Noctilucent clouds, also known as polar mesospheric clouds, are cloud-like phenomena in the upper atmosphere, visible in a deep twilight. They are made of crystals of water ice. The name means roughly night shining in Latin. They are most commonly observed in the summer months at latitudes between 50° and 70° north and south of the equator.
They are the highest clouds in the Earth's atmosphere, located in the mesosphere at altitudes of around Template:Convert/to. They are normally too faint to be seen, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in the Earth's shadow. Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon; there is no evidence that they were observed before 1885.
Noctilucent clouds can form only under very restrictive conditions; their occurrence can be used as a sensitive guide to changes in the upper atmosphere. Since their discovery the occurrence of noctilucent clouds has been increasing in frequency, brightness and extent. It is theorised that this increase is connected to climate change, although since the phenomenon has been unknown until fairly recently it is also plausable more are noticed because now they are known to exist.
Noctilucent clouds are composed of tiny crystals of water ice 40 to 100 nanometers in diameter and exist at a height of about Template:Convert/to, higher than any other clouds in Earth's atmosphere. Much like the more familiar lower altitude clouds, the noctilucent clouds are formed from water collecting on the surface of dust particles. The sources of both the dust and the water vapour in the upper atmosphere are not known with certainty. The dust is believed to come from micrometeors, although volcanoes and dust from the troposphere are also possibilities. The moisture could be lifted through gaps in the tropopause, as well as forming from the reaction of methane with hydroxyl radicals in the stratosphere.
The exhaust from Space Shuttles, which is almost entirely water vapour, has been found to generate individual clouds. About half of the vapor is released into the thermosphere, usually at altitudes of Template:Convert/to.
This exhaust can be transported to the Arctic region in little over a day, although the exact mechanism of this very high-speed transport is unknown. As the water migrates northward, it falls from the thermosphere down into the colder mesosphere, which occupies the region of the atmosphere just below. Although this mechanism is the cause of individual noctilucent clouds, it is not thought to be a major contributor to the phenomenon as a whole.
As the mesosphere contains very little moisture, approximately one hundred millionth that of air from the Sahara desert, and is extremely thin, the ice crystals can only form at temperatures below about Template:Convert/-1 °C (Template:Convert/F). This means that noctilucent clouds form predominantly during summer when, counterintuitively, the mesosphere is coldest. Noctilucent clouds form mostly near the polar regions, because the mesosphere is coldest there. Clouds in the southern hemisphere are about 1km higher up than those in the northern hemisphere.
Ultraviolet radiation from the Sun breaks water molecules apart, reducing the amount of water available to form noctilucent clouds. The radiation is known to vary cyclically with the solar cycle and satellites have been tracking the decrease in brightness of the clouds with the increase of ultraviolet radiation for the last two solar cycles. It has been found that changes in the clouds follow changes in the intensity of ultraviolet rays by about a year, but the reason for this long lag is not yet known.
Noctilucent clouds are known to exhibit high radar reflectivity, in a frequency range of 50 MHz to 1.3 GHz. This behaviour is not well understood but Caltech's Prof. Paul Bellan has proposed a possible explanation: that the ice grains become coated with a thin metal film composed of sodium and iron, which makes the cloud far more reflective to radar.Sodium and iron atoms are stripped from incoming micrometeors and settle into a layer just above the altitude of noctilucent clouds, and measurements have shown that these elements are severely depleted when the clouds are present. Other experiments have demonstrated that, at the extremely cold temperatures of a noctilucent cloud, sodium vapor can rapidly be deposited onto an ice surface.
Discovery and investigation Edit
Noctilucent clouds were first observed in 1885, two years after the 1883 eruption of Krakatoa. It remains unclear whether their appearance had anything to do with the volcano, or whether their discovery was due to more people observing the spectacular sunsets caused by the volcanic debris in the atmosphere. Studies have shown that noctilucent clouds are not caused solely by volcanic activity, although dust and water vapour could be injected into the upper atmosphere by eruptions and contribute to their formation. Scientists at the time assumed the clouds were another manifestation of volcanic ash but, after the ash had settled out of the atmosphere, the noctilucent clouds persisted. The theory that the clouds were composed of volcanic dust was finally disproved by Malzev in 1926.
In the years following their discovery the clouds were studied extensively by Otto Jesse of Germany, who was the first to photograph them, in 1887, and seems to have been the one to coin the term "noctilucent cloud", which means "night-shining cloud". His notes provide evidence that noctilucent clouds first appeared in 1885. He had been doing detailed observations of the unusual sunsets caused by the Krakatoa eruption the previous year and firmly believed that, if the clouds had been visible then, he would undoubtedly have noticed them. Systematic photographic observations of the clouds were organized in 1887 by Jesse, Foerster and Dr. Stolze and, after that year, continuous observations were carried out at the Berlin Observatory. During this research the height of the clouds was first determined, via triangulation. The project was discontinued in 1896.
In the decades after Otto Jesse's death in 1901, there were few new insights into the nature of noctilucent clouds. Wegener's conjecture that they were composed of water ice was later shown to be correct. Study was limited to ground-based observations and scientists had very little knowledge of the mesosphere until the 1960s, when direct rocket measurements began. These showed for the first time that the occurrence of the clouds coincided with very low temperatures in the mesosphere.
Noctilucent clouds were first detected from space by an instrument on the OGO-6 satellite in 1972. The OGO-6 observations of a bright scattering layer over the polar caps were identified as poleward extensions of these clouds. A later satellite, the Solar Mesosphere Explorer, mapped the distribution of the clouds between 1981 and 1986 with its ultraviolet spectrometer. The first physical confirmation that water ice is indeed the primary component of noctilucent clouds came from the HALOE instrument on the UARS satellite in 2001. In 2001 the Swedish Odin satellite performed spectral analyses on the clouds, and produced daily global maps that revealed large patterns in their distribution.
On April 25, 2007, the AIM satellite (Aeronomy of Ice in the Mesosphere) was launched. It is the first satellite dedicated to studying noctilucent clouds, and made its first observations on May 25, 2007. Images taken by the satellite show shapes in the clouds that are similar to shapes in tropospheric clouds, hinting at similarities in their dynamics.
On August 28, 2006, scientists with the Mars Express mission announced that they found clouds of carbon dioxide over Mars that extended up to 100 km above the surface of the planet. They are the highest clouds discovered over the surface of a planet. Like noctilucent clouds on Earth, they can only be observed when the Sun is below the horizon.
Noctilucent clouds are generally colourless or pale blue although occasionally other colours including red and green occur. The characteristic blue colour 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. They are considered a "beautiful natural phenomenon". Noctilucent clouds can be confused with cirrus clouds, but appear sharper under magnification. Those caused by rocket exhausts tend to show colours other than silver or blue because of iridescence caused by the similar sizes of water droplets produced.
Noctilucent clouds can be seen by observers at a latitude of 50 to 60 degrees. They seldom occur at lower latitudes (although there have been sightings as far south as Utah and Italy) and, closer to the poles, it does not get dark enough for the clouds to become visible. 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 can only be observed 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 degrees below the horizon. Although noctilucent clouds occur in both hemispheres, they have been observed thousands of times in the northern hemisphere but less than 100 times in the southern. Southern hemisphere noctilucent clouds are fainter and occur less frequently; additionally the southern hemisphere has a lower population and less land area from which to make observations.
The clouds can show a large variety of different patterns and forms. An identification scheme was developed by Fogle in 1970 that classified five different forms. These classifications have since been modified and subdivided.
They can be studied from the ground, from space, and directly by sounding rocket. Also, some also noctilucent clouds are made of smaller crystals, 30nm or less, which are invisible to observers on the ground because they do not scatter enough light.
Connection to climate changeEdit
There is evidence that the relatively recent appearance of noctilucent clouds, and their gradual increase, may be linked to climate change. Atmospheric scientist Gary Thomas of the Laboratory for Atmospheric and Space Physics at the University of Colorado has pointed out that the first sightings coincide with the Industrial Revolution and they have become more widespread and frequent throughout the 20th century. The connection remains controversial however.
Climate models predict that increased greenhouse gas emissions cause a cooling of the mesosphere, which would lead to more frequent and widespread occurrences of noctilucent clouds. A competing theory is that larger methane emissions from intensive farming activities produce more water vapour in the upper atmosphere- methane concentrations have more than doubled in the past 100 years.
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- Schröder, Wilfried (November 2001). "Otto Jesse and the Investigation of Noctilucent Clouds 115 Years Ago" (PDF). Bulletin of the American Meteorological Society 82 (11): 2457–2468. doi:10.1175/1520-0477(2001)082<2457:OJATIO>2.3.CO;2. Bibcode: 2001BAMS...82.2457S, http://ams.allenpress.com/archive/1520-0477/82/11/pdf/i1520-0477-82-11-2457.pdf. Retrieved on 18 October 2008.
- NLC time-lapse movies
- AIM satellite mission
- BBC News Article - Mission to Target Highest Clouds
- Noctilucent Cloud Observers' Homepage
- Solar Occultation for Ice Experiment (SOFIE)
- Southern Noctilucent Clouds observed at Punta Arenas, Chile
- Astronomy Picture of the Day: Noctilucent Clouds Over Sweden
- BBC Article - Spacecraft Chases Highest Clouds
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