Some Results of the Study of Noctilucent Clouds in Germany during the years 1967-1997  

By Wilfried Schroeder

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, mesosphere

1. lntroduction

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 horizon.

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:

• most NLC have been observed in the 10-day interval June 21-30
• NLC displays may exist for several hours, but there are significant morphological changes at least in parts of a NLC-complex observable within scales of minutes with velocities of the order of several 10 m/s
• the earliest observed NLC in our series is dated May 2, the latest August 18, both coinciding with seasonal changes of mesospheric circulation
• correlations between NLC and solar activity parameters have been claimed to exist. The small number of NLC until 1992 together with the frequent occurrence NLC in 1993, 1994 and 1995 might be regarded as a respective hint
• the southernmost NLC in our sample occurred on June 30, 1995 and has been observed from 48⁰ 7 N.

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 + 1^h 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 systematic data.

3. Transition periocls of the Mesosphere and Noctilucent clouds

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.

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.

4. Conclusions

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 Solar activity.

References:

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

Acknowledgement:

 I am grateful to K. Kaiser, R. Loewenherz, M. Mersich and many other observers who kindly support me with their data.