85-434: The mesosphere ( / ˈ m ɛ s ə s f ɪər , ˈ m ɛ z -, ˈ m iː s ə -, - z ə -/ ; from Ancient Greek μέσος ( mésos ) 'middle' and - sphere ) is the third layer of the atmosphere , directly above the stratosphere and directly below the thermosphere . In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define limits: it begins at
170-563: A Black Brant XII suborbital sounding rocket launched from NASA's Wallops Flight Facility to create an artificial noctilucent cloud. The cloud was to be observed over a period of weeks or months by ground instruments and the Spatial Heterodyne IMager for MEsospheric Radicals (SHIMMER) instrument on the NRL/STP STPSat-1 spacecraft. The rocket's exhaust plume was observed and reported to news organizations in
255-543: A pitch accent . In Modern Greek, all vowels and consonants are short. Many vowels and diphthongs once pronounced distinctly are pronounced as /i/ ( iotacism ). Some of the stops and glides in diphthongs have become fricatives , and the pitch accent has changed to a stress accent . Many of the changes took place in the Koine Greek period. The writing system of Modern Greek, however, does not reflect all pronunciation changes. The examples below represent Attic Greek in
340-466: A diffuse scattering layer of water ice crystals near the summer polar mesopause . They consist of ice crystals and from the ground are only visible during astronomical twilight . Noctilucent roughly means "night shining" in Latin . They are most often observed during the summer months from latitudes between ±50° and ±70°. Too faint to be seen in daylight , they are visible only when the observer and
425-525: A fifth major dialect group, or it is Mycenaean Greek overlaid by Doric, with a non-Greek native influence. Regarding the speech of the ancient Macedonians diverse theories have been put forward, but the epigraphic activity and the archaeological discoveries in the Greek region of Macedonia during the last decades has brought to light documents, among which the first texts written in Macedonian , such as
510-522: A giant balloon from Esrange , Sweden which traveled through the stratosphere across the Arctic to Western Nunavut , Canada in five days. The giant balloon was loaded with cameras, which captured six million high-resolution images filling up 120 terabytes of data storage, aiming to study the PMCs which are affected by the atmospheric gravity waves , resulted from air being pushed up by mountain ranges all
595-550: A prefix /e-/, called the augment . This was probably originally a separate word, meaning something like "then", added because tenses in PIE had primarily aspectual meaning. The augment is added to the indicative of the aorist, imperfect, and pluperfect, but not to any of the other forms of the aorist (no other forms of the imperfect and pluperfect exist). The two kinds of augment in Greek are syllabic and quantitative. The syllabic augment
680-542: A strong Northwest Greek influence, and can in some respects be considered a transitional dialect, as exemplified in the poems of the Boeotian poet Pindar who wrote in Doric with a small Aeolic admixture. Thessalian likewise had come under Northwest Greek influence, though to a lesser degree. Pamphylian Greek , spoken in a small area on the southwestern coast of Anatolia and little preserved in inscriptions, may be either
765-510: A vowel or /n s r/ ; final stops were lost, as in γάλα "milk", compared with γάλακτος "of milk" (genitive). Ancient Greek of the classical period also differed in both the inventory and distribution of original PIE phonemes due to numerous sound changes, notably the following: The pronunciation of Ancient Greek was very different from that of Modern Greek . Ancient Greek had long and short vowels ; many diphthongs ; double and single consonants; voiced, voiceless, and aspirated stops ; and
850-448: Is a result of decreasing absorption of solar radiation by the rarefied atmosphere having a diminishing relative ozone concentration as altitude increases (ozone being the main absorber in the UV wavelengths that survived absorption by the thermosphere). Additionally, this is also a result of increasing cooling by CO 2 radiative emission . The top of the mesosphere, called the mesopause ,
935-418: Is added to stems beginning with consonants, and simply prefixes e (stems beginning with r , however, add er ). The quantitative augment is added to stems beginning with vowels, and involves lengthening the vowel: Some verbs augment irregularly; the most common variation is e → ei . The irregularity can be explained diachronically by the loss of s between vowels, or that of the letter w , which affected
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#17327754195531020-528: Is also sometimes used to refer to altitudes within the mesosphere. This term does not have a technical definition, but typically refers to the region roughly between the Armstrong limit (about 62,000 ft or 19 km, above which humans require a pressure suit in order to survive) and the Kármán line (where astrodynamics must take over from aerodynamics in order to achieve flight); or, by another definition, to
1105-448: Is considered by some linguists to have been closely related to Greek . Among Indo-European branches with living descendants, Greek is often argued to have the closest genetic ties with Armenian (see also Graeco-Armenian ) and Indo-Iranian languages (see Graeco-Aryan ). Ancient Greek differs from Proto-Indo-European (PIE) and other Indo-European languages in certain ways. In phonotactics , ancient Greek words could end only in
1190-455: Is not well understood but a possible explanation is 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, although this explanation remains controversial. 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
1275-428: Is the coldest part of Earth's atmosphere. Temperatures in the upper mesosphere fall as low as about −100 °C (173 K; −148 °F), varying according to latitude and season . The main most important features in this region are strong zonal (East-West) winds, atmospheric tides , internal atmospheric gravity waves (commonly called " gravity waves "), and planetary waves . Most of these tides and waves start in
1360-450: Is the least-understood part of the atmosphere, resulting in the humorous moniker ignorosphere . The presence of red sprites and blue jets (electrical discharges or lightning within the lower mesosphere), noctilucent clouds , and density shears within this poorly understood layer are of current scientific interest. On February 1, 2003, Space Shuttle Columbia broke up on reentry at about 62 km (39 mi) altitude, in
1445-422: Is there any evidence of a dependence on auroral activity. This indicates that control of polar mesospheric clouds is determined by geographical rather than geomagnetic factors. The brightness of polar mesospheric clouds and noctilucent clouds appears to be consistent at the latitudes where both are observed, but polar mesospheric clouds near the pole are much brighter than noctilucent clouds, even taking into account
1530-557: The Aeronomy of Ice in the Mesosphere satellite suggests that noctilucent clouds require water vapour, dust, and very cold temperatures to form. 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 particulates from volcanoes and dust from the troposphere are also possibilities. The moisture could be lifted through gaps in
1615-679: The Archaic or Epic period ( c. 800–500 BC ), and the Classical period ( c. 500–300 BC ). Ancient Greek was the language of Homer and of fifth-century Athenian historians, playwrights, and philosophers . It has contributed many words to English vocabulary and has been a standard subject of study in educational institutions of the Western world since the Renaissance . This article primarily contains information about
1700-736: The Chelyabinsk superbolide entry of February 2013 (outside the NLC season) that were actually stratospheric dust reflections visible after sunset. Noctilucent clouds are composed of tiny crystals of water ice up to 100 nm in diameter and exist at a height of about 76 to 85 km (249,000 to 279,000 ft), higher than any other clouds in Earth's atmosphere. Clouds in the Earth's lower atmosphere form when water collects on particles, but mesospheric clouds may form directly from water vapour in addition to forming on dust particles. Data from
1785-498: The D layer , which is only present during the day when some ionization occurs with nitric oxide being ionized by Lyman series -alpha hydrogen radiation. The ionization is so weak that when night falls, and the source of ionization is removed, the free electron and ion form back into a neutral molecule. A 5 km (3.1 mi; 16,000 ft) deep sodium layer is located between 80–105 km (50–65 mi; 262,000–344,000 ft). Made of unbound, non-ionized atoms of sodium,
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#17327754195531870-606: The Epic and Classical periods of the language, which are the best-attested periods and considered most typical of Ancient Greek. From the Hellenistic period ( c. 300 BC ), Ancient Greek was followed by Koine Greek , which is regarded as a separate historical stage, though its earliest form closely resembles Attic Greek , and its latest form approaches Medieval Greek . There were several regional dialects of Ancient Greek; Attic Greek developed into Koine. Ancient Greek
1955-501: The Pella curse tablet , as Hatzopoulos and other scholars note. Based on the conclusions drawn by several studies and findings such as Pella curse tablet , Emilio Crespo and other scholars suggest that ancient Macedonian was a Northwest Doric dialect , which shares isoglosses with its neighboring Thessalian dialects spoken in northeastern Thessaly . Some have also suggested an Aeolic Greek classification. The Lesbian dialect
2040-674: The Solar Mesosphere Explorer , mapped the distribution of the clouds between 1981 and 1986 with its ultraviolet spectrometer. The clouds were detected with a lidar in 1995 at Utah State University , even when they were not visible to the naked eye. The first physical confirmation that water ice is indeed the primary component of noctilucent clouds came from the HALOE instrument on the Upper Atmosphere Research Satellite in 2001. In 2001,
2125-479: The Swedish Odin satellite performed spectral analyses on the clouds, and produced daily global maps that revealed large patterns in their distribution. The AIM ( Aeronomy of Ice in the Mesosphere ) satellite was launched on 25 April 2007. It was the first satellite dedicated to studying noctilucent clouds, and made its first observations a month later (25 May). Images taken by the satellite show shapes in
2210-603: The epic poems , the Iliad and the Odyssey , and in later poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic and other Classical-era dialects. The origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between
2295-484: The mesosphere at altitudes of around 76 to 85 km (249,000 to 279,000 ft). No confirmed record of their observation exists before 1885, although they may have been observed a few decades earlier by Thomas Romney Robinson in Armagh . There are now doubts concerning Robinson's out-of-season records, partly as a result of observations, from several points around high northern latitudes, of NLC-like phenomena after
2380-501: The present , future , and imperfect are imperfective in aspect; the aorist , present perfect , pluperfect and future perfect are perfective in aspect. Most tenses display all four moods and three voices, although there is no future subjunctive or imperative. Also, there is no imperfect subjunctive, optative or imperative. The infinitives and participles correspond to the finite combinations of tense, aspect, and voice. The indicative of past tenses adds (conceptually, at least)
2465-682: The summer solstice . This holds true for both hemispheres. Great variability in scattering is observed from day-to-day and year-to- year, but averaging over large time and space scales reveals a basic underlying symmetry and pattern. The long- term behaviour of polar mesospheric cloud frequency has been found to vary inversely with solar activity. PMC's have four major types based on physical structure and appearance. Type I veils are very tenuous and lack well-defined structure, somewhat like cirrostratus or poorly defined cirrus. Type II bands are long streaks that often occur in groups arranged roughly parallel to each other. They are usually more widely spaced than
2550-471: The tropopause , as well as forming from the reaction of methane with hydroxyl radicals in the stratosphere . The exhaust from Space Shuttles , in use between 1981 and 2011, which was almost entirely water vapour after the detachment of the Solid Rocket Booster at a height of about 46 km (151,000 ft), was found to generate minuscule individual clouds. About half of the vapour
2635-402: The troposphere and lower stratosphere , and propagate to the mesosphere. In the mesosphere, gravity-wave amplitudes can become so large that the waves become unstable and dissipate. This dissipation deposits momentum into the mesosphere and largely drives global circulation. Noctilucent clouds are located in the mesosphere. The upper mesosphere is also the region of the ionosphere known as
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2720-465: The 1960s, when direct rocket measurements began. These showed for the first time that the clouds' occurrence 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,
2805-1031: The 5th century BC. Ancient pronunciation cannot be reconstructed with certainty, but Greek from the period is well documented, and there is little disagreement among linguists as to the general nature of the sounds that the letters represent. /oː/ raised to [uː] , probably by the 4th century BC. Greek, like all of the older Indo-European languages , is highly inflected. It is highly archaic in its preservation of Proto-Indo-European forms. In ancient Greek, nouns (including proper nouns) have five cases ( nominative , genitive , dative , accusative , and vocative ), three genders ( masculine , feminine , and neuter ), and three numbers (singular, dual , and plural ). Verbs have four moods ( indicative , imperative , subjunctive , and optative ) and three voices (active, middle, and passive ), as well as three persons (first, second, and third) and various other forms. Verbs are conjugated through seven combinations of tenses and aspect (generally simply called "tenses"):
2890-495: The Archaic period of ancient Greek (see Homeric Greek for more details): Μῆνιν ἄειδε, θεά, Πηληϊάδεω Ἀχιλῆος οὐλομένην, ἣ μυρί' Ἀχαιοῖς ἄλγε' ἔθηκε, πολλὰς δ' ἰφθίμους ψυχὰς Ἄϊδι προΐαψεν ἡρώων, αὐτοὺς δὲ ἑλώρια τεῦχε κύνεσσιν οἰωνοῖσί τε πᾶσι· Διὸς δ' ἐτελείετο βουλή· ἐξ οὗ δὴ τὰ πρῶτα διαστήτην ἐρίσαντε Ἀτρεΐδης τε ἄναξ ἀνδρῶν καὶ δῖος Ἀχιλλεύς. The beginning of Apology by Plato exemplifies Attic Greek from
2975-716: The Classical period of ancient Greek. (The second line is the IPA , the third is transliterated into the Latin alphabet using a modern version of the Erasmian scheme .) Ὅτι [hóti Hóti μὲν men mèn ὑμεῖς, hyːmêːs hūmeîs, Noctilucent cloud Noctilucent clouds (NLCs) , or night shining clouds , are tenuous cloud -like phenomena in the upper atmosphere of Earth . When viewed from space, they are called polar mesospheric clouds (PMCs) , detectable as
3060-545: The Dorians. The Greeks of this period believed there were three major divisions of all Greek people – Dorians, Aeolians, and Ionians (including Athenians), each with their own defining and distinctive dialects. Allowing for their oversight of Arcadian, an obscure mountain dialect, and Cypriot, far from the center of Greek scholarship, this division of people and language is quite similar to the results of modern archaeological-linguistic investigation. One standard formulation for
3145-434: The Earth. Since the naturally-occurring clouds only appear in summer, this experiment was conducted in mid-winter to assure that its results would not be mixed with a natural event. Observed from the ground, this phenomenon is known as noctilucent clouds. From satellites, PMCs are most frequently observed above 70–75° in latitude and have a season of 60 to 80 days duration centered about a peak which occurs about 20 days after
3230-506: The Sun is above the horizon. Satellite observations allow the very coldest parts of the polar mesosphere to be observed, all the way to the geographic pole. In the early 1970s, visible airglow photometers first scanned the atmospheric horizon throughout the summer polar mesospause region. This experiment, which flew on the OGO-6 satellite, was the first to trace noctilucent-like cloud layers across
3315-762: The Sun is below the horizon. Research published in the journal Geophysical Research Letters in June 2009 suggests that noctilucent clouds observed following the Tunguska Event of 1908 are evidence that the impact was caused by a comet. The United States Naval Research Laboratory (NRL) and the United States Department of Defense Space Test Program (STP) conducted the Charged Aerosol Release Experiment (CARE) on September 19, 2009, using exhaust particles from
3400-499: The United States from New Jersey to Massachusetts . A 2018 experiment briefly created noctilucent clouds over Alaska, allowing ground-based measurements and experiments aimed at verifying computer simulations of the phenomenon. A suborbital NASA rocket was launched on 26 January 2018 by University of Alaska professor Richard Collins. It carried water-filled canisters, which were released at about 53 mi (85 km) above
3485-433: The altitude profile of scattering from clouds at two spectral channels (primarily) 265 nm and 296 nm. This phenomenon is now known as Polar Mesospheric Clouds. The general seasonal characteristics of polar mesospheric clouds are well established from the five years of continuous SME data. Over that period, data for four cloud ‘seasons’ in the north, and five ‘seasons’ in the south were recorded. In both hemispheres,
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3570-550: The aorist. Following Homer 's practice, the augment is sometimes not made in poetry , especially epic poetry. The augment sometimes substitutes for reduplication; see below. Almost all forms of the perfect, pluperfect, and future perfect reduplicate the initial syllable of the verb stem. (A few irregular forms of perfect do not reduplicate, whereas a handful of irregular aorists reduplicate.) The three types of reduplication are: Irregular duplication can be understood diachronically. For example, lambanō (root lab ) has
3655-518: The ash had settled out of the atmosphere, the noctilucent clouds persisted. Finally, the theory that the clouds were composed of volcanic dust was 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". His notes provide evidence that noctilucent clouds first appeared in 1885. He had been doing detailed observations of
3740-399: The atmosphere which are warmest in summer. Temperatures at latitudes equatorward of the boundary of detection never get low enough for water-ice to form. Polar mesospheric clouds generally increase in brightness and occurrence frequency with increasing latitude, from about 60 degrees to the highest latitudes observed (85 degrees). So far, no apparent dependence on longitude has been found, nor
3825-419: The augment when it was word-initial. In verbs with a preposition as a prefix, the augment is placed not at the start of the word, but between the preposition and the original verb. For example, προσ(-)βάλλω (I attack) goes to προσ έ βαλoν in the aorist. However compound verbs consisting of a prefix that is not a preposition retain the augment at the start of the word: αὐτο(-)μολῶ goes to ηὐ τομόλησα in
3910-437: The bands or elements seen with cirrocumulus clouds. Type III billows are arrangements of closely spaced, roughly parallel short streaks that mostly resemble cirrus. Type IV whirls are partial or, more rarely, complete rings of cloud with dark centres. When mesospheric clouds are viewed above the atmosphere, the geometrical limitations of observing from the ground are significantly reduced. They may be observed ‘edge-on’ against
3995-421: The clouds are present. Other experiments have demonstrated that, at the extremely cold temperatures of a noctilucent cloud, sodium vapour can rapidly be deposited onto an ice surface. Noctilucent clouds are first known to have been observed in 1885, two years after the 1883 eruption of Krakatoa . It remains unclear whether their appearance had anything to do with the volcanic eruption or whether their discovery
4080-521: The clouds that are similar to shapes in tropospheric clouds, hinting at similarities in their dynamics. In the previous year, scientists with the Mars Express mission had announced their discovery of carbon dioxide –crystal clouds on Mars that extended to 100 km (330,000 ft) above the planet's surface. These are the highest clouds discovered over the surface of a rocky planet. Like noctilucent clouds on Earth, they can be observed only when
4165-418: The clouds was first determined, via triangulation . That 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
4250-416: The comparatively dark sky background, even in full daylight. The photometer field of view must be well baffled to avoid interference from the very bright Earth about a degree beneath the cloud layer. It is a much more difficult task to observe the clouds against the bright background of the illuminated Earth, although this has been achieved in the ultraviolet in the 200 to 300 nm spectral region, because of
4335-421: 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
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#17327754195534420-615: The dialect of Sparta ), and Northern Peloponnesus Doric (including Corinthian ). All the groups were represented by colonies beyond Greece proper as well, and these colonies generally developed local characteristics, often under the influence of settlers or neighbors speaking different Greek dialects. After the conquests of Alexander the Great in the late 4th century BC, a new international dialect known as Koine or Common Greek developed, largely based on Attic Greek , but with influence from other dialects. This dialect slowly replaced most of
4505-530: The dialects is: West vs. non-West Greek is the strongest-marked and earliest division, with non-West in subsets of Ionic-Attic (or Attic-Ionic) and Aeolic vs. Arcadocypriot, or Aeolic and Arcado-Cypriot vs. Ionic-Attic. Often non-West is called 'East Greek'. Arcadocypriot apparently descended more closely from the Mycenaean Greek of the Bronze Age. Boeotian Greek had come under
4590-510: The divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period. They have the same general outline but differ in some of the detail. The only attested dialect from this period is Mycenaean Greek , but its relationship to the historical dialects and the historical circumstances of the times imply that the overall groups already existed in some form. Scholars assume that major Ancient Greek period dialect groups developed not later than 1120 BC, at
4675-473: The green color of these dunes has tentatively been explained by the interaction of those solar particles with oxygen molecules. The dunes therefore occur where mesospheric oxygen is more concentrated. Millions of meteors enter the Earth's atmosphere, averaging 40,000 tons per year. The ablated material, called meteoric smoke, is thought to serve as condensation nuclei for noctilucent clouds . The mesosphere lies above altitude records for aircraft , while only
4760-468: The lower layers of the atmosphere are in Earth's shadow , but while these very high clouds are still in sunlight . Recent studies suggest that increased atmospheric methane emissions produce additional water vapor through chemical reactions once the methane molecules reach the mesosphere – creating, or reinforcing existing, noctilucent clouds. They are the highest clouds in Earth's atmosphere, located in
4845-433: The lower mesosphere, killing all seven crew members. Ancient Greek language Ancient Greek ( Ἑλληνῐκή , Hellēnikḗ ; [hellɛːnikɛ́ː] ) includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek ( c. 1400–1200 BC ), Dark Ages ( c. 1200–800 BC ),
4930-428: The lower sky background seen from space. Polar mesospheric cloud observations have revealed that the well-known phenomenon of the northward shifting with latitude of date of peak noctilucent cloud occurrence is partly due to the increased number of hours of noctilucent cloud visibility with latitude and partly due to an actual northward retreat of the boundary towards the end of the season. On 8 July 2018, NASA launched
5015-403: The lowest few kilometers are accessible to balloons , for which the altitude record is 53.0 kilometres (32.9 mi). Meanwhile, the mesosphere is below the minimum altitude for orbital spacecraft due to high atmospheric drag. It has only been accessed through the use of sounding rockets , which are only capable of taking mesospheric measurements for a few minutes per mission. As a result, it
5100-413: The mesosphere from the thermosphere —the second-outermost layer of Earth's atmosphere. On Earth, the mesopause nearly co-incides with the turbopause , below which different chemical species are well-mixed due to turbulent eddies . Above this level the atmosphere becomes non-uniform because the scale heights of different chemical species differ according to their molecular masses . The term near space
5185-505: The mesosphere is coldest as a result of seasonally varying vertical winds, leading to cold summertime conditions in the upper mesosphere ( upwelling and adiabatic cooling ) and wintertime heating ( downwelling and adiabatic heating ). Therefore, they cannot be observed (even if they are present) inside the Polar circles because the Sun is never low enough under the horizon at this season at these latitudes. Noctilucent clouds form mostly near
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#17327754195535270-468: The northern hemisphere, but fewer 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. 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
5355-508: The older dialects, although the Doric dialect has survived in the Tsakonian language , which is spoken in the region of modern Sparta. Doric has also passed down its aorist terminations into most verbs of Demotic Greek . By about the 6th century AD, the Koine had slowly metamorphosed into Medieval Greek . Phrygian is an extinct Indo-European language of West and Central Anatolia , which
5440-461: 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 may 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
5525-487: The perfect stem eilēpha (not * lelēpha ) because it was originally slambanō , with perfect seslēpha , becoming eilēpha through compensatory lengthening. Reduplication is also visible in the present tense stems of certain verbs. These stems add a syllable consisting of the root's initial consonant followed by i . A nasal stop appears after the reduplication in some verbs. The earliest extant examples of ancient Greek writing ( c. 1450 BC ) are in
5610-495: The polar cap. The very bright scattering layer was seen in full daylight conditions, and was identified as the poleward extension of noctilucent clouds. In the early 1980s, the layer was observed again from a satellite, the Solar Mesospheric Explorer (SME). On board this satellite was an ultraviolet spectrometer, which mapped the distributions of clouds over the time period 1981 to 1986. The experiment measured
5695-413: The polar regions, because the mesosphere is coldest there. Clouds in the southern hemisphere are about 1 km (3,300 ft) higher 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
5780-410: The season begins about one month before summer solstice and ends about two months afterwards. Since there are no biases due to such factors as changing number of hours of visibility, weather conditions, etc. this is a ‘true’ behaviour. It is believed to be a result of the fact that summertime mesopause region becomes coldest during this period causing water-ice to form, in contrast to most other regions of
5865-461: The sodium layer radiates weakly to contribute to the airglow . The sodium has an average concentration of 400,000 atoms per cubic centimetre. This band is regularly replenished by sodium sublimating from incoming meteors. Astronomers have begun utilizing this sodium band to create "guide stars" as part of the adaptive optical correction process used to produce ultra-sharp ground-based observations. Other metal layers, e.g. iron and potassium, exist in
5950-457: 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
6035-483: The space between the highest altitude commercial airliners fly at (about 40,000 ft (12.2 km)) and the lowest perigee of satellites being able to orbit the Earth (about 45 mi (73 km)). Some sources distinguish between the terms "near space" and "upper atmosphere", so that only the layers closest to the Kármán line are described as "near space". Within the mesosphere, temperature decreases with increasing height . This
6120-517: The syllabic script Linear B . Beginning in the 8th century BC, however, the Greek alphabet became standard, albeit with some variation among dialects. Early texts are written in boustrophedon style, but left-to-right became standard during the classic period. Modern editions of ancient Greek texts are usually written with accents and breathing marks , interword spacing , modern punctuation , and sometimes mixed case , but these were all introduced later. The beginning of Homer 's Iliad exemplifies
6205-626: The thermosphere 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 , and is extremely thin, the ice crystals can form only at temperatures below about −120 °C (−184 °F). This means that noctilucent clouds form predominantly during summer when, counterintuitively,
6290-467: The time of the Dorian invasions —and that their first appearances as precise alphabetic writing began in the 8th century BC. The invasion would not be "Dorian" unless the invaders had some cultural relationship to the historical Dorians . The invasion is known to have displaced population to the later Attic-Ionic regions, who regarded themselves as descendants of the population displaced by or contending with
6375-520: The top of the stratosphere (sometimes called the stratopause ), and ends at the mesopause , which is the coldest part of Earth's atmosphere , with temperatures below −143 °C (−225 °F; 130 K). The exact upper and lower boundaries of the mesosphere vary with latitude and with season (higher in winter and at the tropics, lower in summer and at the poles), but the lower boundary is usually located at altitudes from 47 to 51 km (29 to 32 mi; 154,000 to 167,000 ft) above sea level , and
6460-467: The uniform size of the water droplets produced. 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. They occur during summer, from mid-May to mid-August in the northern hemisphere and between mid-November and mid-February in
6545-533: 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 Stolze and, after that year, continuous observations were carried out at the Berlin Observatory . During this research, the height of
6630-401: The upper boundary (the mesopause) is usually from 85 to 100 km (53 to 62 mi; 279,000 to 328,000 ft). The stratosphere and mesosphere are sometimes collectively referred to as the "middle atmosphere", which spans altitudes approximately between 12 and 80 km (7.5 and 49.7 mi) above Earth's surface. The mesopause , at an altitude of 80–90 km (50–56 mi), separates
6715-565: The upper mesosphere/lower thermosphere region as well. Beginning in October 2018, a distinct type of aurora has been identified, originating in the mesosphere. Often referred to as 'dunes' due to their resemblance to sandy ripples on a beach, the green undulating lights extend toward the equator. They have been identified as originating about 96 km (60 mi; 315,000 ft) above the surface. Since auroras are caused by ultra-high-speed solar particles interacting with atmospheric molecules,
6800-494: The very small albedo of the earth in this part of spectrum. American and Soviet astronauts observed the phenomenon from space as early as 1970. Most observations are reported from the night side of the orbit and the observer is looking towards the twilight sector. At this time the observer's eye is dark-adapted and polar mesospheric clouds would appear with maximum contrast against a comparatively dark background. Soviet astronauts have reported sightings of mesospheric clouds even when
6885-623: The way up to the mesosphere. These images would aid in studying turbulence in the atmosphere, and consequently better weather forecasting . NASA uses AIM satellite to study these noctilucent clouds, which always occur during the summer season near the poles. However, tomographic analyses of AIM satellite indicate that there is a spatial negative correlation between albedo and wave‐induced altitude. Noctilucent clouds are generally colourless or pale blue, although occasionally other colours including red and green have been observed. The characteristic blue colour comes from absorption by ozone in
6970-480: Was Aeolic. For example, fragments of the works of the poet Sappho from the island of Lesbos are in Aeolian. Most of the dialect sub-groups listed above had further subdivisions, generally equivalent to a city-state and its surrounding territory, or to an island. Doric notably had several intermediate divisions as well, into Island Doric (including Cretan Doric ), Southern Peloponnesus Doric (including Laconian ,
7055-452: Was a pluricentric language , divided into many dialects. The main dialect groups are Attic and Ionic , Aeolic , Arcadocypriot , and Doric , many of them with several subdivisions. Some dialects are found in standardized literary forms in literature , while others are attested only in inscriptions. There are also several historical forms. Homeric Greek is a literary form of Archaic Greek (derived primarily from Ionic and Aeolic) used in
7140-412: 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
7225-420: Was released into the thermosphere , usually at altitudes of 103 to 114 km (338,000 to 374,000 ft). In August 2014, a SpaceX Falcon 9 also caused noctilucent clouds over Orlando, Florida after a launch. The exhaust can be transported to the Arctic region in little over a day, although the exact mechanism of this very high-speed transfer is unknown. As the water migrates northward, it falls from
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