GENERAL METEOROLOGICAL OPTICS 67 
more recently by Fujiwhara and others [c. 21]. There 
is, however, still a thermodynamic problem connected 
with inferior mirages such as can be observed over 
heated highway surfaces. There, the vigorous stirring 
of the air by passing vehicles apparently has little ef- 
fect on the existing density distribution. It would be 
interesting to study the ‘‘tenacity” of the mirage- 
producing air layer. Ives [24] has investigated larger- 
scale mirages of this nature and found that phenomena 
caused by steep lapse rates re-form, after disturbance, 
within a few minutes, while mirages produced by low 
inversions are more readily disturbed and “heal” much 
more slowly. Laboratory experiments, which permit 
control of the variables, and theoretical study of the 
heat transfer rate seem desirable to expand our knowl- 
edge of mirages. 
Scintillation. Scintillation is due to temporal and 
spatial variations of density in the atmosphere. It 
consists of one or more of the following characteristics, 
depending on the nature of the object viewed. If the 
object is a point source, scintillation causes (1) apparent 
directional vibrations (unsteadiness in position of fixed 
objects), (2) apparent intensity fluctuations (light 
source may even appear to flash on and off), or (8) 
color changes (white light shows alternately its in- 
dividual chromatic components). For extended objects, 
inhomogeneities in air density will cause (1) varying 
distortions of the contours or of internal line-structure 
of distant objects, thereby producing apparent expan- 
sion, contraction, or even disruption of the visible area; 
or (2) inhomogeneous brightness distribution, so-called 
shadow bands, over the surface of an object that is il- 
luminated by a collimated light beam. 
Astronomical scintillation involves extraterrestrial 
light sources. Its effect, in general, decreases with in- 
crease in angular elevation of the source. The amplitude 
of the vibratory motion of stars (or of the edge of the 
sun’s or moon’s disk) amounts to a few seconds of 
are at the most, with a frequency of roughly 2 to 30 
sec, although the vibrations are seldom of truly peri- 
odic nature [7, 42, 55]. The relationship between the 
quality of star images in telescopes and weather ele- 
ments has long been recognized [42]; the image quality 
deteriorates as wind speed, turbulence, or temperature 
lapse rate in the lower atmosphere increase [2, 55]. 
Respighi [c. 42] ascribed a greater effectiveness to the 
rotation of the earth than to wind, because he observed 
spectroscopically that stars on the western horizon 
pass through the spectral color sequence from red to 
violet, while those on the eastern horizon show the 
reverse. Pozdéna [43] shares this opinion, stating that 
the lmear speed of the earth’s rotation is much greater 
than the relative speed of the winds. Exner [42], how- 
ever, pointed out that the observed phenomenon was 
due to the prevailing westerlies at higher altitudes and 
suggested that the argument could be decided by means 
of observations in regions with prevailing easterly cir- 
culation. There, the phenomenon observed by Respighi 
should appear reversed if wind is the dominant factor. 
Such a test has apparently not yet been made. 
The explanation of chromatic scintillation, which 
has frequency characteristics similar to those of direc- 
tional vibrations, was given by Montigny [c. 42] and 
is briefly outlined (Fig. 7). The difference in refractive 
index for the extreme visible wave lengths causes a white 
light ray 1, entering the atmosphere at A, to send its 
red component toward O; at the ground, its violet com- 
ponent toward an observer at O. Another light ray 
In, which is lower than I; by a distance D and enters 
—“ SS, 
SS 
VIOLET 2 Ns 
Fig. 7.—Dispersion of light rays by the atmosphere. 
the atmosphere at B, sends its red beam toward O, while 
its violet beam falls at Oo. Other rays between L, and 
Ly send the intermediate colors toward O, so that the 
observer there sees the entire color mixture as white, 
because the angular subtense of the spectrum is gener- 
ally too small to be resolved by the eye. The distance D 
between rays of the extreme colors varies with the 
angular elevation of the light source and the height 
above the earth’s surface. Table VI represents an ab- 
stract of the corresponding table by Exner [42]. The 
TasieE VI. VARIATION OF THE DISTANCE BETWEEN VIOLET 
AND Rep Rays (IN cM) witH ZENITH DiIsTaNcE (0) 
AND Huteut (After Hxner) 
Height in km 
e(°) 
0.1 il 5 10 40 
50 = = 2 3 5 
60 = = 5 8 12 
70 == 3 14 22 31 
80 = 15 58 92 127 
84 4 37 142 224 311 
88 17 151 580 915 1273 
90 50 442 1698 2680 3727 
color separation for zenith distances of < 50° is ex- 
tremely small so that chromatic scintillation of stars is 
generally not perceptible. For greater zenith distances 
and for increasing heights, the rays’ separation rapidly 
increases. Any air parcel that has a density different 
from that of its environment (density schlieren) and a 
diameter less than the rays’ separations, will be capable 
of diverting individual color components into a different 
direction at different instants, thus causing chromatic 
scintillation. The size of these air parcels was variously 
determined as of the order of a few centimeters to a few 
decimeters [36, 42]. The size of the schlieren in relation 
to altitude, and the schlieren velocity, obviously deter- 
mine the possibility and the frequency, respectively, of 
color fluctuations. For example, in order to produce 
chromatic scintillation of a star at 80° zenith distance, 
an air parcel must have a diameter of < 15 emif at 1 km 
height, < 58 cm if at 5 km height, etc., whereas near 
