MONTHLY
WEATHER
REVIEW
VOLUME 99, NUMBER 10
OCTOBER 1964
A STUDY OF MARTIAN YELLOW CLOUDS THAT DISPLAY MOVEMENT
F. A. GIFFORD, JR.
U.S. Weather Bureau Research Station, Oak Ridge, Tenn.
ABSTRACT
Study of all reported instances of motion of Martian yellow clouds yields an analysis of thcir probable nature
and properties. The yellow clouds seem to be initiated by wind-driven sand and teiid to form in low latitudes.
The limb and terminator projections seem to be quite diffcreiit in nature, probably in part aqucous condensations.
Thcse occur primarily in middle latitudes.
1. INTRODUCTION
Among many observations of Martian “yellow clouds”,
that is, transient obscurations of portions of Mars’ surface
that can be observed visually or in photographs made
through yellow filters, a few have occasionally displayed
motion. As the only available direct indications of Mar-
tian atmospheric circulation patterns, these observations
are of exceptional meteorological interest. Unfortunately,
they are also exceedingly rare.
Yellow cloud motions have been observed either as dis-
placenients of obscurations located, roughly, in the central
portion of the disk, or as displacements of limb or termina-
tor projections, Hess [14] presented a streamline analysis
of 1s Martian cloud motions based primarily on the exten-
sive study of limb and terminator projections made by
Douglnss [SI during the apparitions of 1894 and 1S96.
De Vaucouleurs [6] has tabulated a half-dozen instances of
yellow cloud displacements, including examples given
earlier by Antonindi [I]; and Slipher [24] has recently pro-
vided adclitional examples documented photographically.
In addition to the above, there exists a small but sig-
nificant number of isolated reports of yellow cloud motion
scattered through the literature. Furthermore, it is pos-
sible, in certain cases, to infer motions from reported yel-
low cloud observations for which the observer did not
himself make this inference. The following is the report
of an attempt to cull from the published literature all such
reported instances of Martian yellow cloud motions, to-
gether with an analysis of their probable nature and
properties.
2. SOURCES OF DATA
The first observer of Martian clouds was Maraldi, in
1704, according to Flammarion [lo] who provided n coin-
prehensive summary of all Martian observations up to the
turn of the century. Schroeter, during the apparitions of
1787 and 1792, attributed apparent differences in the rota-
tional period of Mars, as judged by the displacement of
markings, to cloud motions. Later, P. Secchi suspected
cloud motions as the cause of short-period changes in the
appearance of Martian dark markings, and this was con-
firmed by Lockyer. To obtain quantitative infornitition
on yellow cloud motions it is necessary to turn t o more
recent sources.
In addition to the texts already mentioned, files of
astronomical journals, observatory annual and periodic re-
port series, and other pertinent literature sources have
been consulted in search of instances of Martian yellow
cloud motions. Attention has naturally been directed
primarily to those sources richest in reports on Mars: for
example, Lowell Observatory Annals [SI; reports of the
Observatoires Jarry-Desloges [20]; reports of the Mars
Section of the British Astronomical Association; reports
of the Mars Commission of the French Astronomical
Society; W. H. Pickering’s [23] invaluable Monthly Re-
ports on lMars; and, more recent, the Mars reports by the
ALP0 in The Strolling Astronomer [2]. In addition, many
individual monographs, observntory reports, and pub-
lished articles dealing with the subject of Martian clouds
have been searched for reports of cloud motion.
43 5
436 MONTHLY WEATHER REVIEW Vol. 92, No. 10
Fcb ............................. Mar .............................
A4pr .............................
29-31 Mar .............................
14-15 Mav ............................
2-7 Aug ...............................
21-22 Aug .............................
3. OBSERVED MARTIAN YELLOW CLOUD MOTIONS
The results of this search for reported examples of
Martian yellow cloud motions appear in tables 1 and 2.
Table 1 includes reports of yellow clouds observed on the
disk o€ Mars, as a result of their obscuration of surface
details. Table 2 contains exttiiiples of cloud motions
determined from observations of limb and terminator
projections. The two groups have been separated be-
cause they are distinct from the observational standpoint;
as will be seen below, they seeni also to represent physi-
cally distinct phenomena on Mars. For clarity, in what
follows the clouds of table 1 will be referred to as “yellow
clouds” and those of table 2 as “projections”. 01 the
latter group only one example, the cloud of October
20-27, 1924, overlaps Hess’s collection and so the re-
mtlinder of these examples, 35 in number, are all additional
examples to the 1s he studied. The extreme rarity of
such cloud motion observations is evident from the fmt
that over the period of S7 years spanned by the observa-
tions, only 53 examples of cloud motion are found. This
means that their average hequency is little more than one
per opposition period, altliough actual occurrences tend
to be bunched around the perihelic oppositions, i.e. those
of 1892, 1909, 1924, 1939, and 1956. It might be thought
that this is an effect of more intensive scrutiny of Mars
during these favorable apparitions; but, against this, no
cxamples were found for the 1909 or 1939 oppositions,
although adjacent years are in each case represented by
cloucl drift occurrences.
Two additional interesting phenomena involving the
possibility of atmospheric niotion should also be noted,
nltliough they are not cloud drifts, a t least in the sanie
sense as the esaiiiplcs given in tables 1 and 2 . Dollfus
[7] inentioiis a cloud, invisible to the naked eye but de-
tectable by its polarization property, that drifted fro~ii
Margaritifer Sinus arid Miire Erythraeuin to the Sabaeus
Sinus-Hellespontus regions in several days, wliich he
observed in May 1952. Also, Pcttit and Richardson [22]
mention the fact that the striking W-shaped blue cloud
15/15 11/20 O/-20
Or30
01-40
-7O/-70
15/0
of June-July 1954 shifted in position, nioving slightly to
the southwest over a 30-day period, a t a speed of 0.34
mi. hr.-l This phenomenon occurred between longitudes
60” and 120”.
1926 25-28 OGt ...........................
...................... 1937 25-29 May--. 1941 12-28 Nov.-.- ......................
1943 3-5 Oct .............................
4. RELIABILITY OF YELLOW CLOUD DRIFT
ESTIMATIONS
......................... 31 July-1“Aug 10)-30 22-24 Feb 10/25
31 July-10 Aug ........................ 3O/-40 3-4 Aug ............................... O/-20
.............................
Before attempting to interpret the cloud drift observa-
tions it is quite iniportant to review certain aspects of
their background so as to give some indication of their
probable reliability. In the first place, nll the cloud
observations reported in tables 1 and 2 were mnde by
experienced observers of the planet. However., the
conditions of the observations differ considerably; and,
particularly, they have been interpreted as cloud drifts
in various ways, which should be clearly understood a t
the outset.
Some of the cloud observations have been interpreted
as cloud motions by the observer hiniself, wheretis others
have been so identified by the present writer. An incli-
cation of this is given in the tables. Naturally, iustmces
of cloud motion described by the original observer must
be given more credence. Cloud drifts are ordinarily
detected by noting displacements that occur on successive
nights, although i t sccms soinetiiiies to be possible to
estiiiiate motion during the course of a single night’s
observing. Naturally the question of whetlier one is
really viewing the sanie cloud in a displaced position, or
R different cloucl arises; the original observer cttn best
answer this question.
On the other hand ninny of the limb and terrninittor
projections reported, for example, by thc Observatories
Jarry-Desloges to have occurred on successivc nights
seem fairly certain to bc gcnuine cases of cloud motion
and I have so interpreted them. If a significant projec-
tion was notcd, and one night later another was reported
in a nearby location, thc displacenient has been attributed
to cloud motion. This procedure cicarly involves the
exercise of subjective judgment about what cloud drifts
are physically possiblc.
1954 2 June .............................. 1956 22-25 Bug ..........................
195G 28 Aug.4 Sept .....................
1958 12-15 oct ...........................
1961 19-21 Jan ...........................
TABLE I.-List of Martian yellow clouds that displayed motion
A I B I c -I -0 I E I F I o
13 Mar .............................. 41-10 29-31 May.. .......................... -421-38
13-14 Oct .............................. 40/40
2-6 June ............................... -38/10
29-31 July ............................. 201-25
24-25 May ..........................
27-29 July’ .........................
29-30 July- .........................
1911 11-18 oct ........................... 1911 13-14 Nov ..........................
1922 9-12 July ...........................
1924 9-10 Aue ...........................
11-12 15-16
20-21
I I l-
Aeria ..................................... Ccrberus-I to Elysiun. ................... Libya t o M. Ciiniiieriuiii ................. Libya to Eridania ......................... IIella%. ...................................
Margaritifer S. to Clirysc .................
Isidis R . to Libya ......................... Libya-Isidis to Ausonia. .................. Candor to Niliacus 1,- ....................
Libya to I’haethontis ..................... Libya to 1.Lesperia ......................... Ulysses to I’hoenicia L ....................
Argyre to Noachis ......................... Noachis to Sprtis Major ...................
Isidis R. to IIcsperia ...................... Casiusto Elysiuin ........................
W/lG .............. SSE/4 ............. WNW/12 ._______.
NW/13 ............
w/2o .............. SE/9 .............. “E/22 ........... NNIV/55 .......... SW/8 .............. N\V/10 ............ N\1’/25.. ..........
“W/- ..........
SlVIlS ............. N\V/24.- .......... Wj20 ..............
\I’ S \V/15 ..........
A Terrestrial date of observation. B Eciuivalent Martian Southern Ilemisplicre date.
C Beginning and cnding latitude (niinus indicates Southern Hemisphere). 1) Beginning and ending longitude.
E Region on Mars.
F
0 Reference.
Direction from which motion occurrcdpaveragc speed, m.p.h.; csliinatcs are inostly by the writer, based on rcportcd cloud positions, except where average direction and speed were given in the rcfcrcnce.
*Identified as a ease of cloud motion by the present writcr.
October 1964
1915-28-29 I)ec.* ........................ 1924-10-13 Oct .......................... 1924-20-27 Oct .......................... 1937-2-5 May ..........................
F. A. Gifford, Jr.
TABLE 2.-List of Martian projection clouds that displayed motion
27-28 Oct .............................. 25-26 June ............................ 30 June-5 July ........................
10-12 Feb .............................
43 7
A I R I C I I ’
1890-5-8 July ...........................
1894-25-26 Nov .........................
1900-7-8 Dee ........................... 1903-26-27 May- ....................... 1911--Fr7 Oct .* .......................... 1912-15- 16 .Ian.* ........................
1913-31 Dec.11 Jan.’. ................... 1914-2Fr26 Jan.*.-. .....................
1892-2.3 July ........................... 1892-11-13 July .........................
1914 28-29 Jan.*---. .................... 1014-2-3 Feb.*____ ...................... 1914-G-7 Feb.*t ......................... 1914-10-11 Feh: ........................ 1914-17-18 Beb.*t ....................... 1915-24-25 Jan.*t .......................
21-22 Mar ........................... 12-13 -4pr.. ._.......... ...............
19-POApr ............................. 10 Aiig ................................
23-24 Jan ..............................
18-19 Aug ............................. 23-24 Sept ............................. 7-8 Oct ................................ 20-21 o c t .............................. 21-22 oct .............................. 23 Oct ................................. 25 Oct ................................. 27 Oct ................................. 30-31 oet .............................. 20 oct .................................
24-25 Oct ..............................
~
40140 -501-50 -471-48 -321-23
-41-1 l812G -451-40
-401-45 -451-45 -451-45 -451-45
-151-15 -IO/-40 -451-45
-42!-42 -45!-40 -l O /l O -441-32 -401-40 loll0
~~
52/41
3351326 3441357
44/49 3361322 40132 2951255 3401300
65/50 10125 3471356 3331318 345/10 260/193
155/150 1651135
313/319
2871236
2851270
21oj190
D Beainnina and ending longitude. .~
E Region o n Mars. F Direction Irom which motionoCcurred/average speed,m.p.h.; estimates are mostly by
The Jarry-Dcsloges observers noted the position of
projections only approximately, by general references to
surface niarkings, and as a result it is doubtful whether
the inlcrred drift directions arc reliable to within less
than k 1 5 ” or the speeds to f 4 0 or 50 percent. The
remainder of the drifts reported i n tables 1 and 2 can
probably be assigncd, to be conservative, a reliability of
perhaps &IO” in dircction and f 2 5 percent in speed,
although thcse estimates are quite subjcctive. Drifts are
in all C ~S C S averages, computed Srom the initial and final
reported cloud positions, without rcgiwd to such interesting
day-to-day variations as havc occasionally been reported,
for ext~mplc, by de Vaucouleurs [6]. Most cloud positions
arc reported in terms of R4artian surface markings; and,
in the absence of other indications (such as an accom-
panying sketch, or a detailed dcscription), clouds have
been tissumcd to be located a t thc liititude and longitude
of the approsimatc center of the rcportcd area or marking,
as dcterniincd by referencc to the maps given by Sliphcr
[24], dc Vaucouleurs [6], and Antoniadi [I]. Most of the
tabulatcd lntitudcs and longitudcs havc becn rounded off
to the nenrcst 5” so as to rcAcct the probable accuracy
involvcd, although in a few cascs thc available information
has warranted a more precise position indication.
Thc Martian dates arc computcd, following custom,
according to an cquivalcnt Martian calcndar whose
southcrn hcniispherc vernal equinox occurs on March 20.
In practice this date is Sound by dctcrmining, from an
ephcmcris Sor the observation datc, the longitude of the
sun as seen from Mars (the planctoccntric longitude of thc
sun), tidding or subtracting 180 O, and finding the nearest
terrestrial date having an equal solar longitude; this is the
equivalent htnrtian southcrn hemisphere date.
5. COMPOSITION OF MARTIAN YELLOW CLOUDS*
Mars’ surIt’tecc consists of large dark areas, the “maria”,
Tempe .................................... W/13 .............. Hellespontus .............................. W/18 .............. Noachis ................................... E/7
Protei ..................................... SEI13 ............. Sabaeus to Icarium ....................... WSWj27 .......... Chryse .................................... SW/15* ...........
Hellas to Ausonia ......................... W-I77 .............. Noachis to Hellos. ........................ W148 .............. Argyre .................................... WIlG .............. Argyre .................................... E/l4 .............. Argyre .................................... El10 ............... Deucalionis R ............................. WI38 .............. Sahaeus S. to Areyre ...................... NE155 ............ Hcllas to Eridania ........................ W/85 .............. P haethontis ............................... W/5 ...............
Phaethontis to Icaria ...................... W133.- ............ Aeolis to Elysium ......................... SEI30 ............. Yaonis to Hellas .......................... SI5 ................ Hellas ..................................... W/8 ...............
Syrtis Major to Isidis ..................... W(6 ...............
...............
the writer, based on reported cloud positions, cxcept where average direction and s p e d were given in the reference.
‘Identified as a case of cloud motion by the present writer. tdudged by the writer B doubtful cloud motion case because of limitations of the
Q Reference.
observed data.
superimposed on an cvcn more extensive bright colored
background, which is usually refcrred to as "desert". In
fact the available evidence, chicfly Dollfus’ [7] polarization
studies, indicates that thesc dcscrt areas are composed ol‘
fincly divided mineral material, possibly limonite. The
occasional obscuration oE surface details by what must be
dust storms, and thc gencral kick of moisture on thc planet,
iilso support the desert hypothesis.
Taking all the evidence, both direct and indirect, into
account, it is reasonable to suppose that the Marf,itin
deserts are in fact broad expanses of finely divided mineral
inaterial that might as well bc called said (although its
composition may differ from the terrestrial variety).
This RiIartian sand must have been produced by vtLrious
weathering processes, just as is the terrestrial kind, except
thiit tlie action of flowing water I I ~S not been involved to
any extent, at least recently. Without attempting t o
speculate about the details, it seenis also fair to twunie
that these weathering processes shoulcl produce :L rilngc
of sizes of sand grains. According to Bagiiolcl [3], in-
diviclual grains of sand lying on tlie ground are acted upon
by two forces. The wind blowing over them escrts a
drag force which is proportiorid to the cross-scctionnl
area of the grain, and to tho square of tlie “friction
velocity”, ?I*, which is a cliaracteristic velocit,y assocititecl
with the turbulent air flow, of llie form
Ppv2d2;
p is sir density, d is the particle diameter, and P is sonic
constant. The tendency of this ixir drag force to tuiiible
the grains is opposed by n verticiil force, the resultiirit of
gravitation and buoyancy, equd to
*Conclusions similar in many respects to rcsults of this scetion werc also rcadicd (iiidc-
pentlcntly) In the very interesting study by J. A. Ryan “Notes on the Maitian Ycllow
Clouds, Part I (Prchmmary Copy)”, Enginccring Paprr No. 1990, Douglas Airu‘alt Co ,
Santa Monica, Callf., 1964, 24 pp. (rnimco.)
438 MONTHLY WEATHER REVIEW
- v* u(z)=-ln (z/z,) k
Vol. 92, No. 10
where g is the gravitational acceleration and u is sand
density. When the moments associrhed with these forces
are just equal, a threshold value of v* is defined that is
just sufficient to cause sand grains of diameter d to move.
Equating the moments, it develops that
where z is height above ground, zo is a measure of' the sur-
face roughness, and k=0.4 is von Karman's universal
constant. This will amount to winds of 1 or 2 m. set.-'
at heights of a meter. For higher wind speeds, larger
sand grains begin to move, up to an ultimate value deter-
mined by the size ol the largest grains prcscnt. Sand
grains become airborne and progress downwind in bounc-
ing trajectories, a process known as "saltation". As .*=A[(?) yd]" (1)
where A, a constant, involves in addition to constants
already introduced, the (approximately constant) angle
between the vertical and the line joining adjacent grain
centers, because the nionients depend on this angle.
Now it is known from careful experimentation that the
plienonienon of turbulent clrag on sand grains is associated
with a threshold value of the Reynolds number, Re,
equal to about 3.5. For snialler values, vortex shedding
stops and the flow around individual grains is laminar.
Thus we can also mite
Re =v,d/v = 3.5, (2 )
where v is the air's kineinntic viscosity. Combining
equations (1) and (2 ) we find a threshold sand grain
diameter, d*, the snidest for which movement occurs :
(3)
p being negligible coniparecl with U.
The dynamic viscosity, p=pv, depends only slightly on
pressure and temperature and so it follows, using sub-
scripts M for Mars and E for Earth, that, very closely,
(4)
Martian gravitation is about 40 percent of Earth's; the
exact value of the Martian surface pressure is at the
moment subject to some debate, but it probably lies
between 1 and 10 percent of Earth's. Consequently we
find from equation (4) that the critical diameter for sand
motion on Mars is between 3 and 7 times larger than on
Earth.
From equation (1) it also follows that
(5)
from which it can be concluded that the critical friction
velocity for sand movement on Mars is between approxi-
iniitely 3 and 15 times that on Earth.
At the Earth's surface the threshold diameter for sand
grain motion is about 0.1 mm., corresponding to a value
of V~ of about 15 cm. see.-' The corresponding wind
speed for sand motion, ii,(z), is given by Prandtl's well
known logarithmic velocity profile law,
descending grains hit and rebound, sometimes dislodging
others, more and more sand grains become involved in the
motion up to a fixed quantity, governed by the capacity
of the sand to absorb momentum from the air. This
amount naturitlly depends on wind speed, and the quan-
tity of sand driven by a given wind can be calculated.
I t seems likely that thc yellow clouds of table 1, which
obscure the surface fcaturcs of Mars, are initiated by the
process just described. The wind speeds required to
initiate sand motion on RIiirs, even if according to equa-
tion (5) they are considerably greater than on Earth, are
comparable with the drift velocities of the yellow clouds
of table 1, which avcragc 1 s mi. hr.-l (8 m. sec.-l>. To
show this we conclude first, from equation (6) that
(7)
Over a desert the roughness length, z,, is a measure of the
size of the sand grains that make up the desert surface.
On Earth a representative value of the roughness length
over a level desert is 0.03 cm., according to Pasquill 1211.
By equation (4) we might estimate that on Mars a cor-
responding value would be 0.3 cm. Assuming further
that the driving sand motion takes place very near the
surface, at or below itbout z =I m., (this is what happens
on Earth) we find that
Thus, corresponding to ratios of v*,,Ju*~ of 3 to 15, we
find that iiM/iiE equals about 2 to 10. The critical values
of ii for sand motion on Earth being about 1 or 2 m.
set.-' a t z =l m., the observed average Martian yellow
cloud drift of S to 10 In. set.-' could certainly correspond
to the velocity of low-level, wind-driven sand.
The initiation of sand motion within a few meters of
the Martian surface involves particles having diameters
somewhat Iarger than on Earth, perhaps 0.5 to 1 mm.,
according to equation (4). This will be accompanied,
of course, by the raising of finer grains of dust to much
greater elevations. Dust will remain suspended in an
atmosphere for long periods when its settling speed is
smaller than the vertical wind fluctuations due to turbu-
lence. The actual numerical values suggested in the
above analysis are somewhat tentative, but there appears
October 1964 F. A. Gifford, Jr. 439
240°-%90 ..............................................
250'-259'. .............................................................
Z1X~-269~. .............................................
270°-2790.. ............................................................
280°-289'.
2900-2990 ..............................................
.............................................
to be little reason to suspect the general nature of the
Thus we can formulate a picture of the Martian yellow
TABLE 3.--Frequency of average seasonal surface tenaperature of the place of origin of moving clouds results apart from the possibility of minor adjustments.
clouds as being composed of wind-driven sand grains
Temperature (O K .)
_____
6 10
30
12 10
40 37 10
44 ................
moving by saltation within a few meters of the Martian
surface, accompanied by an overlying dust cloud, of
much smaller particles, extending, perhaps, to many
thousands of meters. The composition of the Martian
limb and terminator projections of table 2 must be much
different from this. It seems probable that these clouds
are, in part, aqueous condensations. Since they are
reported to extend to very great elevations, over 50 km.
a t times, and have on occasion been observed as completely
detached from the Martian surface, they are evidently
quite different in nature from the low-lying yellow clouds. Original latitude (N or S)
6. LOCATION OF MOVING CLOUDS
Yellow clouds Projections
(percent) (percent)
In table 3 the frequency of origin of the moving clouds
is presented as a function of surface temperature a t the
location of their origin, as determined from tables 1 and 2,
and the temperature values presented by Gifford [12].
There seems to be a preference for the low-level yellow
clouds to originate in regions of higher surface temperature
than do the projections. This is consistent with the
hypothesis that the former are desert sand and dust
storms. On the other hand it is also true that tlie warm
regions are in general the ones most easily observed from
the earth. In table 4, the frequency of occurrence of the
moving cloud origins is given as a function of latitude; and
in table 5 the same information for the terminal position of
the moving clouds appears. These tabulations indicate
clearly that the projections are mid-latitude phenomena,
whereas the yellow clouds are primarily found in lower
Martian latitudes.
Summarizing these positional aspects, we can conclude
that the moving yellow clouds tend to form in low lati-
tudes; near the thermal equator. The projections, on the
other hand, occur primarily in the middle latitudes and
their motion does not show any clear latitudinal
component.
- >IN0 ..................................................
500-590. ...............................................................
4Oo49O ................................................
3O0-3Yo ................................................
200-290--.. ............................................
10~-190 ................................................
............................................. 00- YO..-
7. MOVING CLOUDS AND MARTIAN CIRCULATION
PATTERNS
Hess [14] showed a possible streamline map of Mars
based on cloud mo\-ements determined from projection
observations, for the southern hemisphere summer. The
projection data of table 2 extend the observational
material to all other Martian seasons, but there are too
few cases in each season to permit an attempt to draw
maps similar to Hess's.
The yellow cloud motions of table 1 exhibit a tendency
that msty well be related to a Martian circulation pattern;
namely they tend, generally speaking, to drift equator-
word, or a t least toward the position of the thermal
equator. If the yellow clouds are analogous to terrestrial
G
5
6 65
19 5
6 ................
31 25 31 ................
................
_______
(16 cases) (20 cases)
Final latitude (N or S) Ycllow clouds Projections
(percent) (pcrccnt)
>so? .................................................
50'-59'. ...............................................................
40'49'..-.- ...........................................
30'-39'. ...............................................
20"~29 O.. ..............................................
10~-190 ................................................
-
................................................ 00- 90
G 5
5
12 50
25 10
31 10
19 15 6 5
(16 cascs) (20 cilses)
_ _____
cyclonic storms, this behavior presents a curious problem
since terrestrial low-latitude cyclones tend to move pole-
ward. Two possible alternative explanations have sug-
gested themselves. Perhaps the yellow clouds tire
associated with polar outbreaks, similar to "northers".
This would explain their direction of motion witl-iiii the
framework of a terrestrial analogy. On the other hand
the following ideas mxy apply.
While this report was being written, the important
Mars studies by Miyamoto [IS] were received. In these
he identifies four drifts observed since 1956. One is a
yellow cloud that appeared over the Neith-Casius regions,
moving to Elysium, around December 10, 1962 (planeto-
centric solar longitude equals 21 ") , which exhibited a.
drift from the west quite like that of the 1961 example of
table 1. A second yellow cloud drift is reported as having
occurred between January 29 :Lnd February 7, 1963, from
Noachis and Sabaeus Sinus across the equator to the
Aeria-Neith regions, from a southwesterly direction.
At approximately this time a large, persistent, bright
cloud appeared over the Tempe-Arcadia-Propontis regions.
440 MONTHLY WEATHER REVIEW Vol. 92, No. 10
This complex formation developed from the east according
to Miyamoto, as judged by its position a t the successive
appetwances of these longitudes until April. The fourth
cloud is the well known great storm of 1956. The general
development, during August to September 1956, of this
tremendous, planet-wide disturbance has also been widely
interpreted as progressing from east to west, although in
table 1 the present writer has indicated that individual
yellow clouds or storms forming part of this development
moved from the west, following the argument by Heintz
[13], who based his conclusions as nearly as possible on
day-to-day observations. There may in fact be no s e a t
inconsistency between these viewpoints. The motions of
individual storm systems lasting for a few days need not
necessarily be the same as the general development of a
hemispheric disturbance that persists for over a month,
even though the two may be dynamically related.
Miyamoto has suggested that such storms as these two
great disturbances may be related to the breakdown of a
symmetric regime of the general circulation of Mars into
a wave regime, as predicted theoretically by Mintz [17].
Gifford [Ill pointed out that the yellow clouds, such as
those documented in table 1, are generally speaking too
small to be related to large-scale baroclinic wave in-
stability. Their generally equatorward drift may imply
that they are steered by the tropical portion of a sym-
metric general circulation cell. On the other hand both
the 1956 storm and the great Tempe-Arcadia development
of 1963 are large enough to correspond to the dominant
wave number on Mars, which was calculated by h4intz
to equal 3.
ACKNOWLEDGMENTS
I am very grateful t o the Lowell Observatory for providing
quarters and making their invaluable library facilities availablc to
me during thc litcraturc search reported hcre, and for the help
and cncouragemcnt provided by members of the staff there. Like-
wise I wish t o express appreciation t o the Jet Propulsion Labora-
tory for supporting, and to the U.S. Weather Bureau for granting
me leave from my duties during this work.
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1963, pp. 610-612.
1911,393 pp.
[Received A p r i l 10, 1964; revised July 2, 196.4]