APRIL, 1904 166 MONTHLY WEATEER REVIEW.
STUDIES ON THE CIRCULATION OF THE ATMOSPHERES OF THE SUN AND OF THE EARTH.
By Prof. FRANK H. BIGELOW.
V.-RESULTS OF THE NEPHOSCOPE OBSERVATIONS IN THE
WEST INDIES DURING THE TEARS 1899-1903.
METEODS OF OBSERVATION AND REDUCTION.
The observers of the United States Weather Bureau OCCII-
pied eleven stations in the West Indies during the years 1899-
1903, and the opportunity was utilized to make a survey of the
motions of the atmosphere in that region of the Tropics by
means of nephoscopes.
The instruments were of the Marvin pattern, and the metliocl
of observation, to obtain the aziniuth and velocity of motion,
was identical with that described in t8he Report of the Chief of
the Weather Bureau, 1S9S-99, T’ol. 11, chapter 3. The reduc-
tions were, however, carried out more perfectly than in any
previous research of the kind in the following manner: (1)
The three readings at each observation were reduced to a mean
azimuth and velocity, for entry on the coinputing sheets. (3)
Each vector V, q was separated into its rectangular conipon-
ents, + S, + E, where S is positive southward on the meridian,
and E is positive eastward on the parallel of latitude. (3)
The algebraic sum of each set of components was then taken
from month to month. a d the groups for corresponding
months during the years 1699-1903 combined. (4) These sums
were divided by the number of observations to obtain the
mean velocity component per observation. The observations
were taken for two years in the afternoon hours, and for the
other two years in the forenoon hours, so that the diurnal
variation was practically eliminated by putting the entire data
for each of the twelve months in a single summation. (5 ) These mean ordinates were plotted month by month for the
nine cloud levels, and average curves mere drawn through
them. I n spite of the fact that the number of observations is
large, when taken as a whole, yet, when subdivided among so
many strata, there is more scattering of the orcliiiate points in
certain levels than is desirable. The average curves approxi-
mate the normal components which would be derived from a
very much more extensive series of Observations. The full
report will contain the observed ordinates and those obtained
by graphical adjustment. (G ) The resultant polar coordinates
1; ~p were then computed from the rectangular coordinates.
Up to this point the numerical data hac1 been carried forward
in the number of millimeters passed over in twenty-five sec-
onds on the scale of the nephoscope. (7) These numbers were
now reduced to velocities in meters per second, by multiply-
ing them by the factor 6 H, where His the adopted height of
the cloucl stratum. The values of H were adopted from the
Washington cloud heights, after considering the heights ob-
tained a t Manila during the same cloud year, 1696-97. These
velocities and azimuths were transferred to charts, drawu to
the scale, 1 millimeter = 1 meter per second. I n preparing
these charts for publication in the MONTHLY WEATHER REVIEW,
the original scale has been reduced to 0.6 niilliineter = 1 meter
per second, as shown by the “velocity scale in meters per
second” a t the bottom of each chart of the sets Chart SI1 and
S I I I , the scale of Chart SIV being unchanged.
CHARTS OF THE RESULTING VELOCITIES AND DIRECTIONS OF MOTION
FOR THE WEST INDIES.
The vectors ( ?< p) have been plotted for each station, so that
the mutual relations of the resulting motions can be properly
compared and studied. There are several remarks to be made
about the observations themselves. A t Willemstad the ob-
servers hare frequently misnamed the r i t n i tilira clouds as C U M ulo-
stratrrs. This becomes apparent on plotting the vectorb. The
angles are correct, but the length of the arrows is too great.
I have accordingly interpreted the values of TT, and P under
strato-cumulus as belonging to the cumulus level, and have
used the reduction factor 0.5 instead of 0.9 in drawing the
charts.
At Bridgetown the vector systems of the alto-stratus and
t,he cirro-cumulus levels have apparently been interchanged.
As they now stand a t Bridgetown they are inconsistent with
the flow of air as determined a t Basseterre, Roseau, Port of
Spain, and Willemstad; but if they are transposed, then there
is harmony. The observation sheets indicate that the ob-
servers hare an unusually large number of cirro-cumulus en-
tries and comparatively few alto-stratus, so that apparently
they were accustomed to name many alto-stratus c l o d s as
cirro-cumulus clouds. It is not easy to secure identical esti-
mates of cloud forms a t so many independent stations as we
have used, and these fern instances of apparent discrepancies
are gratifying evidence of the general excellence of the results
in other respects.
On comparing the charts here presented with those pub-
lished by Prof. H. H. Hildebrandsson for the international
committee, it appe,ars that he has computed only the angle of
the azimuth of motion without the velocity, and that the same
fieliematic velocity is entered throughout the year. Apparently
the actual velocities have not been computed for that report
a t any station. The importance of having the velocity as well
as the direction of motion is e&~ent, and this is emphasized
by esainining the great variations between the summer and
the minter velocities and between those at the different cloud
leiels of each stsation of the West Indies. Comparing the
Hildebrandsson and the Bigelom results for Havana (Plate
VII, International Report), with fig. 22, Chart SI1 A, the azi-
muth directions are not in agreement in the autumn; com-
paring those for the llntilles (Plate 111, International Report),
with figs. 28, 29, 30, Charts S I I B , and S I I C , Basseterre,
Roseau, and Bridgetown, the legend ‘‘ Nuages superieurs ”
should apparently be changed to ‘‘ Nuages intermediaries” or
“ Nuages inferieurs.”
The vectors of the West Indian stations have been plotted
in three forms: The first, Charts SI1 A, SI1 B, SI1 C, figs. 23
to 33, wherein the vectors of the same month throughout the
nine levels terminate on the same vertical line; the second,
Charts SI11 A, SI11 B, SI11 C, figs, 33 to 43, wherein the vec-
tors for June terminate on the same vertical line ancl the
ot,hers in succession, so as to form a continuous broken line; the
third, Charts S I V A, S I V B, S I V C‘, figs. 44 to 61, showing ap-
proximate normal vectors for winter and summer. The first
en:tbles us to study the nioveuents simultaneously occurring
in a given month from the surface wind to the cirrus level, and
from this many important conclusions can be drawn. The
second makes more distinct the general course of the move-
ment in the several strata throughout the year, and especially
the nature of the currents that depend upon the forces
producing the westward drift of the lower levels and the
eastward drift of the upper levels, together with the tran-
sition levels between them. The third system of charts
is a composite of the mean winter and the mean summer
systems, respectively, some of the minor irregularities being
rectified in the adopted vectors. January and February con-
stitute the middle of the winter group, while July and Au-
gust are a t the miclclle of the summer group. I n adopting
these vectors regard \vas hac1 to the most probable balanced
system which is indicated by the entire set of vectors. If the
reader has doubts as to the accuracy of these final results, the
original material of the third set of charts is to be found in
the first and second sets, or in tables from which all the charts
have been plotted, which will appear in the full report. Nu-
merous studies in the dynamic meteorology of the Tropics
are now practicable for the first time, but as it will require-
much careful labor to execute them, only some general re-
marks are required in this place.
APRIL, 1904. MONTHLY WEATHER REKIEW. 167
THE ARCH SPANNINCi THE TROPICS, WHICH DIVIDES THE EASTWARD DRIFT
FROM THE WESTWARD DRIFT OF THE GENERAL CIRCULATION.
The general theory of the circulation of the atmosphere
shows that in the teinperete zones of the Northern and the
Southern hemispheres there is a strong prevniling eartmsrd coin-
ponent producing :tu eastward drift, while in the Tropics there
is a prevailing westward component canring a westward drift.
The tropical westward drift is, however, limited in altitucle, and
a t a certain elevation the drift reverses from a westward to an
eastward direction. The position of the curve which separates
the eastrvarcl from the westward elrift, varies with the 'ieason of
the year. When the stin is far to the south ancl the northern
winter prevails, the arched curve must be skewetl to tlie sontli,
and when the siin is far to the north nuel tho northern suiii-
iiier prernils the arc11 ib sliewetl towarc1 the north. This is
on the assuiliption that tlie foot of tlienroli rests on iienrly the
snnie latitude in winter niill huiuiiier a t aiip given region of
tlie earth. The high pressure belt, which fixes tlie positicin
of the arch on the surface of the enrtli, f u r the enstern p r t i o u
of the United States lie8 soiiirJwhere lwtneen +3U" an(1 +85"
north latitucle, and crosses the ltlnntic coast at :tl,ont Floridla
and South C:trolin:t. I t is de\ir;Lble to cleteriniiie fro111 our 01)-
servations itr exact lvc:ztion, but ar this is of sulJorclinate iiii-
portance for our iininecliate purpose i t can be passecl over iii
this coiinection. Further acconnt,s of t81ie iiintheninticd sig-
nificance of the tropicd clihcontinuous surface Letween the
prevailing eastmarc1 niicl
consulting the full rrl)ort, aiid 111)- 1)aper iii tlie MIJNTHI.P
WE~THER R E V I E ~, .Jniiuary, 1904. T'ol. SSSII, 1). 15.
One special piirlrose o f the TT'eht I d i n n nephoscope survey
for lfi!)!)-lL~o:? was to cleteruiiiie tlii.; surface of separation ill
tlie higher levels, and its rnri:ttiou with the heitson of the )em.
The velocities ancl direction (i f iiiotioii on either sicle of i t , nud
the nunieroiis inetroro1o::ic~tl iiifereniw tlint c m i br c1r:i~vii
from these coiiclitioiis, iiinile the work of iwiiiinry i1uIiort:ziic.e
in the (lei elopiiieut of tlie tlynniiiics of the atiiiohpliere. The
series of C'ltarts, SI1 A to XIV C ' , f i g h . 22 to G1. are iiow n\nil-
alble for this purpohe. The twelve iiiontlis. i~.; o1)servetl. iiiny
be tnlien in twci groups, of uliicli the winter grouli i:, <lis-
trilJutec1 allout Januttry :iii(l Felirtmrp as the centrnl uioiiths,
anel the suiiinier group nlwut July a i d August. If tliere hail
been siinultaneons o1)servntiou~ in tlie Southern Heiuisphere
nt latitudes correspoucliiig t o tliose that were occupiecl 1)y tlie
West Inclinii statioiis iii the Northerii Heiiiisl)liere, we slioulcl
then have the data applying siiuultaueonsly to the entire tropic
zone. The same result cnn lw closely aplnwxiiuntetl by treating
the winter group as represeutinq tlie iinrtli tropicid zoue, aiitl
the suiiiiuer group zzs representing the south troliical zone,
eltiring the northern 11 iiiter. Heiicae, lry iisiiig the rrsnlth from
Nc weiiiber to April in the iiortliern lntitndes, and :t4hiiiiiiiig
that duriiig this 1)eriod the conditions ol,serve;L for the iiorth-
eru latitudes during May to ()ctolwr then prevailetl in the
southern lst8itudes, tlie spc*lironocs notioil of the oiroiilatioa
call be foiincl for the northern winter tlirooghout the Tropic?r.
By reversing this process the correhponcliug results for the
northern sitinmer are o1)tniiied.
An inspertion of tlie vectors of inotioii in tlie winter i~~oiitlis
sho~rs that) for Havana, Cienfnegos, niitl Santiago, Culm. with
mean latitncle W", tlie revers:cl is about mielway lJetn eel1 the
mmulo-stmtub and alto-c*uiiiuliis levels, u r a t appi*o\ini:ttely
3500 meters alwve the sen level. At I<ing.ston, Santo Doiningo,
San Juan, Basseterre, Roseau, and Bridgetown, 1i:zving tlie
mean latitnde of 1TQ, tlie reversal is appnrently in or a1)ov.e
the alto-stratus level, or about 6400 meter\ elevation. At
Willeinstacl and Port of Sp:ziii, nith iiieaii latitude of 12'. tlie
reversit1 is in tlir cirro-cniiiulus level, a t alwut SUO0 meters
elevation. This is slic )wii oii t81ie iitrrtherii wiiitrr brniich o f
fig. 62, '' Mean nltitucles a t which tlie we\tward drift reverses
to the eastward drift in the Tropics."
estwmd drifts may be fouucl
23- a
FIG. G2.--hIenu :tltitudils at which tlie westward drift reverses to the
ra~twarrl drift in the Trollies.
AII e x m h t t i o n of the rectors for tlie miiinier group of
i i i m t l i h intlicates tliat, geiiemlly tliew is a teiiileucy to rever-
sal in tlie cirrur aut1 cirro-stratus levels, nt about 10,000 meters
ele1ntion. This. however, is iiot definitive, and I have iudi-
c t z t d the prulx~1)le top of the nrcli in a brolieii line rather than
to lw4tilely assign the limit. The sui~iiuer westward circula-
tion is ~e r y feeljle tliroiighnut the coluiun from tlie Hea Ievel at
least to the lO.OU()-iueter leJel, nncl it niny be that it extends
even to n higher lerel over tlie effective therim1 equator, which
l n ~h nliout forty-five &ys beliiiicl the position of the sun in
1:ttittde. 111 tlir very ccmp1e.r circulation actuallg existing in
the Tropics it, is iiot pos5ible to iuake very exact statements
reg:zrcling such :t loosely defined 1)ouiiclary as that which
renlly :,sixmites tlie eastward and westward currents of the
atmohphere uiideriieath the suii. The calms of the tliermal
eclmtor move northward and southward with the sun, ancl ap-
1xzreiitly the westwnrd drift extends ulnvard uiilwoken to about
six miles. though tzt that level there are evidences that the
en.;twarel drift is iualiing itself felt. Tlie east and west cur-
rents play against each other a t these high levels in a some-
whnt irregular iiimiier.
THE LETEL\ OF MAYIMtIM HI )RIZONTAL YE1,OPITY.
,in esaiiiination of Cliarts SI11 A, SI11 B, SI11 C, figs. 33 to
43, Ibriiigy out clearly the levels of the maxiinurn hurizontsl cur-
rents. Tlie circulation generally increases in westward veloc-
ity froin the surface to the strato-cumulus level, then falls off
to a iuiiiiiuum in the alto-stratus level, wliere the direction is
irregular, nnd then increases to a masiiiiuiu eastward velocity
in the cirrus level. The stations of Cienfuegos and Santiago,
C'U~J:~, together with Kingston anel Santo Doiningo, give rela-
tively small resultnut velocities in the lower levels, from
wind to C I I I ~I I I U ~, as compared with Havana, San Juan, and the
southeastern group, Basseterre, Rosenu, Bridgetown, Port of
Spain, and Willeinstacl. Tlie former group nf stations develop
greater irregularity in their azimuth directions than do the
latter group, and consequeutlg their resultmts are much
c1i~niriislietl in niagiiitude. This probably iii(1icates some ac-
tion of the continental inass of North America in disturbing
the nrehtwtrd drift, which prevails steadily in the Windnmd
Islnncls, and in changing its general direction from the south-
east, which is the natural flow from the trades, to the northeast
in that region. The strato-cnmulus level in the more eastern
statioiis has a powerful westward current, which falls off cle-
cideclly in the vicinity of Cuba. Oii the other hand the north-
168 MON!CHLY WEATHER REVIEW. APRIL, 1904
eastward or eastward velocity i n the three upper levels-cirro-
cumulus, cirro-stratus, and cirrus-is a t a maximum over the
Cuban stations, and tends to diminish toward the southeast;
that is, from the Antilles to Port of Spain. The trade from the
southeast holds quite uniformly in the lower levels of the east-
ern group of stations, ancl the northeastward upper trade pre-
vails in the upper levels of the western group. Between the
lower and the upper levels there is a region of transition whose
nature is quite clearly indicated At Port of Spain the south-
east trade prevails throughout the gear with maximum in the
cumulo-stratus lerel, diminishing and partially reversing in
the cirro-stratus and cirrus levels. At Willernstad, Briclge-
town, and Roseau there is an exclusively northern component
in the alto-cumulus and alto-stratus levels. At Basseterre and
San Juan this component becomes northwestmard or shows
signs of rerersing. g t Kingston ancl Sailtiago the azimuths
are irregular and the velocities sniall, and a t Cienfugos ancl
Havpa the eastward drift practically dominates. Beyond
these statements it is not very safe to go a t present. The cir-
culation is complex and depends largely upon the relation of
the Atlantic high area, belonging to the general high pres-
sure belt of the Northern Henlisphere, to tlie adjacent conti-
nents. The normal system beems to Le like that of TVillemstacl,
Bridgetown as corrected, and Roseau, where the movements
from the southeast in the lower levels change to movements
from the south in the middle levels ant1 from the southwest in
the higher levels. The southwest antitrades are conspiciimis
over Cuba, but they become west and even northwest anti-
trades over Ban Juan ant1 the Ifrindward Islands. One is sur-
prised to find that the southeast trades prevail so steadily in the
northern zone, winter and bummer, even up to latitude+17°,
including Port of Spain, Tf’illemstacl, Bridgetown, Roseau, Bab-
seterre, and San Juan. It is eviclently of priiuarg importance
that meteorologists should extent1 this nephoscope surrey to
the Azores, Ascension Island, St. Helena, the South Americaii
Continent, Central America, and Mexico. The entire circula-
tion of the atmobphere can thus lie carefully cleterminecl by
means of the method here illustrated.
TEE WINTER AND TEE SUMMER CIRC‘ULATII )NS.
At a glance the great contrast between the motions of the
air in winter ancl summer is apparent. Over Havana the
northeastward velocities are above 30 meters per second in
winter, and in the summer they become about five meters per
second and are directed westward. The sumiiier vectors on
Chart SI11 A, SI11 B, and SI11 C1 form an irregular broken
line which in some cases become a very good loop. This loop-
ing or tangle in the line is quite cha:acteristic of the suniiner
vectors in the middle levels a t Havana, Cienfuegos, Santiago,
Kingston, Sen Juan, Basseterre, and of the vectors in the
high levels a t Roseau, Bridgetown, Port of Spain, and Willem-
stad. These loops ancl tangles sometimes make it clificult to
determine from the original observations what is the true
mean curve to be drawn, because the ordinates are quite scat-
tering ancl irregular in the middle cloud lerels. I n the lower
and higher leyels there was little clifticnlty experienced froin
this cause in drawing the mean lines. As a general principle
none of the broad statements which meteorologists have been
accustomed to make regarding the trade-wind system seeins
to hold over a very large region. The changes froin one lo-
cality to another are numerous ancl iiq~ortant, showing that
the circulation of the Tropics is really very much localized.
There exists no system of cyclones and anticyclones to dis-
turb the general circulation, but this circulation is itself much
more complicated than it is in the temperate zone. The
dynamics of the two systems are very different, and depend
upon a complex distribution of temperatures and pressures.
Every effort should be macle to determine what these are be-
fore resorting to analytic discussions, which will be of little
permanent value until a11 the principal facts are known.
THE CAUSE OF TEE WEGT INDIAN HURRICANES.
The hurricanes which devastate the southeastern districts of
the United States in the months from July to October originate
in the West Indian region, ancl, as much conjectural writing
has been published in order to account for them, it is import-
ant to throw what light is possible upon the subject. In the
complex circulation shown to exist over the Carribertn Sea, it
is easy to suppose that gyratory local circulations can be set
up which will develop into cyclonic: action. The summer cir-
culation is irregular, as befits a belt of calms such as prevail
in the ~l~~lclrii~ns, or it has a feeble westward direction. I n
the minter tliis inotion has become powerfully eastward in the
upper levels. in cousequence of the overspreading of the cold
sheet of air from the temperate zone. which is controlled by
the eastward drift of higher latitudes. In the autumn, espe-
cially in September. a niarked cliange takes place, by which
t,he stagnaiit or mestward moving air is sliarply propelled east-
ward. This ib seen 1 ~y examining the months of August, Sep-
tember, and October, in the alto-stratus and the higher lei els
on C’liarts SI11 A, SI11 B, SI11 C’, figs. 33, 34, 35, 36, 39, 40,
and 41, for Havana, C‘ienfuegos, Santiago, Sail Juan, Basseterre,
Roseau, and Bridgetown. This indicates the locality where
hurricanes are especially generated, and agrees with otherwise
well known facts. They seldom occur as far south as Port of
Spain and Willemstad, but a t these stations there is no indi-
cation of a sharp rerersal in the cloud region. Tlw Irwl.s~fi.oi)c
dto-sti*ntiis to virm fr0t)l .f i J /C l . f f ) h / I’ ))lib’.V hi!///, UrP f h f J S P f’/iIt.‘f!!/
do not have the same reversal currents, but their \ectors are
very steadily directed from the southeast to the northwest
throughout this season of the year. A hurricane is built up
on esactly the smile mechaiiical principle as a tornado, namely,
by the conflict of two currents flowing together from differ-
ent directions ancl having different temperatures, only the
hurricane is much deeper than the tornatlo. the hurricane
forming a tube from four to six miles long. while tlie tornado
tube seldom exceeds one iiiile in lengtli. I n the tornatloes of
the United States the cool wind froin the northweht, flows
against and over the warm current from the houtli or south-
east as they meet in the central valleys. Between theu, a t the
height of about one mile, a vortex tube is formed. mliich, by
its gyratory action, extends clomnm arc1 through the lower
strata, which latter must be in a more or less quiescent state
or else drifting slowly forward from top to Lottom. I n the
case of the hurricane in the high levels we have the cool east-
ward drift of autumn strengthening ancl spreading into the
tropic zone, with a northeastward or eastward current, as sho\vn
a t Havana, C‘ienfuegos, Santiago, San Juan, Basseterre, Roseau,
and Bridgetown, Charts SI11 A, SI11 B, and SI11 C!. This
ineets the southeast trade with currents moving northwest-
ward, as shown a t Willemstacl and Port of Spain, Charts SI11 C,
figs. 42 ancl -13, and between them R gyration is set up which
penetrates downwarcl four to six miles, and SO lxoduces a vor-
tex tiibe of large dimensions and great power a t the surface,
such as hurricanes exhibit. This conclusion is an esact agree-
ment with tlie results obtained in the Report of the Chief of
the Weather Bureau, 189s-99, vol. 2, as given on chart 35 for
the tropical hurricane and described 011 page -157. It was
there shown that the rectors of motion in the cirrus level re-
quire the existence of a vortex tube a t least five miles long.
The usual drift of hurricanes froin the place of generation i R
a t first westward or northwestmard, and this is because they
are carried along with the prevailing currents in the lower and
middle levels. It seems then that hurricanes build up in the
higher levels by the counterflow of currents there prevailing,
that they penetrate through four to six miles of lower strata
to the surface, and are borne along westward by entanglement
in the lower currents through which they penetrate. When
these change their direction to the northward and northeast-
APRIL, 1904. MONTHLY WEATHER REVIEW. 169
ward the hurricane track recurves with them. On the other
hand the hurricane itself disappears in higher latitudes and
is transformed into a shallow cyclone, because there the coun-
tercurrent flow in the higher levels ceases. These conclusions
can be further illustrated by reference to Charts S I V A, S I V B,
and S I V C, mentioned above.
APPROSIMATE NORMAL CIRCULATION I N THE WEST INDIES DURING THE
'SINTER AND SUMRIER, REqPECTIvELT.
On Charts S I V A, S I V B, S I V C, figs. 41 to 61, which show
the average normal circulation in the West Indian district of
the Tropics, special attention is directed to the vectors in the
four upper levels-alto-stratus, cirro-cumulus, cirro-stratus,
ancl cirrus-for the summer months, figs. 44 to 53. These charts
were drawn by inspecting all the available data froin the eleven
stations and carefully determining the most probable mean rec-
tors that would make a natural, well-balanced system, wherein
irregularities clue to imperfect observations monlcl be rectified.
A comparison with the vectors of Clliarts S I I A , SIIB, ancl
SI1 C, shows that the changes which have been introduced are
all of n minor nature, and it is supposeecl that a larger number
of observations with the nephoscopes would produce a systein
of vectors very closely approsimating those here adopted. I n
the lower levels, froni the surface wind up to and including
the alto-cumulus level, the currents are similar, except that in
the strato-cumulus level the velocity is a t a niasimum. From
this level i t diminishes both upward and clownmard.
It should be remembered that in &scussiug the neplioscope
observations of 1696-97 for the strictly cyclonic and anti-
cyclonic components in the circulation of the middle latitudes,
me reached the same result regarding the prevailing level of
maximum velocity, namely, that the masimuiu velocity is in the
strato-cumulus level. Compare chart 68, page 695, Report of
the Chief of the Weather Bureau, 169H-39, Vol. 11.
I n the upper levels of the Tropics, on the other Iiantl, a
new circulation is prevailing. which is peculiarly interesting
in connection with the causes that generate hurricanes. In-
stmead of one single westward drift, ns in the five lower leiels,
there exist two countercurrents in the four upper levels. The
western group of stations-Havana, Cienfnegos, Santiago, :md
Kingston-hare their vectors pointing sonthward ; the exsteru
group of etations-that is, Saiito Doiningo, San JLKLI~, Babse-
terre, Roseau, Bridgetown. Port of Spain, and Willemstad-
have vectors pointing genernlly northward. Between them
there is a distinct region of counterflnw, and, consequently, a4
area of low pressure. If we assunie that in the upper strata,
where the meclianical friction is a very suiall quantity, ancl
where the internal mixing froin local minor cyclohes is n e ~l i -
gible, the vectors are directed nearly parallel to the isolmrs,
then we can easily construct their configuration, though w e
can not assign numerical values to them without further in-
vestigations. On the eastern side there is a high area, which
is a portion of the western end of the prevailing Atlantic high
pressure. On the western side there is another high pressure
area, whose origin is not so easy to understand. Over the
North American Continent in sunmer the heated surface con-
ditions produce a general low pressure area in the lower
strata. nncl simultaneously a high pressure area in the upper
strata. It is very likely that the western high pressure in the
upper strata over the West Indies is redly the southern ex-
tension of the continental high pressure area prevailin$ in
sumnier over the United States. Some further computations
on our nephoscope observations in the United States will be
required to determine the exact facts.
Between theqe two high pressure areas in the West Indies
there exists a low pressure area, with countercurrents on
either side, SO that all the conditions are present that nre
needed to produce a c!y4>ue ('11 //r ~ !(p /J f ' t ' strvfn. If the prevail-
ing pressures and currents become intensified a t any time, the
high-level cyclone is strengthened, and it then penetrates with
its large vortex tube to the surface as a regular hurricane.
The entire circulating structure is borne along northwestward
in the prevailing drift of the lolwr levels till it recurves in the
southeastern part of the United States. It is evident that the
locality of the formation of the center of cyclonic motion may
shift eastward and westward over the West Indian region, de-
pending upon the state of the atmosphere a t the time, the posi-
tion of the two great high pressure areas, and the conflicting
currents in action. The normal type here produced is in
reality made up of iiumeroufl fluctuations on either side of the
mean. I n forecasting for hurricane conditions it becomes
necessary to watch carefully the motions of the four upper
cloud levels, in order to learn the practical signs foreshadow-
ing such a hurricane condition.
On Charts S I V B, S I V C, figs. 53 to 61, "Normal vectors
for winter," the interest is of a clifl'erent character from
tlmt explained in connection with the summer type. Here i t
is the reversal from the westward drift of the lower strata to
the eastwarcl drift of the upper strata. From the surface up
to and inclucliug the strato-cumulus level the configuration is
generally the same throughout the West Inclian region, Then
the reversal vectors first set in a t the western stations, Havana,
Cienfuegos, Santiago, in the alto-cuinulus and alto-stratus
levels; tlie other stations become involved later in the higher
cirro-cumulus, cirro-stratus, awl cirrus levels, where the regular
antitrades prevail. The azimuths of the higher vectors show
that the iiorthwarcl component nearly vanishes in the cirrus
level over the eastern stations. It will be necessary for mete-
orologihts to outline the eastern portions of the Atlantic high
area in the levels up to 6 miles before executing conclusive
discussions of the important dynamic probleins suggested by
these vectors.
THE MEASUREMENT AND UTILIZATION OF FOG.
RJ P ~R < 1 Lb,sh &f i l l , d .i t 4 r o d I.*min TItmr\ta a11, hnu Iurgai, Cnl., A p l 12, 1 9 ~.
Seein:: that the lJrer&lenCe of fog here in the early summer
is a very great help to vegetation, but that only a very small
part of this gets into the rain gage, i t seems a pity that our
climate does not get the credit of this moisture in the preci-
pitation records. Why should not an instrument be made, to
imitate, to some extent a t least, the action of the leaves and
twigs of trees, and arrest the passing fog particles and make
them render an account of themselves in the measuring tube ?
I would suggest a wire framework of the same area as the
rain gage. and say one font high niid made to fit on the rain
gage. This wire-work cylinder to be crossed and recrossed with some durable filaments (e. g. horsehair) dividing up the
cubic content into cubes of one-half an inch or less.
From a day of drifting fog I am convinced a great deal of
mater woulcl be intercepted and deposited in the measuring
tnbe.
Perhaps i t might not answer to make the fog depositor a
pernianent extension of the rain gage, as it might interfere
with its function as a ineasurer of legitimate rainfall. I n this
case perhaps i t might form a separate instrument and its data
1)e entered in a coluinn by themselves.
A simple cyliiicler of very fine meshed wire gage of the same
diaiiieter as the rain gage to fit on the top might he better than
the horsehair-ixossed space, because I iniagiue that this would
not interfere a t all with its functions as a rain gage pure ancl
siiuple.
If such fog depositors were used thronghout the country
the comparison of the returns woulcl be interesting, and this
much abused arid section " so called, would make ,z very re-
spectnble showing of fog deposit that wonlcl level up to an
appreciable degree its total precipitation to that of tlistricts
with a larger rainfall.
It must occur to everyone who reads t.he above article that