THE THEORY OF THE SEASONAL FREQUENCY OF NOCTILUCENT CLOUDS OCCURRENCE
The seasonal frequency of noctilucent clouds has been discussed on the
basis of visual data from the Northern Hemisphere. It is shown that there is a
strong dependence on the atmospheric constitution at the mesospheric level and
the seasonal transition in the mesopause. Before spring transition and after
autumn transition of the mesosphere at the 80 km level no NLC are observed on
the Northern hemisphere.
Conversely we can conclude the same distribution for the Southern
hemisphere and the strong relation between transition periods and occurrence of
Noctilucent clouds, Transition periods of Mesosphere, PMC
Formulation of the problem
Any successful theory of noctilucent clouds (NLC) must first of all
explain their monthly appearance and then
the special effects which have been noted in
past observations; that is the sporadic appearance of NLC, their absence
during the months of October-April in the Northern hemisphere, the nearly
constant height of about 82 km as well as the morphological features.
The fact that an explanation
is only partly successful, may be due, on the one hand, to insufficient
knowledge the physical composition of of the various layers of altitude and, on
the other hand, to the scarcity of mathematical-physical estimates. The
hypothetical character of past evaluations
inclines one to replace these speculations by a more general treatment, which
will still be subject to certain limitations. In doing this we will not
obtain a clearly proven theorem but an explanation which may eliminate many of
Past attempts at explaining the appearance
In the literature we meet the expressions "Dust"- and "Ice
hypothesis". By that it is meant that NLC are either the product of cosmic
particles or are formed by the addition of a new influence,namely lower
During recent years NLC have
been described in many publications. (Chapman and Kendall,(1965) Hesstvedt,(
1967), Soberman (1966), and Webb.(1965). Haurwitz (1964) also made a
contribution to the morphology of NLC (wave characteristics). Regarding the
meteoric comopnents, Soberman made some interesting comments on the formation of
"Basically, the new theory is that a small updraft between 50 and
80 km keeps meteoric dust from drifting to Earth. Over a period of days or weeks,
a large concentration of the dust accumulation in this altitude region. An
updraft lasting long enough for such an accumulation to take place,
could occur only certain conditions. These conditions, summer and high
latitude, are exactly those under which NLC occur.
... Since this altitude change involves a corresponding temperature drop
from a few degrees above zero (centigrade) to - 100 degress C, any water vapor
present would freze on the condenstaion nuclei provided by the meteoritic dust
particles... These particles, illuminated by the Sun's ray coming from far below
the observer's horizon, would be seen from ground as NLC as they pass through
the twilight region", Soberman,1966, p. 101-102)
Figure 1 graphically present Soberman's ideas.
Chapman and Kendall,(1965) raise the following points:
"we suggest the occurrence of noctilucent clouds may require the
following conditions to be satisfied simultaneously:
(1) The mesopause temperature minimum Tm sinks to a sufficiently low
(2) The turbopause descends to about the level of the mesopause.
(3) Convection carries an adequate amount of water vapour up to the
turbopause, and turbulence creates a dust ledge there" (page 130).
The views of Webb regarding an "upward movement of captured
meteoric particles are, in a sense, analogous to those
given byHoffmeister and Schröder earlier (see Schröder, 1966).
Webb outlined also a system of circulation which is responsible for the
movement of these particles into the main region of the NLC.
A cosmic origin is found only in paper (Hoffmeister, 1960
) That paper also outlines a "system of circulation " for the
movement of cosmic dust particles.
The aforementioned papers
give the impression taht in order to explain individual question certain
assumptions must be made. This complicates the explanation, and, in addition,
involves assumptions which are largely questionable. However, these papers point
to a lesser or greater dependency on cosmic particles.
For methodological reasons
we shall consider in his study only the results obtained by British, German and
earlier Soviet observers before 1970. At this time we had a good regular watch with well qualified observers from
different sites in these conuntries, and
the data are mostly valid reliable.
On the reliability of data
For the following deliberations it is particularly important to assess
the reliability of the visual observations. In Britain, observations of NLC have
been made by Paton and colleagues since 1939. In addition, observations were
made both at regular meteorological stations and by many volunteer observers.
The observational procedures for both groups were the same. Since the
observations also include a watch for auroras they extended over the entire year.
In Germany, in the years after 1885, there existed under the direction
of Jesse, a working group in the Commission
of astronomy and cosmical physics (VAP). However, for many
years later only sporadic data were recorded. It should also be pointed
out here that the " Catalogue of German Observations of NLC 1885-1956"
(Zeitschrift für Meteorologie, 1967) needs to be revised. Well documented were
the long-term observations by Hoffmeister and colleagues in their programme of
increased airglow which also included NLC. As to the Roennebeck data (Northern
Germany) , the following selection criteria should be observed: meteorological
and geomagnetic numbers, position of the sun, morphology Of clouds, as well as
knowledge gained from the medical and psychophysical sciences (e.g. the adaption
of the eye by normal sight (10/10) etc.
Seasonal frequency of NLC
Table 1 records the observations.
1. Seasonal frequency of NLC
The following comments complement Table 1: For Britain, Paton remarks:
"Our observations of the occurrence of noctilucent clouds, using data from
Britain, Scandinavia and Denmark, show that the clouds are never seen before May
16th and seldom after August 3rd". Only in 1966 were NLC still seen in
mid-August. For Germany we find the same picture, with some differences in dates
for the the beginning and the end of period. Most NLC were
observed in the 10-day interval June 21-30. From the earlier USSR there
were repeated reports of NLC observations during the autumn months , but these are
questionable and very rare.We have disregarded them.
Of special importance is the question of the appearance of NLC in winter
of the Northern hemisphere. It should again be pointed out that in Britain and
Germany observations are made throughout the entire year. In Germany (Roennebeck,
53.2N) in addition to NLC, auroras as well as the appearance of increased
airglow were recorded. Reference
should also be made here to the statistics of increased airglow (see
Hoffmeister, 1960) which covers a period of over 30 years and which was
published in full.When we examine the results of these three undertakings,
all independent from each other, we can say that during winter months no NLC
appear. The possibility that, for one reason or another, NLC were "overlooked"
may also be excluded. If NLC did really occur in winter at any time
one of these groups would have surely
4.The meteor phenomenon and
impetus to NLC research
An impetus to NLC research was given by the rocket launchings in Sweden
in the year 1962 and later (cf. Hemenway et. al. 1964).
The electron microscope analyses that were made at that time showed only
weakly defined structures around the particles. Only after chromium vapor was
applied did a few particles show a halo. This pattern is supposed to be the
result of a tendency to change of a volatile material (cosmic particles). The
particles showed radii of about 0.08 to 0,8, only a small number showed a radius
smaller than 0.05. The original chemical analysis has in the meantime been given,
in revised form, by Skrivanek. The
halo particles are said to contain a relatively large amount of sulphur.
Otherwise, only silicon and a small
component of esther iron or calcium
were identified. But we know from meteor physics that sulphur is not a regular
component of meteorites. From the halo-shaped structures on the nitrocellulose
film it was deduced that some of the particles were ice-coated. In addition it
was found that the number of particles was 1000 times greater in the presence of
NLC than in their absence (see Hemenway et. al. 1964)
These results were widely reported in the sixties and later. However,
the results of 1962 have so
far not been confirmed by later
studies. This is a all the more remarkable since further ascents took place in
Sweden in 1964.
Since then much rocket data has been collected from mesospheric levels
and there have been many attempts to link NLC formation to different particles.
But no clear explanation of the
formation of clouds has been given, up to now. It is of interest that many
proposed factors for NLC growth have been noted, but a simple physical
explanation is still not available.
It is therefore questionable
whether the cited halo particles are really present in NLC and, if they
are, it is still unproven that we are dealing with traces of cosmic dust.
Moreover, it should be borne in mind that there is no dust cloud such as was
repeatedly assumed in earlier works. The number of particles which fall into the
earth's atmosphere is much smaller than was heretofore assumed. This explains
the difficulty of finding a mechanism which cumulates the particles at the 80-84
km level. The fall-rate of extraterrestrial dust is almost identical with the
dust density in planetary space, as demonstrated by the zodiacal light
phenomenon For further discussion on the problem of cosmic dust see also
Grjebine (1967 ).
Various calculations of the processes of micro- meteorites in the
Earth's atmosphere have been done, e.g. by Oleak (1956/57) Regarding the
intensity of vaporization he writes: " It is evident that the largest loss
of mass occurs at heights proportionately greater the smaller the particles and
the greater the initial velocities." (Oleak, p 140).
Regarding the intensified nocturnal air glow we find that the
corresponding layers of altitude (90-180 km) will be penetrated in a few minutes.
But this cannot be reconciled with the observed fact that the luminous streaks
are not only visible for many hours but remain unchanged. His study shows that
" even 'small' particles... are not directly resonsible for the appearance
of the luminous streaks since they are virtually completely vaporized at great
altitudes. "(Oleak, p. 143)).
This conclusion can also be
applied to NLC because, for example, they can often be observed for several
hours. If we compare these facts we see no basis for the participation of cosmic
particles in the direct formation of NLC. It is also of interest that the meteor
streams of comets (e.g. Perseids) cannot make an active contribution to the
intensified nocturnal airglow (see , Hoffmeister, 1960, Schröder, 1966).
Two further results argue against cosmic components. In a study of the
"Quadrantids" current, which occurs in January, Glöde ,1966 reaches
the following conclusion:
"...the additional amount of material falling into the Earth's
atmosphere during the shower must also be smaller than that from the sporadic
meteors. Then we could not expect that the "Quadrantids" would have
secondary geophysical effects which could be noted in addition to the always
present effects of sporadic meteors. Nor could we generally use the effects
which might or might not have been produced as criteria for the action of the
meteor dust in the higher atmosphere". Glöde, p. 163)).
Past research (Kohl, 1968)
also shows that depolarisation can be interpreted as an additional
primary scatter effect of cosmic dust particles. In general it is surprising
that no direct relation has been found
between the meteor stream activity and the development of NLC. In summer
different streams are active but they show no direct correlation with
the frequency of NLC. If we remember that
NLC displays are observable for some hours, and very often show abrupt
changes in the different forms, it seems that
a "layer" must be present in the NLC zone. This layer will not be
persistent and is very dependent on the
wind streaming at these height which very often and suddenly destroy the
NLC-layers and produce the different various cloud forms. So, there is a
accumulation layer of material at 82km which
comes from meteors or as a result a
part of pollution, e.g. such as that observed after the great Krakatoa eruption
It required two years for
the volcanic material to reach the
82km layer and create the conditions for the formation of the NLC which were
first detected in June 1885.Because
N LC are only observed during a very limited time-span it can be see that the
particles alone are not enough for the formation of the clouds. Humidity and low temperatures are also required, conditions
not appreciated in the time of O. Jesse, so that he was unable to construct a
more detailed theory of NLC.
The idea that meteor particles play an important role was not new.
Various researchers since 1885 had discussed this aspect of the theory, but it was not enough to explain all the details
shown by the visual and photographic observations (cf. Schröder, 1966). A very
difficult point is the abrupt variation in forms which can be observed during a
few minutes. This can only be
explained by turbulence in the wind streaming at the NLC layer which
suddenly destroys the homogeneous layers.
No increased meteor
streams or enhancement occur in late June/early July .The normal flux of
meteoric material at those times also
plays a part in the structure of the upper atmosphere in their composition with
different particles.Because the studies by Oleak et. al showed no direct
increase with micrometeoric
influence it is open to question whether meteors contribute to NLC formation.
We conclude therefore that, there is no definite proof of cosmic
influence in the formation of NLC.
structure of the mesosphere
Haurwitz (1964) pointed
out in his study the inadequate knowledge at that time in the field of
mesospheric physics. Hesstvedt (1962, 1967) makes a similiar observation. In
fact this inadequate knowledge means that proof of the various views cannot be
carried out with precision. There
have many interesting measurements
in the past, e.g. by the Kühlungsborn group and other scientific institutes.
But, in general our knowledge of the mesospheric processes is insufficient to
account for the behaviour observed in NLC
This comment also applies to polar
mesospheric clouds. In many cases one must depend on assumptions which may
quickly be proven false by measurements in the future.
a. Temperature condition
shows the of temperature at
various height levels of the atmosphere. This
average course of temperature variations shows a steep decline during
summer in the region of the mesopause. A few measurements of the temperature
profile of NLC werde recorded during the 1962 rocket ascents. A value of Tmin=
133K was obtained when NLC were present and one of Tmin=140K when they were
absent (see Witt et. al., 1965). Further measurements were obtained in that
years in Barrow (71N) As to the significance of temperature values in the
formation of NLC the authors state: " Thus the coldest temperatures did not
necessarily produce noctilucent clouds, but the clouds were always accompanied
by mesospause temperatures less than 150K" (Theon et. al. 1967, p. 419).
Even though in paper (31) other new factors were introduced into this problem
only one fact really matters: the appearance of NLC is tied to low temperatures,
a result which has been confirmed during the recent years. These, however, are
present in the Northern hemisphere only in summer and solely in the region of
the mesopause. This clear
relationship is also shown by recent data .
Researchers had available only scant material on the H20 mixing ratio in
earlier years. Hesstvedt then pointed out that there are no objections to the
theory of formation of sublimation (see Hesstvedt 1967, see figure 3).
Various discussions have been published on the use of different mixing
ratios and relations to the freezing point (see Gadsden and Schröder, 1989) We
quote here a few comments made by Hesstvedt since they best describe the general
situation in his time:
"...that the more photochemical model presents highly misleading
results for H2 and for water vapor between 70 and100 km...the water vapor
still takes up 60% of the available hydrogen. At 95km, 22% of the hydrogen will
still be in the form of water vapor" (Hesstvedt 1967, p. 5)
From the formula for the growth rate of particles we find that the
initial radius of a particle increases from 0.05 within of two hours .Recent
studies gives a more detailed picture of the processes but do not succeed in
explaining all the problems.
The model of mesospheric circulation
Concerning the reaction that arose at the time of the finding of a
temperature decrease in the mesopause Faust remarks: " This finding, which
at the time caused much surprise and led numerous arguments, can be easily
understood by means of the cited model of circulation" , (Faust 1967, p.
6). Since the model by Faust and
Schröder occupies a special position in understanding NLC, we shall describe it
in more detail. Figure 4 shows that in summer, above 60km altitude, upward
currents set in as soonas the transition of summer circulation is completed.
These rising currents produce a cooling off at altitudes greater than 60km.
Theoretically the spring transition of winds begins in high latitudes and
spreads to lower ones.When the summer circulation is in full swing rising
movements at higher latitudes are matched by downward movements at lower
latitudes. Of special interest, is
the fact that the spring transition of winds starts at a different time each
year over a four to six week period. During the winter there are downward
currents at 80km altitude in high latitudes, an occurrence which is
automatically reflected in the model by Faust and Schröder. In the meantime, a
generally accepted explanation of this occurrence has been given by Kellogg
(1961). In Faust (1967) the
following statement is made:
" Above the approximate 60km level downward air currents must
predominate during winter in high latitudes, a conclusion that can also be
arrived at by assuming a reversal of the large scale vertical movement in the
60km wind maximum on the basis of the "zero layer concept". In this
downward movement, air particles, which had been stimulated in the ionosphere
and which have again released their stimulated energy in the mesosphere, are
carried downward" (p. 155).
Although the model is known in its broad outlines, information is
lacking on individual variations. First of all it must be understood that the
model is valid only under average conditions. This is why it is understandable
that the zero layer circulation may at times be stronger along one circle of
longitude than along another. Maybe this explains the single "out-time"
observation also of NLC in spring and autumn.To draw reliable conclusions from
the situation would necessitate repeated rocket measuremnts from the polar areas.
These were not available in the sixties and earlier seventies. At that time only
the stations in Fort Churchill and Fort Greeley were in operation.Now, we have
many more observational points and a various satellite and rocket programme by
many research groups. They have developed an interesting insight into the
formation of the mesosphere, but it has not been possible to present a generally
accepted theory of the NLC and later PMC.
It appeared desirable to apply the conditions of Faust's and Schröder's
model to NLC. Past results (see Schröder,
1968) showed that the continuous observations of NLC in the Northern hemisphere
occurred only after the spring wind transition, mainly 4-6 weks later (Schröder
1968). It was also noted that NLC disappeared almost exactly at the time of the
autumn wind transition (cf. Theon et. al. 1969) in mid-August. From the
available material it can be concluded that a temperature effect must be present,
particularly so since the connection between the start of the East stream and
the sudden appearance of NLC is unmistakable. These views are also supported by
the knowledge that winter temperatures of the mesopause in the Northern
hemisphere are some 60 degree higher: this results from the downward movement
shown in Faust's and Schröder's
It is also of interest to examine the effectiveness of Faust's and Schröder's
ideas on the basis of NLC data.We should therefore attempt to stress more
closely the formation of average conditions. Past views of NLC observations lead
to the assumption of a frequency zone (NLC-belt) in the vicinity of 50-70N/S. Maximum devations of this
range occur between about 45N or 75N. The research of Astapovic (see Schröder,
1966) has also shown that
no NLC occur south of 45N: this result is also confirmed by American
data. All of the studies in the sixties show that outside this region no NLC
were observed in the mesopause region.
Transition periods of the mesophere and NLC
It has been demonstrated from earlier data (Schröder,1968) that there
exists a strong relationship between the periods of transition in the mesophere
- in spring and autumn - and the appearance or disappearance of NLC. NLC have
never been observed before the spring transition was complete. The delay between
transition and the apperance of NLC shows a variation of some weeks (3-6). The
frequency of clouds varied extremely in spring from year to year, so that there
is no definite day of first appearance.
A rather different result has been found for autumn: If the transition
period in the 80km region is complete, NLC vanish and cannot be observed.
Throughout the last 100 years no
positive observations after the autumn transition have been noted for the
Northern hemisphere. This is a key factor in the relationship between the
changes in the mesosphere and the appearance of NLC.
The data (1962-1968 and 1991-1996) shows a variability in May over for 4-6 weeks. The NLC period
is nearly constant in August in the first 20 days. After August 19 no NLC have
been noted showing that the
transition was then complete for
the mesopause region and the physical conditions for the formation of NLC no
From this it follows that NLC
can only be oberved visually during the full mesospheric circulation times which
is limited by the transition times in spring and autumn.This result is valid for
both hemispheres, e.g. we have NLC
in summer in the Northern hemisphere, and during
the Austral summer in the Southern hemisphere (cf. Schröder,1968). As we
shown there seems to be a little deviation from this rule altough an occassional
NLC was observed at an earlier or later time.
This can be explained by the duration and complex exchange of the
mesospheric layers in the the
spring and autumn transitions.
I am grateful to Prof. N. J. Skinner for translation help and Prof. J.
Verö for his assistance.
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