On the
Diurnal Variation of Noctilucent Clouds
By WILFRIED SCHROEDER
In the last part of their paper, Jensen and Thomas and
Toon (1989) describe and discuss the diurnal variation of noctilucent clouds
recorded by visual observations. It seems to be useful to make a more careful
overview of the ground based data which have been sampled during the last 100
years. Furthermore, an insight into the general problems associated with visual
data may be useful for further interpretations.
Noctilucent clouds were first observed in Germany in
1885, two years after the great Krakatoa volcanic event of August 1883. Around
that time there were anomalous twilight phenomena and observers interest in
these atmospheric processes were stimulated (see Gadsden and Schroeder, 1989).
Originally, people spoke of the “glowing clouds” or they were sometimes
called “silvery clouds”. It was probably O. JESSE who introduced the term
“noctilucent clouds” (Leuchtende Nachtwolken). Jesse, who was then active at
the Berlin Observatory, also performed the first photographic measurements of
the clouds.
With the “Vereinigung der Freunde der Astronomie und
kosmischen Physik”, a German society of friends of astronomy, a working group
was formed under the leadership of ARCHENHOLD, FOERSTER and JESSE, with the goal
of observing noctilucent clouds. They worked mostly between 1889-1899. More or
less sporadic observations were made in Europe and USSR up to the end of the
Second World War. Only since the IGY (1 957) has there been more systematic
surveillance of these phenomena in different parts of the world.
The observations are predominantly visual but there are
also many photographs and other special measurements. Taken as a whole, these
data have allowed no clear understanding of the complex problem of noctilucent
clouds, mainly because of their sporadic nature. This includes the various
climatological aspects, i. e. the diurnal and seasonal variation.
In their last section Jensen et. al. (1989) discuss the
diurnal Variation of noctilucent clouds. It seems to be necessary to make a more
accurate distinction between the terms “brightness” and “appearance”.
For visual observations, a scale of noctilucent cloud intensity exists; it has
the following five points:
1 - very weak NLC, barely visible against the
background of the twilight sky, detected only through very careful examination
of the sky;
2 - NLC
easily detected, but having low brightness;
3 - NLC clearly visible, standing out sharply against
the twilight sky;
4 - NLC very bright;
5 - NLC extremely bright and noticeably illuminate
objective facing them.
In many papers the diurnal Variation was reported in
terms of the “first sightings” per night and this may possibly have led to
the conclusion that the clouds are seen more often after midnight than before.
For instance, Archenhold (1928) reported: “in 1889 - 1894, the clouds were
observed only six times before but 33 times after midnight”. At first Vestine
(1934) accepted this result, and some decades later Ludlam (1 957) wrote:
“Except in very intense displays, the clouds have been seen more frequently
after than before midnight ......”. A more careful examination of the various
data was made later by Fogle, Chapman and Echols (1965) and by Schroeder (1968).
It was found that 83% of the North American displays and 60% of the displays
over the USSR during the IGY (1957 - 1958), were first seen before midnight.
Furthermore, Pavlova (1962) reported that the total number of occasions of the
observations of noctilucent clouds was 1,56 time greater in the morning than in
the evening twilight.
These data relate to the times of first “sighting”
(detection by visual observations on each night).
If we look at the “brightness” of the noctilucent
clouds, a difference can be pointed out. Jesse (1890) reported in one of his
first analyses on the variation of brightness of the clouds “we found an
increased brightness of all noctilucent clouds in the morning hours”. This was
the first report of increasing brightness of noctilucent clouds during the
morning hours and this effect has been confirmed in many notes after 1889. The
fact that noctilucent clouds are generally brighter and most widespread after
midnight may account for the differences found between the earlier and more
recent studies on the daily variation of noctilucent clouds frequency (Fogie and
Haurwitz 1966; Schroeder 1968).
Using the visual estimation of noctilucent cloud
brightness, it should be noted that the clouds are usually very patchy, and
observers often note the intensity of the bright patches in the display. For
visual observation the structure in noctilucent clouds has been classified in
four different types (cf. Gadsden and Schroeder, 1989):
I. Veils;
II: Bands;
III: Billows (or waves);
IV: Whirls.
It is possible that in complex displays all four forms
are observed simultaneously. The genesis of these formations shows remarkable
differences in brightness and lifetime; therefore visual observations are very
often reported only as a general term the “brightness” of the clouds, rather
than referring to the different structures.
Generally it has been reported by observers that
noctilucent cloud brightness (and their different morphological forms I-IV) vary
considerably during the time of observation.
Table 1 presents a summary of visual estimations of
noctilucent clouds based on reports received from regular meteorological
stations and individual observers. In general the brightness is reported in the
1-5 scale of intensity described above. It must be considered that the
notification of brightness are related to different parts of a display; the
clouds are mostly patchy and therefore a general brightness estimation for it is
impossible by visual observations.
The conclusions of Table 1 are the same as have been
reported by earlier observers: (a) in general the noctilucent cloud displays
show a variation of brightness; (b) most of the clouds have been noted after
midnight with the intensity 3-5; (c) a maximum before local midnight is not
found. The intensity 1-4 has been observed for all different forms of the clouds.
Intensities 4 and 5 were very often associated with the forms IIa (bands with
diffuse edges), IIIa (billows/waves consisting of straight and narrow, sharply
outlined parallel short bands); IIIb (wave-like structure with undulations in
the short-bands) and IVb (whirls having the form of a simple band of one, or
several bands with a radius of curvature of 3 – 5o).
It seems to be useful to comment further on the visual
data. Most of the displays showed a variation of brightness during the
observation epoch, i.e. no display showed a constant brightness. For the
Variation during the night (and/or) in twilight we must consider some subjective
aspects and the adaptability of vision of the observer during darkness. During
the evening (and morning) twilight the human eye cannot adjust fully to lower
illuminations and is less capable of detecting low contrast forms of noctilucent
clouds against the sky background.
It would be useful to continue the visual observations
of noctilucent clouds, including ground-based data and measurements, paying more
attention to the variability of the different forms.
During recent decades the relationship between
increased noctilucent clouds and the influx of cosmic dust has been discussed (cf.
Gadsden and Schroeder, 1989). Noctilucent clouds appear at nearly the same time
as the May and June increase of meteor streams (e.g. Aquarids, Arietids and Zeta
Perseids). The cloud period diminishes shortly alter the great injection of
August Perseids. In general, the meteor flux is only one possible factor in the
development of noctilucent clouds. But, indeed, the observed diurnal variation
in brightness of increased airglow and noctilucent clouds, is of interest.
Considering the conclusions of Jensen, Thomas and Toon
(1989) and the data from noctilucent clouds and increased airglow, it would be
valuable to continue this research in future.
The diurnal variation of noctilucent clouds has been
noted in general terms. Further research should aim to present more detail on
the dependence of brightness of noctilucent clouds on observational (local)
time. From it some refinement may be possible, including the assumptions of
Jensen, Thomas and Toon
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