into tropospherestratosphere, middle tierThermosphereandexosphere The specific height of the escape layer is unknown, and there is no obvious boundary with outer space).The noctilucent cloud can be directly observed with the naked eye only when the sun is 6 °~12 ° below the horizon in summer in high latitude (50-65), that is, the lower atmosphere is in the shadow of the earth, while the noctilucent cloud in the upper atmosphere is exposed to sunlight.
The origin of noctilucent clouds is still controversial in the scientific community. The most mainstream theory believes that it is mainly composed of extremely fine ice crystals. This empirical theory is only valid in the troposphere.
Above the troposphere is the stratosphere (stratosphere), and above it is the middle layer, butstratosphereIt is characterized by upper heating and lower cooling, so there is little convection in the vertical direction, but mainly the horizontal atmospheric movement (so passenger aircraft flight is generally selected in the stratosphere). In fact, the temperature of the mesosphere is very high rather than very low.Scientists believe that this may be due to the attenuation of gravity waves in the vertical direction.The gravity wave may weaken in some places due to various abnormal reasons, thus leading to the upward momentum transfer in the lower atmosphere.In this way, those gravity waves with strong phase velocity in the easterly direction will transfer the easterly momentum to the upper atmosphere.
The eastward flow will be affected by the equatorialCoriolis forceThis will cause the atmospheric flow with water vapor in the vertical direction (it should be noted that the water vapor in the upper atmosphere may not come directly to the ground, but may be formed due to the photodegradation of methane).Because only the stratosphere at high latitudes in summer allows gravity waves with phase velocities eastward to pass into the middle layer (the winter monsoon is opposite, so the eastward momentum cannot pass into the middle layer), the temperature of the middle layer of the hemisphere in summer will decrease due to the loss of momentum (accordingly, the temperature of the middle layer of the hemisphere will increase in winter),As a result, the temperature drop in the middle layer (130 k) is caused.In fact, in the summer of the entire polar region, satellites can observe large mesospheric clouds, but regions with too high latitude are in polar day in summer, so noctilucent clouds are not easy to be found.Under such conditions, noctilucent clouds can only be observed in some high latitudes in summer.
Phenomenon description
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Nocturnal cloud is a kind of cloud formed in the middle layer, and its height from the ground is generally about 80 km.This rare cloud can only be seen in the summer at high latitudes (50-65).
The noctilucent cloud is usually light blue or silver gray, which is the result of the scattering of sunlight by ice crystal particles in the noctilucent cloud. The radius of such ice crystal particles is generally 0.05~0.5 μ m.The noctilucent cloud looks a bit likeCirrus cloud, but it is much thinner than it, and the color is silver white or blue, which appears when the sun rises in the morning or sets at duskhorizonWhen the included angle is between 6-15 degrees.This is very natural, because too early it will be too thin to see, and too late it will fall into the shadow of the earth.
Main types
According to the observation results in July 2012, there are four main types of noctilucent clouds (NLCs): veils, bands, waves and whirls.
In 1883Krakatoa Volcano, IndonesiaTwo years after the eruption, humans observed this nocturnal luminous cloud for the first time. Since then, noctilucent clouds have appeared more and more frequently.An atmospheric physicist at Hampton University, Virginia, USAJim RussellIt means that they appear more frequently, for a longer time and at a lower latitude than before.
Noctilucent clouds
But why noctilucent clouds appear has always been a mystery.Several satellites have observed noctilucent clouds, but none of them has been specially studied in space.The upper air ice detection satellite in the middle atmosphereVandenberg Air Force Base, California, USASoon after the launch, they began to answer these questions.A jet plane will take it across the Pacific Ocean to an altitude of 12 kilometers, and release the spacecraft there.Then, the upper air ice detection satellite in the middle atmosphere will ignite the engine and finally enter a channel about 600 kilometers above the earthPolar orbit。
The upper air ice detection satellite in the middle atmosphere will study noctilucent clouds over the Arctic and Antarctic.Russell said that there is a big difference between the noctilucent clouds in the northern and southern hemispheres.The noctilucent cloud over the North Pole looks a little brighter than that over the South Pole and appears at a lower altitude.But in these two places, the formation of noctilucent clouds requires three conditions: low temperature, water vapor and dust particles, so that water vapor can condense into tiny ice crystals.
Upper air ice detection satellite in the middle atmosphereThree sets of scientific instruments are carried to carefully view the noctilucent clouds from different angles.The diameter of ice crystals forming noctilucent clouds is generally 50nanometerThe sunlight reflects on these particles, so that they can only be seen at dawn or dusk when the rest of the sky is dark.Russell said that this effect is like watching a plane flying in the sky reflect sunlight after the sun has partially set.These clouds will also fluctuate and give off a shimmer.Russell said, "They are quite bright. They are as bright as silver and blue. They also shine. They captured this scene."
Sky observers in high latitudes tracked the peak season of noctilucent clouds - from May 15 to August 20 in the Northern Hemisphere, with the most frequent activity in early July.The results from the upper air ice detection satellite in the middle atmosphere may finally clarify the exact conditions for the formation of noctilucent clouds, especially the increasingGreenhouse gasesWhether the noctilucent clouds will appear more frequently.
Boulderuniversity of colorado A joint investigator of the upper air ice detection program in the middle atmosphereCora RandallHe said that more and more greenhouse gases, such as carbon dioxide, will warm the climate in low altitude areas, while the temperature in high altitude areas where noctilucent clouds form will fall.In addition, the high content of the greenhouse gas methane will actually lead to the formation of more water vapor in high altitude areas.Randall explained that it seems that the presence of low temperature and more water vapor may lead to the formation of most noctilucent clouds more frequently.She said: "The question is, what is the comprehensive factor that causes the noctilucent clouds to appear more frequently and become brighter?"
Record formation
On December 23, 2017, Elon Musk's SpaceX "Falcon" 9 rocket was launched from Southern CaliforniaVandenberg Air Force BaseTake off and carry 10 communication satellites into space, forming noctilucent clouds in the atmosphere.[3]
At about 8:00 p.m. on April 27, 2018, the noctilucent cloud version of the track cloud appeared above Beijing.[3-4]
In June 2019, the atmospheric phenomenon commonly known as noctilucent clouds appeared in Saskatchewan, Alberta and other places in Canada.[1]
influence
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The existence of noctilucent clouds ranges from a few minutes to several hours.The brightness of strong noctilucent clouds is equivalent to the international second-class aurora brightness;Weak noctilucent clouds are generally invisible to the naked eye, only ultraviolet orInfrared optical instrumentObserve.
The information about noctilucent clouds mainly comes from the detection of rockets and satellites.The study of noctilucent clouds can reveal the laws of atmospheric structure, atmospheric fluctuations and chemical processes in the mesopause.[5]
world record
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The noctilucent cloud is the highest cloud at an altitude of about 80 kilometers, higher than 99.9% of the earth's atmosphere.(Guinness World Records)[6]
