Results of the Study of Noctilucent Clouds in Germany during the years 1967-1997
Summary. The visual and photographical
data from German network (and partly from Austria) have been collected. The
climatological variations are demonstrated, and the possible relation to the
transition periods in mesosphere in spring and autumn are described. The tables
showed details of the records.
Keywords: Noctilucent clouds, climatology,
Noctilucent Clouds (NLC) are regularly observed
phenomena in the period May to mid-August north of about 480 N. The
maximum of their occurrence is centered around the summer solstice or a few days
later. Begin and end of the Noctilucent Clouds occurrence coincide with wind and
temperature changes in the Mesopause region. Ground observations are only
possible when the Sun is between 60 and 160 below the
There are only very few systematic series of observations available, although no particular equipment is required. In Germany, series of data exist for the periods 1885 - 1897 and 1956 - 1965 (Schroeder, 1967). Systematic visual and photographic observations were started again in 1982 by volunteer observers. Later, operators of fireball patrol cameras regularly checked for Noctilucent Clouds in the twilight.The data also include information if definitely no NLC were visible in a clear sky. The available data sample allows some conclusions:
Next, we provide some details about the data sample and
some additional information about the obtained results.
2. Observations and Results
The observations reported in this paper are carried out
in Germany from latitudes between 480 and 540 N. Observers
are distributed over the entire latitude region.
In the period 1968 to 1989, there were mainly reports
of witnesses. A limited systematic patrol started in 1990. The History
Commission of Geo- and Cosmical Physics initiated a regular visual and
photographic observing program from 1991 onwards, with a growing number of
participants. Observers operating fireball patrol cameras have to open and close
the shutters of the meteor cameras just in the twilight period which permits
systematic observation of NLC. The entire sample stored in our data base
contains 592 records (both positive and negative). Of these records, 547 cover
the period 1991-1995.
There are no quantitative analyses possible for the
years prior to 1991. This was the first year with more than 30 observational
reports. Observers are now reporting the time and additional information for
both the appearance and the certain absence of NLC. This allows to derive
information on the frequency of NLC occurrence and - if the network of observers
is close enough - their changes and development.
Assuming a constant height of the NLC of 83 km (Schroeder,
1975), it is possible to determine velocities in the NLC from single station
photographs. First such measurements have been carried out from series of images
taken in 1994 and 1995. The velocities found from measurements of various
features within the NLC display varied between 10 and 30 m/s, coinciding with
figures reported by other authors.
Our earliest field observation is dated May 2 (1994),
the latest August 18 (1995). Another late and extended display has been observed
from a far northern position during a flight over Northeast Canada and Greenland
on August 17, 1994, and was in fact almost over the pole region. Both dates
coincide with the changes in the mesospheric circulation. There are no
significant differences in the relative rate of NLC before midnight and after
midnight (local time; i. e. roughly UT + 1h for our data). For this
comparison we used a relative rate which is the ratio of observed NLC (divided
by the number of observations for the same period). The periods were chosen from
dusk to midnight local time and from midnight to dawn. Considering all
observations collected within the systematic program from 1991 and 1995, the
respective rates are 0.19 before local midnight and 0.21 after local midnight,
respectively (Fig. 1).
Restricting to the years 1993 - 95 only, the rates are
0.20 and 0.27 respectively. Considering the number of reports one might state an
even distribution of NLC displays observed before and after midnight local time.
However, it should be noted that the rate of NLC observed in the morning
steadily increased from 1991 to 1995. Again, the small sample must be kept in
mind. The highest relative rate per 10 days interval (sum of NLC records for the
entire night divided by the total number of patrol observations in the same
interval) was found for the period June 21 - 30 (Fig.
2). Here we again restricted to the years 1991 to 1995, in which the entire
period from May to August is covered. NLCs are very rare before May 20 and after
July 31. This confirms other investigations of NLC and the suspected connections
with the Polar Mesospheric Clouds (Thomas, 1991). As for the higher rate in the
last 10 day bin, we think this may be an effect caused by the small number of
reports, perhaps superposed with the more educated observation, the shifted
twilight periods, and the relatively large number of Perseid observers being
active towards mid-August all night.
Calculations from a series of NLC photographs taken
during the night July 11-12, 1994 from Potsdam, Germany (l = 130
0 E, j = 520
4 N) yield velocities of 10 to 30 m/s for various parts within a NLC display,
assuming a height of 83 km for the clouds.
The most extended and intense NLC displays have been
observed over several hours and from different stations in Germany on July 11
-12, 1994, June 29 -30 and June 30 - July 01, 1995. Most remarkably, the NLC of
June 29 - 30 have been observed and photographed from a location at (j
= 480 7 N (Austria). There are very few reports from similar
latitudes - one from Basle (Switzerland) is listed in the 1981 NLC summary
(Mcintosh and Hallissey, 1982). A list of observations we have on file for the
period 1985 to 1995 is given in Tab. 1
Another suspection regards the anti-correlation of NLC
frequency with solar activity. Although our series of systematic observations is
too short to allow respective conclusions, it should be noted that the rate and
intensity of NLCs as observed from mid-Europe was striking in 1994 and 1995
compared to the years before. However, a reliable conclusion requires further
3. Transition periocls of the Mesosphere and
It has been demonstrated from earlier data (Schroeder,
1970) that there exists a strong correlation between the periods of transition
in the mesosphere - in spring and autumn - and the appearance or disappearance
of noctilucent clouds. Noctilucent clouds have never been observed before the
spring transition was complete. The delay between transition and appearance of
noctilucent clouds shows variations of some weeks (3 - 5). The frequency of
clouds varied extremely in spring, so that there is no definite day of
appearance in different years. Another result has been found for autumn: if the
transition period in 80 km region is complete, noctilucent clouds vanished and
cannot be observed. During all the last 100 years no positive observations after
the spring transition has been noted for the northern hemisphere. That can be
understood as a key factor of the relation between the change in mesosphere and
the appearance of noctilucent clouds.
Table Appearance of NLC in spring and
autumn (selected data)
The table shows that the variation in May varied for 4
weeks. The NLC-period is nearly constant in August in the first 20 days. After
August 19 no noctilucent cloud has been noted. That showed that the transition
in these times was complete for the mesopause region and the physical condition
for the formation of noctilucent clouds are not more exist.
First of all, the data of NLC observations currently
stored in our data base allows some basic conclusions about the frequency of
their appearance and the annual duration of the NLC period. Highest relative
rates have been found in the 10 days interval June 21 - 30. The observers of our
net are situated close to the southern border of the NLC visibility. An
observation of NLC at 480 7 N in the morning of June 30, 1995,
belongs to the southernmost reports of this phenomenon. A first attempt to
derive velocities within a display of NLC yielded an average figure of 30 m/s.
Although the systematic observations are too short for definite conclusions, the
significantly enhanced rate of NLC in the 1994 and particularly 1995 seasons are
striking and may be regarded as a hint on an anti-correlation of NLC rates and
Hallissey M., 1982: Noctilucent clouds over western
Europe during 1981. Met.
Magazine 111, 122-125
Schroeder W., 1967: Results of the study of Noctilucent Clouds over Germany during 1885 - 1965. J. Geophys. Res. 72, 1971.
Schroeder W., 1975: Entwicklungsphasen der Erforschung der Leuchtenden Nachtwolken. Akademie-Verlag, Berlin.
Schroeder W., 1970: Noctilucent clouds and transition periods in spring and autumn in the mesosphere. Gerlands Beitr. Geophys. 64, 328.
Thomas G. E., 1991:Mesospheric clouds and the physics of the mesopause region. Rev. Geophys. 29, 553.
Thomas G. E., and J. Olivero, 2001: Noctilucent clouds as possible indicators of global change in the mesosphere. Adv. Space Res. March 7, 2001
grateful to K. Kaiser, R. Loewenherz, M. Mersich and many other observers who
kindly support me with their data.