GENERAL METEOROLOGICAL OPTICS 73 
Description. At, or shortly after, sunset, the antz- 
twilight arch, a purplish band of some 3° or more in 
width, can be seen to rise above the solar counterpoint 
on the eastern horizon. At about 1° sun’s depression, 
the gray-blue dark segment or earth’s shadow begins to 
rise beneath the antitwilight arch. At approximately 2° 
sun’s depression, the purple light appears as a purplish 
area above the solar point in the western sky. This area 
has a vertical angular extent of 10° to 50°, a lateral ex- 
tent of 40° to 80°. Its upper boundary, which has an 
elevation of about 50° at the beginning, descends stead- 
ily to the horizon. The purple light reaches its maxi- 
mum intensity at about 4° sun’s depression and usually 
disappears at about 6° sun’s depression. The rising anti- 
twilight arch usually fades from view when the purple 
light is at its height, and shortly afterwards, the dark 
segment becomes indistinguishable from the rest of the 
darkening sky. Its transit through the zenith generally 
cannot be observed, but it reappears as the bright seg- 
ment’ or twilight arch above the solar point, when the 
sun’s depression is about 7°. The bright segment disap- 
pears below the western horizon at about 16° sun’s 
depression. 
Fig. 11—Schematie diagram of major twilight phenomena 
(elevation angles are a = antitwilight arch, 6 = maximum 
purple light intensity, y and y’ = upper and lower boundary 
of purple light, 6 = antisolar point, and e = dark segment). 
When the sun’s rays are partially obstructed by 
clouds or mountain peaks, the purple light may assume 
a ray-structure because of the interruption by the sha- 
dow bands (crepuscular rays) which seem to converge 
towards the sun. Similarly, the continuation of these 
shadow bands (anticrepuscular rays) on the eastern sky 
may give the antitwilight arch a fanlike appearance. 
Colored illustrations of the various twilight phenomena 
can be found in [16]. 
During brilliant twilight phenomena, a secondary 
purple light may become visible after the main purple 
light has disappeared. Also a secondary antitwilight 
arch and dark segment may develop within the primary 
dark segment. These secondary phenomena are much 
more diffuse in outline and show fainter colors. 
8. The descriptive term ‘‘bright segment’? is preferable 
because of its fundamental identity with the dark segment and 
the basic difference from the “‘antitwilight arch.”’ The term 
“earth’s shadow” is physically incorrect [40] and does not 
readily permit differentiation between the phenomena on 
either side of the zenith. 
For the measurements of twilight phenomena the 
following spatial and temporal aspects are generally 
considered: The elevation angle of the upper boundary 
of the antitwilight arch, of dark and bright segments, 
and of purple light; and the sun’s depression at the 
time of beginning, end, and maximum intensity of the 
purple light. In addition, photometric measurements of 
the light intensity in various spectral ranges along sig- 
nificant portions of the sun’s meridian are of major 
interest. 
Results of Observations. The development pattern 
of the purple light has been found to be practically 
the same everywhere except at altitudes above 2000 
m where the purple light ends at somewhat greater sun’s 
depressions and where its upper boundary reaches 
ereater elevations. A greatly detailed analysis of visual 
observations, such as made by Gruner [14] and Dorno 
[11], seems hardly warranted in view of the fact that 
the sun’s depressions are computed without regard to 
the variable refraction, that the intensity is estimated 
according to a memory scale, and that the measurement 
of elevation of the diffuse boundary of the delicately 
tinted phenomenon is affected by subjective factors 
[50]. In Europe, a maximum frequency of bright purple 
lights occurs in autumn, a minimum in spring; this 
fact has been attributed to the corresponding frequency 
of anticyclones with clear skies in that area [51]. Other- 
wise no significant relationship between weather and 
purple lights has been found. 
Secular variations of intense purple lights have been 
observed associated with dust produced by voleanic 
eruptions [c. 42, 50]. There exists, however, a time lag; 
for example, after the Katmai eruption in summer 1912, 
purple lights did not reach their maximum intensity 
until summer 1913 [11]; this delay was obviously due 
to the time involved in the sedimentation of dust parti- 
cles necessary to produce the optimum concentration 
and size distribution for the formation of the purple 
light. For this reason, an absence of dust layers cannot 
be deduced from an absence of intense purple lights 
[50]. 
Although visual observations have long been recog- 
nized as inadequate, objective methods have been em- 
ployed only in relatively recent times [13, 14, 32]. The 
techniques involved still need improvement and stand- 
ardization. The results obtained at different stations 
from photoelectric [13] and photographie [32] intensity- 
measurements with color filters show a maximum red 
content of the sky light between 4° and 5° sun’s depres- 
sion, corresponding to the visually observed maximum 
relative intensity of the purple light. The absolute 
luminous density of the sky light decreases steadily in 
all spectral ranges with increasing sun’s depression in 
contrast to the visual impression [14]. Whereas the re- 
sults from visual observations were essentially con- 
firmed by objective methods [13], the latter have shown 
the presence of the purple light when the spectral dif- 
ferences in intensity were below the threshold of visual 
perception [32]. 
Gruner [14] has indicated a method for determining 
