ALBERT

Notes: Summary The global distribution of the lunar barometric tideL 2 is investigated by spherical harmonic analysis, based on 104 stations for the annual mean, and on 85 stations for the three seasons. The main wave ofL 2 is the one with wave number 2, but for a detailed study of the irregularities of the global distribution ofL 2, waves with other wave numbers have also to be considered. Even the main wave ofL 2 is asymmetric to the equator with the two lunar-daily pressure maxima occurring earlier in the Southern than in the Northern Hemisphere. The amplitudes at the same distances from the equator are greater in the Southern than in the Northern Hemisphere. These hemispheric differences are most pronounced during the D season. As found in earlier investigations the phase consiant ofL 2 is always greater during the J season than during the D season. But the amplitudes are greatest during the J season only north of 30oS. Farther south the amplitude maximum occurs during the D season.

Notes: Summary The global distributions of the annual and seasonal means of the diurnal (S 1) and semidiurnal (S 2) surface pressure oscillations are investigated by spherical harmonic analysis. The main waves are,S 1 1 (with wave number 1) forS 1 andS 2 2 forS 2.S 1 1 is much less predominant among the waves ofS 1 thanS 2 2 among those ofS 2. As in the case of the lunar semidiurnal barometric tideL 2 the pressure maxima occur earlier in the Southern than in the Northern Hemisphere. In the case ofS 2 the standing waveS 2 0 and the waveS 2 3 are also of interest besidesS 2 2. Although the present analysis extends only from 60°N to 60°S, whileS 2 0 is largest at polar latitudes, its results show thatS 2 0 should be smaller at high southerly than at high northerly latitudes, as has been observed. Thus this observed asymmetrical distribution ofS 2 0 may be due to causes outside the polar regions rather than to their geographical differences. The best approximation to the observed distribution ofS 2 0 is obtained by including a mode representing an oscillation independent of longitude and latitude indicating a small semidiurnal variation of the mean global surface presure, which is an unlikely result on physical grounds. The seasonal variation ofS 1 1 expressed in percent of the annual mean is smaller than that ofS 2 2, and both are less than the unexplained seasonal variation ofL 2 2. The main wavesS 1 1 andS 2 2 are expressed not only by associated Legendre functions, but also by Hough functions.

Notes: Summary When two tropical cyclones are present simultaneously in the same region they show as a rule a counterclockwise rotation around each other. A theoretical explanation of this motion is given. The theory permits the computation of the rate of rotation. A discussion of nine examples with sufficiently reliable and complete observational data gives a satisfactory agreement between theoretical and observed rates of rotation.

Notes: Abstract The amplitudes and phases of the semidiurnal pressure wave have been computed for sixteen high-latitude stations. The data were analyzed to obtain a picture of the geographical and seasonal variations of this oscillation at high latitudes. At most of the stations the seasonal variation of the amplitude was found to have a maximum during the equinoxes and a minimum in the summer. No definite seasonal variation was found for the phase, although a tendency was found at stations above 70° N for the latest phase to occur in summer and an earliest phase to occur in winter. A simple expression is given for the geographical distribution of the annual mean of the standing semidiurnal oscillation at high latitudes.

Notes: Zusammenfassung Auf Grund von Beobachtungsdaten und theoretischen Erwägungen wird eine Darstellung der Hauptglieder der täglichen Temperaturschwankung und ihrer Verteilung über die Erde in Form von Reihen von trigonometrischen undLegendre-Funktionen gegeben. Ein Vergleich mit einer rein theoretisch erhaltenen Darstellung vonKertz zeigt in großen Zügen eine befriedigende Übereinstimmung beider Ergebnisse.

Notes: Summary Observations of the diurnal (S 1) and semidiurnal (S 2) wind oscillations between 80 and 100 km show that their amplitudes are of the same magnitude as the mean winds. Both periods show pronounced seasonal changes in the same sense as, but much larger than would be expected from surface data forS 2, and in agreement with the idea of a thermal cause forS 1. From the observed wind oscillations the pressure oscillations can be computed. The pressure amplitudes are about 5 percent or, especially forS 2, more of the mean pressure, indicating a fifty to hundred-fold increase compared to the low atmosphere, as suggested by theory. But the phase ofS 2 does not show the expected reversal relative to the ground.S 1 seems to be much more regular than at the earth's suface where it is greatly disturbed by local influences. The few data on the lunar semidiurnal (L 2) oscillation show also the increse of the amplitudes with height.

Notes: Zusammenfassung Die Untersuchungen von Helmholtz, Diro Kitao u. a. über die Bewegungen gerader Wirbelfäden umeinander werden auf endliche Wirbel von kreisförmigem Querschnitt ausgedehnt. Die Wirbelgeschwindigkeit im Innern braucht dabei nicht konstant, sondern nur radialsymmetrisch zu sein. Wirbel mit positivem (negativem) Rotationssinn bewegen sich in positivem (negativem) Sinne umeinander. Ist die horizontale Divergenz in den Wirbeln überwiegend positiv (negativ), was auf Vertikalbewegungen zurückgeführt werden kann, so entfernen (nähern) sich die Wirbel.

Notes: Conclusions Over the past ten years, our knowledge of the characteristics of NLC has advanced considerably, but much remains to be done before a satisfactory understanding of the nature of these clouds is obtained. The intensive study of NLC since the I.G.Y. shows that these clouds occur over North America as frequently as in Europe and the U.S.S.R. Further, NLC displays are generally quite persistent and last for periods up to and greater than 5 hours, but individual parts (particularly the billow structure) often form and decay within a few minutes or tens of minutes. The rapid structural changes in the clouds indicate that the layer in which they are formed is well stirred and often in wave motion. In the Northern Hemisphere NLC are observed predominantly between June 1 and August 15, with the peak of activity occurring around 20 to 30 days after the summer solstice and the brightest and most widespread displays taking place between July 1 and August 15. The optimum latitude for NLC observations is around 60° N. NLC occur far more frequently than previously supposed — during the month of July, NLC are seen nearly every night in some part of the Northern Hemisphere. An observer at 60° N might expect to see NLC on about 75% of the clear nights during the month of July. Occasionally NLC displays extend over an area in excess of millions of square kilometers. Recent studies of NLC in the Southern Hemisphere have resulted in the proof of the existence of NLC there and in the determination of some of their characteristics. Southern Hemisphere NLC were found to have a general drift motion toward the west-north-west. NLC were observed there (at 53° S) during the period December 25–January 20, with the brightest and most widespread displays occurring during the first four days of January. A comparison of these results for 53° S with those obtained from stations at 53° N suggests that NLC in the Southern Hemisphere have the same apparent frequency of occurrence with respect to the solstice as NLC in the Northern Hemisphere and that the clouds are likely to be seen at 60° S from December 1 to February 15. Geometrical considerations of NLC observations and observational results show that the clouds are likely to be seen only during the time periods when the solar depression angle (SDA) is between 6° and 16° and that they are most easily detected at SDA from 9° to 14°. At SDA greater than 16°, the 82 km level where the NLC are formed is no longer illuminated by the sun even at the observer's horizon. An atmospheric screening height of around 30 km appears to be operative in the case of NLC. The collection and statistical analysis of all available data on NLC provides the following picture of their characteristics in the Northern Hemisphere: Color: bluish white Height: (average) 82.7 km Latitude of Observations: 45° to 80°, best at about 60° Season for Observations: March through October, best in June through August Times for Observations: nautical and part of astronomical twilight, SDA = 6° to 16° Spatial Extent: 10 000 to more than 4000 000 km2 Duration: several minutes to more than 5 hours Average Velocity: 40 m/sec towards SW; individual bands often move in different directions and at different speeds than the display as a whole Thickness: 0.5 to 2 km Vertical Wave Amplitude: 1.5 to 3 km Average Particle Diameter: about 0.3 microns Number Density of Particles: 10−2 to 1 per cm3 Temperature in Presence of NLC: about 135° K The available evidence suggests that the dust particles in NLC are of extraterrestrial origin and that they have a volatile coating, the nature of which is uncertain at this time although it is largely assumed to be water substance. The fact that no uncoated particles with diameters greater than 0.20 micron were found in the NLC samples obtained over Sweden in 1962 indicates that particles of this size are absent in the regions above and below the cloud layer. This result suggests that the larger particles may be formed in the NLC layer by coagulation of the smaller ones and that these particles are retained in the NLC layer by some mechanism such as large-scale vertical motions. Calculations of the fall speed of NLC particles indicate that the particles are likely to be of low density (below 1 g/cm3) and/or non-spherical in shape. In view of the large uncertainties remaining as to the nature of NLC particles and the characteristics of the region in which they form, a definitive theory explaining their formation must await further experimental data. For a satisfactory understanding of NLC more and better information is required on the following variables: 1) Height, thickness, vertical wave amplitudes, horizontal wave lengths and wave speeds of NLC, 2) Brightness, polarization, and spectrum of NLC, 3) Wind and temperature in the NLC region both in the presence and absence of NLC, 4) Water vapor concentration around the mesopause in the presence and absence of NLC, 5) Concentration, composition, and shape of particles in the NLC region both in the presence and absence of clouds, 6) Nature of the volatile coating on NLC particles, 7) Height of the turbopause when NLC are present and absent. The first two items can be measured from the ground, but the others will presumably require rocket measurements. A knowledge of the wind and temperature distribution would permit a decision as to whether the observed wave forms are internal gravity waves or interface waves. A knowledge of the temperature and water vapor content during the presence and absence of NLC would also be helpful in the investigation of condensation processes on NLC particles and the changes of NLC appearance. Polarization measurements at scattering angles greater than 90° would assist in determining whether NLC become visible because of an increased concentration of particles at the mesopause or because of an increase in particle size due to coating. Better information about the nature of the particles would help in making more definite theoretical deductions from ground-based optical measurements and more reliable theoretical estimates about the sinking velocities important for the theory of NLC formation. A measurement of the height of the turbopause and the turbulent state of the atmosphere in the region in question, by means of artificial vapor trails, could make an important contribution to the Chapman-Kendall theory which postulates a descent of the turbopause to the NLC region. Because of the sometimes observed disappearance of NLC when auroral displays occur, a particularly interesting experiment would be a sequence of temperature measurements in the NLC region when aurora and NLC occur together in order to see whether a warming of the region due to auroral heating can, in fact, be discovered and whether such a warming leads to significant changes or even disappearance of the NLC by removing the coating from the nuclei or by greater turbulence which would reduce the particle concentration. NLC are, even in the latitudes and seasons when they occur, relatively rare phenomena, but their study is related to many other problems connected with the mesopause, the lowest layer of the ionosphere, the lowest fringe of the auroral layer, and with the influx of cosmic dust. Thus their continued exploration can contribute greatly to our knowledge, not only of this particular level, but of our whole atmospheric and space environment.