MONTHLY WEATHER REVIEW CHdaLES F. BBOOKS, Editor.
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CLOSED Ocr. 4, 1919 IRBUED Nov. 3, 1919
, AUGUST, 1919. VOL. 47, No. 8.
W. B. No. 693.
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I
EFFECT OF WINDS AND OTHER WEATHER CONDITIONS ON THE FLIGHT OF AIRPLANES.
By CHARLES F. BROOKS and OTHERS.
[Dated: Weather Bmesu, Washington, Oct. 3,1910.1
SPNOPNIR.-BY means of data collected from numerous sources relative to nieteorological phenomena observed in flying, an at,tenipt is niade in this paper to explain on a ecientific basis, for t,he benefit of the aviator, the phenomena he has observed, and a t the same t.ime to gather from these erperiences such facts as are of value to the meteorologist in amplifying his knowledge of what actually exists in the upper air. Tlie disturbances of the air due to davtime convection are one of the. prime sources of bumpiness. Especially on hot summer days do strong, rapidly rising currents of air penetrate to great altit.udes and. where encountered, jolt the aerop ane. Where the cooler air is descending. the effect is similar to t.hat of falling into a hole. Tlie height to -which the eFfecB of surfwe roughness esteiid when the wind is blowing depends upon the speed of t,he sinface wind and t.he height, of the obstruction. In the free air, aviators observations show how the layers of air flow owr one another, the interface sometimes being marked by clouds and sometimes entirely invidde. At ~iicli levels are encount- ered billows or waves. and considerable difficulty is Roniebimcs experienced in flying tluough such regione. Clouds, rain, and fog all contribute t,o tlie discomfort and danger of flyinq. I)erhxps tlie most interesting are tlie experiences in the thimder- storm and tlie up-and-down wjnda which accompany such storms. AR tlie driving wedge of cold air a t the Hurface advances ahead of the st,orni, the air into which the shrni is moving is forced upward. The Iimxinium t,urbulence is found in the region of the sguzll cloud, but t!ie force of the rising air ahead of the st,orni is Rufficient to carry up airplanes considerably, in s ite of the effort,s of the pilots to keep the nose of the plane down. !he dangers from lightning and hail, are also c1uit.e aa important as those from the capricious winds.
INTRODUCTION.
. In a mutual discussion of the meteorological aspects of aviation the aviator and the meteorologist obt.ain much valuable information. Tlie aviator tclls the meteoro- logist his esperienccs wi tli various air conditions, ancl the meteorologist attempts to explain how such conditions nrc. ~w)diici:d, how bcst to :woid the unfavornhle o n ~s ,
and how to take aclvantap of the favorable. Furt,her- more, t.he infoinistion which the aviator cmi give th:? meteorologist helps to esplain niany doubtful wcatliw phenonima which t,lie nwteorologist has not bccn able
to obser~e pelsonall>- a t closc range. To obt,ain facts rsseiitial for this apcr, bout 50 esperienced aviators
nieteorologists themselves have niade flights or h a ~c prevniled on ot,hers to investigate carefully ccrtain points about which further information was needed. In additmion, we haw wed freely the numerous reports,
 Meteorologicnl mpects of aviation written by Dr. Grifith Taylors stmudent aviators and puldishetl in the Australian Monthly Weather Re Iort (I ) ; n.ud also haw
published in a.eronau t.ical journals and elsewhcrr.. We have attemptctl to classify these espericnces unclcr
t.hc headings: (1) surface winds-effects of local hcat,ing, mcl cffccts of surface configuration; (2) mincls of the
free air, turbulent wind boundaries, and flying in clouds and rain; (3) thunderstorms. With each group we have
offertd esplanations. The esplaiiations are, in a large
haw been consu f tccl. In several cases Signal Corps
gu therecl iiumerous sccount.s o 1 aviat,ors espcriencw,
11088a--1-1
part, presented here without any claim to originality; most are enibodiecl in numerous papers which have already been puhlished here and abr0ad.l In quoting the assertions of aviators the aiithors do not necessarily subscribe t.0 the entire correctness of the inferences car- ried or implied.
SURFACE WINDS-LOCAL CONVECTIONAL CURRENTS.
Erperienres qf a.&tors.-A most c.oninion experience
of aviators is daytime bumpiness. The bumps in the air niny be described in ternis of those felt while riding in different kinds of automobiles on roads of varying roughness. Tlie aviator may be ex jeriencing moderate bum s and
extinguisher into niy lap or to set the ship near1 up on end. Then there may he a little more smooth H ying until suddenly tlie support seems to disappear, the pro- peller appears t,o give no headway, ancl down goes the airplane. A bump may announce the bottom, and the aviator slowly climbs again to his proper level As seen from the ground on a bumpy clay, the airplane tilts from side to side to a maximum of 30 from the hori- zontal, and now nncl trlien some sharp up or down motion is discernible. The tilts may give the ship a side slip, which will sometimes be sufficient to remove it from one side of a IT formation to the other. When such a side slip oc.curs too near the ground, one of the airplane accidents char- acteristic of a hot day in the South occurs.
In general, there appear to he differences of opinion among pilots RS to the intensity or magnitude of the vertical movement on bumpy days; this is probably due to the different localities in which the flyin mas done,
or to the pilots esperience. (Esperiencef pilots are usually the most conservntive.) I t has been suggested that observers are much more sensitive to these atmo-
s Iheric phenomena than t,he pilots, since they have not t A e management of the plane to oc.cupy their attention. All say that for ordinary bunips a change of 50 feet is conuiion. A change of 200 feet is e-xperienced at times, and occasionally an aviator comes in with a story of having risen or fallen 500 feet. Others have reported sudden drops of still greater magnitude, but it is possible that only the initial fall was due to the bump and the consequent loss of altitude was due to the aviators inability to right the airdane immediately. The larger
due to humps. !Most of the aviators say that unless flying in formation, it is im lossible to tell the amount of up or down motion with a f, unip or in a hole without watching the altimeter. Flying in a s uadron forination on a bumpy day in Tesas is describe8 as an interesting
suddenly encounter one s \ iarp enough to  throw t fl e fire
and faster the air )lane t h e less is the change of altitude
~~
* See Bibllognphy on p. 532.
623
FIG. 1.-Late afternoon, double layer of clouds: broken alto-stratus and stratus.
FIG. 2.-Thick layer of strato-cumulus; cirro-stratus above.
524 MONTHLY WEATHER REVIEW. AUGUST, 1919
sight, but a difticult perforniaiice. The different ships are going u or down and getting out of forniation con-
. &i aviator who was flying second on one leg saicl that the leader, who was about 100 feet in front and n lit,tle off to the side, went up suddenly about
300 feet, but that hi.Q ship keet on the sanie leveJ and felt no disturbance. On another occasion the air being very bumpy, the first aii$me in a V formation suc!cienly rose 300 feet, ancl a few seconds later t.he second diti t'lie sanie. Evidently the edges of the conditions v.-liit:h caused bunips are a t tiriies very sharp. Most bunips in the air occur near the ground ant1 on
bright, hot clays. In the northeastern Pnit,ed St.:ttcs,
3,000 to 5,000 feet is tlie upper limit of hum Iiness on ti
suninier day, though in very hot weather, \~ulnpiness, sometimes est,reme, occurs at elevations even above s.000
feet. In Tesas the air is. bumpy sometimes to more than
10,000 feet. In winter the usual limit of bmnpiness at Ellington Field, Tes., is 1,500 to 2,000 feet. Orcliiiarily, however, t,he es treniely buiii ,y conditions are wibhin
500 to 2,500 feet of the grounh or just under and about cumulus clouds.
A cloud of the cumulo-cinibus t.:ipe is often met aitli, mid, i s B rulcnl to avoid it the engine is throtbled down, ancl a cliye is m:uk i~iiie 30, t.0 50 feet lielow it.. As the nia. hine asxiinies its normal flight p:ith :1q:1111.
while under t.he clnnrl. it. is suhjel.tcd to very st,rong .gusts. s:-;inetin>~~s spasnioclir in duration ancl at all times varialilch in clireection. A g i i i .
under the mme c1out.l niiother movement of an os1 illaiivy i..h:irwwr is encouiitrrcd. The i~scillatioiis, as a rule, are 7;er;- S!CW. for i !-!e ~ ~T .I -
d w e in the aeroplane colxitleralile iindulation abut the j:il.. l:iiig nsi.;:
Phiis, for a fern selmiids t!ie mad~ilie is subject. t.n :I &on;: lifl. t.1ii.n tlle
nose is forveil i.lown. sonietiines cauainp 1% skcp cliw-e. Srill yct miothcr r1ist.inc.t and even more interesting c1isturl:~p~e is met. JiPrp. in t.Le shape of a r y k l vibratory rAfl'eit on the mm:liine. T l h is i.xol.ial.il~ 1 1 1 ~
ta a 1icwizoiit.nl liead or following tviiicl ~VIIOSC q ~e d rqiiihy a~tcr~iat.es 1,et.wveen a low{+ nliiiimiini and a high masimum--lit.itf. F. JI. ill+
:Vu~u,narri, dv,straliim Jlotttltly IVeolltcr Wrport August 191.2. j>(:!Tc +jf.
tilluY of a
For esnriil ,le :
In the interior of Tesas flying is generallj- not at- tempted in the hottest hours ai! bright suninier days
because the air is dangerously bumpy. Here is a case of
a crash ascribed to "heat bumps":
Lieut.. F. W. Keller and Sergt. E. Chapmsn on leaving (*l:xrksc!:de, Miss., June 17, 191s. "had dificully in n t k ~i n g altitude bec:iusc of
the lieat, waves. [Their airplane] while making a turn, went into a tail spin and crashed [liillinp bot.11 nien]."--dssoc. Press.
It is impossible to say what proportion of flyin: fLtt,tdi- ties have resulted froiii vertical currents on hot days. L4viators are unaninious in saying that Texas has t,he bunipiest flying of any part of the United States. The placws where buni s occur are well known to aviators. Roads, railroa& edges of plowed fields,
forest edges and clearings, barn roofs, hangars. ditrlies, hordera of swanips, shore lines, all giw l>u1iljis, the sourcw of which can be ident.ified enerrtlly t,o 700 or S O 0
3,500 or inore feet. Tie 11unips associated with inact~tlmi roncls and other hot. )laces are not necessarily c1irei.tly
t.he velocity of tlie wind and the height of the airplane. The effect of ditches seems to depeiid on the nature of t,lio ditch. Some avirttors at :Rivh Field dlomecl for fall of
about 15 feet in crossing a suiiken road in t,lie vicinity of
the field. An aviat.or said that over a certain ditch new
tlie field where he was flying lie nlways experienced R
conaiderithle bunip: but, he could not say whether or not, there WRS n slight fall before the hunip. Bailroads have the sanie effects as roads. Creeks seem to have clomli curreiitrs over them. The following was told by Liiwt. Morgan, one of the most cxperiencecl flyers a t C!arrut.liers Field :
feet, and on c h n l n i da s occasion 3 ly to 2,500 ant1 e ~r n
o ~e r the road, hut to 3 eeward a t a distance c1e~~entlin;z em
His airplane which had landed on the bank of small st.rem on a very hot dnj- liar1 immi&raljle clifirult,v in ri&g because the propeller did not si& trt talcc. h1>1:1 rd t.lie light. &r. Af1.i.r thc ship had risen perliym to .500 feet. it Pullricnly began t.0 fall. Tile propdlcr, even though going
itf 1,4INl re\olu:ions per minute, swliir.~l t,) do nc>t.liing, and t.he elevators did nt.lt. rei;pnn(l until the ship wit9 50 fert fnm the ground. l ' l i c ~~ ron- dit.ic;nr 1:ea.ame ni.nmsl mil the airplane began to rke shwl:i after Iiav- in?: Eallcn 1.0 a lieirl~t. ili only 10 feet,.
An wintor trainecl at Rich Field said that he did niost nf his flying owr the open woods just south of
JTSCO; lie chose the woods because he feared less from tljc 1~~:npy conilit,ioii of the air over the woods than froni coIlisiniia with others over the sinoother fields. Over thse. mmcls the hunipiiiess estencls with a certain degree of rouglinees t,o 1,%j or 1,SW feet, and is perce tilde to
!2,5(:1i1 fert. Ey coninion agreement, a green fie P cl is said to he the best lancliag place and a plowed field the worst (if forcsts and rough round are excepted). In the air,
clouds are generally avoided because of their well-known
1'01 1,011 chnract er . As to conditions along the coast, the following state- nient by Doiiald B. Kimball, a naval aviator, shows the effect of difference in convection over water and land sui-faces:
(-hi c:li?nr. hot clays in si:innier a light breeze off the ocesn will often tlpring il.!l in the morning, the velocity of which increa.ses toward after- iiiion. Such d3y.a niake ideal flying for seaplanes i f the pilot keeps his ni:tcliii?e clear of islands or peninsulas. The air over the shore line of islsiiils or protrrding necks i s especially treacherous on hot days, for viulen t. h i n ~~i f i may catch t,he pilot, iinawares after navigating through t,he ~iiioi-~tli air over t,he ocean [especially in such places where the water is rat1ii-r cold rc?lative to the beach, m at Pan Fiego, Calif. The shore- liuo bunip~ on t?hesapenke Bay. and tlie Gulf of Mexico may be sc'~ri:~Iy noticealde in late summer.] As a rule. t.he depth of these 11~ml1s extends not,iccnljly t.o about 1.500 feet. wry rarely above 2.000
feet, the vialonce varying indirectly with altitude. These conditions are sonizwhntnt. altered i f cumulus clouds tend to form. The region just under and wit.hiii tlie clouds is probably bumpiest of a11 a d thexe nppe:i.rs to Be a sliarp decline in violence on climbing above the clouds.
i.111 n. clear, hot. dny it is not. an inc common feat for a perfectly balanced machine t,o fiy several miniites a t altitudes under 1.000 feet without having the foot, cont.riJls niorecl. On such a day I once even saw a
pilot. slep froni the front sent to thr! wing and t,hcnce to t.he rear seat at iiii nltitiiile of dmut i s feet. Such a feat noiilcl be alniost suicidal in land flying on n hot day.
0tt.o Neumer, of the Sigiinl Corps Meteorological Serv-
ice, in c-Iiscus:;ing typic.sl flight! conditions over the heat1 of C:hc.;npenke Bay in v;inter, states that conrect.ional currents or cilt:lie:< have lit,t.le effect upon t,lic movement
of t,he plnne, weii on p~ssing under or through cumulus clouds. The crossing of t,he shore line is prxc.tically i mpcrce.p til:ile,. I5 Ihrcr.tion~ of h i t i p i i ~ eR2.- -The local vert,ical currents \i-liic i occiir on warm d t t p are the result of tlie unequal limt,iiig of the lrn-er air. The air nest. to the o.rouiit1 gets
lint: a i i t i , t,lierefore, ex )a.ncls. Over ti, ])are, Try .fieill tlia iwatiii? i:; greilt.est,. hius, tlie surface air locitllg may
iiccon-ic! con4erttldy lighter than the cooler air a t the
sanie Ievcl (-11' cTen.above; niid so at the first opportunity wmc of t l i i b : lietnier air nioves laterally or conies don-11
ai!<l ft-ircm some of t.he lighter air to rise. On a quiet
!hi.:,- at. t.hc e:irt.li's surface the moveniend of this cooler
:],;I- is 1na.rkrci 11;i light,, variable wiiick inbersperserl with
clilrus. The 1)roceaS of ~lisplacenient~ is iut,erniitt.ent. Wlim t.1ivi.e is :I. i d lhving, t.he occasions when the ciwlcr air mows t,ox ~r t l the ivarnier places are marlied 1)y gusts or :~l n c k I\-id. These usts are the result not
iicii t.:: of ci~i~i~ect,i~i11;11 minds ant1 of the general wind,
liut, nl.:v of tile quick, iloii-n movenieiit of air, which
1lii:j :t hi;:licr \-chwit,y than t,he frict,ion-limited wind
c,mnulus cloucls and t 5 e region iininediately under the
o t i l j * of the comliiiieil st,rengt,h o f the horizontal compo-
11est t.0 the grou1:tl.
AUGUST, 1919 MONTHLT WEATHER REVIEW. 525
The reason for the bumpier conditions in the middle of tlie clay and for the greater bumpiness in Texas than elsewhere is now apparent-the hotter the lower air relative to that aboye, the stronger are the convec.tionn1 ciirrents and t,lie gustier the wind. The reason for the decrease in buniihiess aloft is that near t,he earth there
ity gradients. Amain, the upper limit of bum liness is the upper limit o r the convectional columns of A etaclieil masses of rising air; and in much of Tesas, at least, the t.op of the convectional upcurrents are marked almost daily by tho to Y of cumulus clouds. (Cf. figs. f5 and 7 .)
open spots in wood-, etc., depends on the differences in the
are likely to be t h e steepest temperature and wind reloc-
The locations o P bumps over roads, railroads, glc>ivecl fields,
swiftest of these to produce effects on an aeroplane more or less disconcerting to the ilot. Down currents, marking $e movement of relatively cool nir over ponds (D), lakes, and clumps of trees (E, fig. 7) , do
not have the marked velocity of upward currents, but are
more dangerous. Their boundaries may be sharp enough for an airplane to fly with m e wing in and on? wing out of the descending current, as over a reservoir in a wheat field in France. Down currents may extend up to 6,000 feet over woods. Stunt fliers try to get into a rising column for a tail spin; and they say that a hole or pocket (i. e., a down current) is often fatal. Perhaps down currents were responsible fnr PC)XF :If thc 35 per cent of training fatali-
1 I
C D E A
tiegree of heating of diff erent parts of the landscape. The
\ roacl gets hotter t.han the fields on either aide, and so the air from both sides flovs toward, and dis hxes upward, the
hotter than the surrounding fields, because their clark, loose surfaces absorb so much more heat, and conduct c-lownward so much less heat, than the surfaces of unplo\nxl fields. Open woods fayor bumpiness because sonie 1)art.s become hotter than others. Isolnt1d hills, especially short or c.oiiic.al ones, should he avoided during warm, still dayFs, for on such occasions their sides are cert.ain to be warmer than the adjncent atmosphere at the same level and hence to act like so
mnny chimneys in producing updrafts. The rising currenbs are interesting, not only becmse
of t,he way in which they disturb aeroplanes, but also 1,ecause they may be utilized by avitttors who are trying t.o make altitude in the shortest time. Many use ti rising
air column that forms the cumulus, but they claim that many ncciden ts have occurred because of the uncertain- ties of its direction and act.ion. Measurementa on pilot, ldlooii~, says Prof. Huniphreys, (2) and also measure- niei1t.s taken in maimed balloons, have shown vert.icn.1 vt?loci t.ies [of coiivectional currents] both up. and down, of niorc than 3 meters per second. The sonring of large birds is R further roof of an u ward velocity of the same
water drops and hailstones are not only temporarily sup- pnrt,ed, but even carried to higher levels, shows t,hat up- rushes of wt least 8 to 10 or 12 meters per second not only may, but actually do, occur. An aviator who has flown over an active forest fire never does so agnin, if ossible.
There are, then, upward currents (some of whi& might be called air fountrtins) of considerable velocity whose sides a t times and placw may be almost 8s sharp1 scpa-
fountain of water, and it is altogether possible for tlie
air oyer the road. (A, fig. 7.) Plowec \ fields are likewise
(B, fig, 7.)
(C, fig. 7.)
order of mngnitu R e, while the P act tshatt in cumulus clouds,
rated from the surrounding atmosphere as the sic P es of n
B FIG. 7.-Locol convectlonal currents on calm day.
ties ascribed to tail spins during the war. In diving, a
very thrilling sensation is encountered if the ship enteis n down current-the airplane seems to drop away from tlie pilot. Other air movements, which are perhaps ecually dis- tressing t.0 aviators, are tsheamountain breezes (t own mver vdleys or other low portions of the country. These
suddenly from dmost calm air in a proachng the mout g
winds, sometimes of gale force locally, may be met ve
of n valley a t times when the ear& is cooling by radia- tion.
EFFECT OF TOPOGRAPHY ON WINDS.
The effects of heating and cooling of the etirths surface upon the flow of air is not the only cause of turbulence, for there is tshe effect of t,opographv, which introduces phenomena quite ?s dangerous and distressing to the airman as convection. Avia.tors exlvet.ience.s.---.Rougl~ess~ (including trees and buildings) of the surface roduce eddy motion, m%.ich
in a strong w-incl to 3,000 feet or more. Extremely rough conditions are experienced in a gale, as the follow- ing accounts show :
in a niodcrate wind will reac. i! to 1,000 or 2,000 feet and
Sirhjert: Re ort to Post-Office Dept] of flight from Philadelphia
Left Hustleton at 2:15 p. m. March 28, 1919, in plane No. 30, Curtian
R-4. Wind velocity, reported bv Philadelphia Weather Bureau, 48 miles er hour; direction approximately 300 [WNW.]. Climbed to ~,W O g e t before reaching Trenton. Steered com ass come of 250, rtllnwing 40* drift angle. severe hmps felt at all &tudes. Looking ahead snow could he seen. Directly to the rear, atnioe here clear. Papsed over Trenton at 2:30. Having a short time beire run into snow, which made it necessary to fly at 1,000 feet, at which altitude c-odd just see the ground. Could not see enough of the ground to rocqnire any landmarks. Flew entirely by compaes. The nest landmark picked up waa the Woolworth Tower, New 1;irk City. Paased over it about 300 feet to the north; altitude about
WlO feet. KO very severe Lumps felt directly over the city, but when paFlsing over Brooklyn very severe bumps were encountered. Them
to New york, Laic6 %, 1919.
626 MONTRLY WEATHER REVIEW. AUQUST, 1919
threw the plane about to such an extent that I WBB unable to watch my compags for several minutes. Extreme aileron and rudder were needed a number of times to brin the plane back to normal flight. Flew with motor wide open during t k s time (1,600 revolutions per minute). While pming over Brooklyn the motor cut out three times. Turned on the gravity tank and it picked up again immediately. Then shut
off the gravity tank each time it came back. Presumably this was
caused by the severe bumps. One pocket over Brooklyn let the plane drop approximately 100 feet. It was 80 rapid that I waa thrown full wei ht against the aafety strap. T%e snow became heavier, and in order to see the ground had to fly below 500 feet. The air continued very rough, and was only able t,o look at compass occasionallv. At about 3:05 icked up a railroad and
attempted to follow it, thinking that it might yead to Jamaica, but the country was not familiar and <t was very difficult to follow the rails. Do not believe I waa more than 800 feet. The air was very rough. Attempted to do thL, but on seeing a field would be driven ast and it would be la9t before I could turn and land. Then heade! around into the wind and flying just over the building and trees, approached several fields, but they were all too small. At thie point passed over a small body of water and felt the worst bump of the trip. 1J7m not more than 50 feet and waa thrown into a vertwal position and hack to normal almost hefore I could use the controls. What appear& to be a fair field was directly ahead. Throttled motor and zunied over a fence. On the farther side the Found rose in a gradual slope, and pulled back to land. .Tuet as the wheels were about to touch, a current of air coming from the other side of the hill caught my plane and lifted it ahnut 20
feet. Then the plane settled vertically to the ground, crushing the landing gear and breaking the ropeller. This occurred at 3:20 . m. on a field owned by John c!. Bafer, Great Xecli, L. I.-Johia If. A#h~~.
At 3:15 decided to pick a field and land.
.4t College Park, Md. [Mar. 25, 1919].-Filot B i d e rose only 50 feet when his plane was forced downward aa if some giant had placed his hand upon it and preased it earthward. His landing gear was smashed. Pilot DeHart, attem ting to leave Belniont Park, Nem l-ork Cit.y. for Philadelphia, coulfnot get his plane off the ground on account of the downward swirls of t.he ~~nd.--~~shiirgtoii Post, Jlfiwh 29,
1919.
During a gale the edge of the woods near Houston, Tex., is marked to a height of 1,000 or more feet. Bad bumps have been felt 2,500 feet above and to leeward of hangars on a windy day. Neumer re orts that on days with high winds in
to be moving in great horizontal eddies or rolls similar to the rolling of ocean waves. The airplane rocks just like a rowboat in the sea. roduce sea-
s ring of 1918: and another on a mail plane flying from &is to London in the summer of 1018-0wing to the straight head wind and deep air pockets, my observer and myself were really seasick, as though pitched and tossed on a heavy sea,  said Lorgnab after landing. Many pilots have been questioned regarding the effect of topography u on the action of their planes at various
the ground, there are also the effects introduced by build- ings, groves of trees, and other obstacles over which the wind must pass. The general opinion is that these effects do not extend upward nearly so ftlr as strong convect I 1011
does, except, perha s, in very rough, mountainous Colin-
altitude to which such disturbances may estend is pro- portional in general to the wind velocity and the size of the surface irre larities. A combination of a stron north wind and Eight sunshine on June 13,1918, cause
eastern Mary P and the air [up to 10,000 feet] seems
siclmess. A case occurred in southeast 4 exas in the Such motion is sometimes sufficient to
altitudes. In a % clition to the natural configurations of
try or with winds o P gale force. It appears also that. the
s bumpiness which was extreme even as
the maximum altitude reached by J. mail plane flight from Washington to day. - - . . . ..- .- -
1 Notein Scientiflo bmericrm, Aug. !24,1918. p. 147.
Here are some aviators accounts:
Almost invariably offshore winds are accompanied b bump and eddies which cause fatiguing flying. A 10-knot shore ireme makea more trouble than a 20- or possibly 35-knot sea breeze. Evidently this is due to the uneven contour profile of hills, trees, and housee OIL
the land as compared with the aea. Air bumps from offihore breezes appear to be of a different nature from those occurring over islands in a sea breeze. Under the former conditions the bumps come in gush and in fairly steady succession. An inexperienced ilot may attempt to keep his machine on an even keel by adjusting &e controls for each bump eepsratdy, but he Boon learns that control, especially with his ailerons, is practically un- necessary. Alertness in the use of elenatom:: m alwa advised by the instructor. The golden rule of flying is, Retain &n g sp+; therefore the burn which raises the ncee of the machine and dimmmheu the apeed must beklanced by a corresponding lowering of the elevatm to regain that speed. Bumps from gustv offshore breezes extend to varying heights. I have encountered them aa high as 5,000 feet. They are not always Btrongest at the lower altitudes. I have seen days when the riding zas rougher at 1,500 feet than at 500 feet. It is rather treacherous to be off I close to a protected shore in an any degree brisk shore breeze for there is a sharp outline at the le\-el of the height of protection. For instance, in taking off toward a hangar, it is not uncommon to pass suddenly from a relatively smooth region to a very bump one in swooping up over the edge of the hangar.--Domald B. K @d When passing over buildings, groves of trees, or simlar obstacles, him 1s sre generally experienced, their extent bein iduenred rnaiihy by the strength of the wind As obstwles d t k s character
:we apprnwhed with a following wini the machine will lift and drop with e r p l soddenness on the other side. If approached head to wind the conerse will take place-the mrtrhine will probably dro sometimes ncsrly to the ground, just before reaching the obsta-%: and rise as i t leaves it. This Reems to be due to the fact that when thewind strikes the obstwle i t shootsupward. Fig. 8.1 Itwould be diflitdt to 887: what height this updraft gener 85 ly rewhes, but the writers expedenw is that i t goes up well above the wtual height of the c1,stwle. and is flattened out on the other side, where there is often an mimistttl;able d o ~i i current.-Limt. 8. W. Addison. (1)
c
FIG. S.--Effwt of minor surface irre~ul3rltiesonli~sof wliid How in alight wind. (Fruiii Linke. (IO))
The deff ect,etl vert.iw1 c-urrent,s roilsequent upon a surface breeze meeting a belt of trees have often lifted the writers machine, just in t.he m n i ~ manner as a long grnuEd roller does a small boat. This is (my to anticipate when a breeze is notic*eable on the ground, but is at. first, apt t,o tmie a1 a surprise, when on the ground practirdly no
air movement. CBII Le cletect.ed.--Crrpt. B. B. Storrer. (1)
A wind makes flying much harder for the airman, not. becaiwe of the a d u d wind it,self. but. on arcount of it,s const,ant,ly c-hanging velocity ;md clirm6on. The vhange of direction of a w i d is its worst feature. and caus(1s 1,he rnmhine to yaw* badly, and so makea it difficult to keep the mwhine on its course. The wind also rhan B in direction vertically. and. when we experience this chanp, tE eqqilibrium
of the mal-hine is up&. When flying at a low altitude I have not.iced that m y irregu1arit.y on the surface will cause a change of direction iipwtrd or downward and cause t.he machine to rock, sometimes verv badly. Ii the objecta on the ground are of a prominent nature, sucb
as the many belts of trees on the aerodrome, I have ex erienced a.
r*ondit,ion which vauctes the machine to flop ? about in alfdirections, and is mopt distracting. I attribute the (mise of this to a horizontal e&ly heing formed near these objecta.-D. P. Flockart. (I )
The turbulence caused by hills or mountains depends largely on the local topogra hy and on the winds con-
cerned. Point Lorna, near 8 an Diego, Calif., gives birth to marked turbulent conditions over and to leeward of it.
AUCIUBT, 1919 MONTHLY WEATHER REVIEW. 537
Dr. F. A. Carpenter describes the conditions as follows (he was in an airplane at 2,000 feet altitude headed seaward toward Pomt Loma):
Suddenly there were two distinct wallo s and I felt the fuselage beneath me respond as if struck by a stuted cliib. There I V ~R evi- dently first a sur tnen a drop, and it waa the desiwiding current of air that d e p r i v z the airplane of the su porting medium. 1ienc.e the shock. * * * Although thisrninsura (Point Loma) is levs thm 500 feet high, it 80 effertively eflecta the prevailing northwesterly wind that the u ward surge has been noticed by aviators at an altitude of 4,000 feet. f t is no wonder, then, that these desreiirling aincls, called woollies (from their churning up the water into isolat~d masses which look like tufts of \vool), are dwaded alike by yaditamcn and birdmen. They have heen known to carry away topsails from too closely venturing schooners and student aviators always give the vicinity of Point Loma a wide herth.
An aviator flying a t 3,500 to 4,000 feet inland from San Diego with the wind, passed over some mountains 2,000 feet high. On the lee side he was forced down, or fell, 1,500 feet. The prevailing cloud sheet a t 4,000
feet gave from below no notable indication of such a
current, but it seems probable that the upper surface would have showed where the wind was up and where down. The upper surface of this lrvelo (stratus) cloud sheet is usually uneven; but this aviator on other occasions had not hap ened to find how to tell from the
The following, from log of balloon flight made by Lieut. W. F. Reed, jr., on flight from Akron, Ohio, to Erederichburg, Va., Se tember 16-17, 1919, shows how
form of the cloud su Tp ace the underlying topography.
the wind tends to hug t Yl e surface in going over a ridge:
6:20 a. m., 2,400 feet.-Passed. over sh-arp nclge. The balloon wa.q
allowed to approarh the ridge +bout disrhargmg hxllnst: it loolied ag though we would bump the chffs, but the upward cwrrent carried us over the to . The trees on the crest looked at first like mere weeds, but a c?wr view showed them up aa full-grown trees and we
could hear the wind rustling through the leaves. Ke went domn with the current on the leeward side at an altitude of 200 feet above ground until we were halfway down.
Going against the wind up over a mountain region the aviator may have difticulty in keeping his altitude. Lieut. Vance reported:
I had one unusual experience with bum y wind. It was in October, 1918, when I was l e h g a formatmn nortfwest in the face of a north wind, from Birmingham, Ala., t o Fayette, Ala., over very hilly coun- try (hills about 1,000 feet covered wiJh fore&). I attained an alti- tude of about 1,500 feet before 8trilun NW. and travelled for more than an hour to climb, and actualfysettlingallthetimo. Itwns during the f o z s in Minnesota, and all the air was smoky, very dense. All the other flyers of the squadron had the same experience.
At the time when the smoke was over the South,
an extensive anticyclone covered the eastern United States-possibly the settling experienced was due to the general downward movement of the air in the anticy- clone rather than due to topographic influences. The air over a mountain region is usually turbulent. Ca t. F. N. Bartlett, in a flight from Scott Field, Ill.,
flying over the Ozarks between Eberts Field, Ark.: and
Post Field, Okla. This turbulence attendant upon flying over the Ozarks was strongly manifest in a recent balloon trip from Fort Omaha, in which it was desired
to maintain a constant elevation of 5,000 feet above sea level. This was found absolutely impossible and the balloon often was dragged down to within a short distance of the ground, or caused to ascend again, almost out of the control of the pilot.3 The accompanying bal- loon at 10,000 feet was but little affected. On windy darns over rough topography, bumpiness has been generally observed to 4,000 feet.
to !k ells Field, Tex., a year ago had nine hours of difficult
I The Aviator and the Weather Bureau. Ban Diego. 1917. pp. 1WO. a Sea psge 685, rdermce to Lirut. Reynolds sccwnt. balloon artlclr
Lieut. R. 0. Searles, the flight commander of the De Havilaiid s uadron, which made the trip from Elling-
34th and 26th of February, 1919, i t was not ossible to it was
emily possible on the 25th. Sergt. E. B. Scott indicates this was doubtless due to the pressure distribution on those dates, for on the 25th the adient wind was not
tion of the canyon, whereas on the preceding and follow- ing days the pressure gradient wm steeper and the wind direction was such a3 to produce great turbulence in passing across the canyon. It has been reported that in a strong south wind the back-and-up current of the eddy in the lee of the south wall of the Canyon has carried tin
A cans u the cliff and into the yard of the hotel. near Zelly Field, Tes., has the reputation of being a an- gerously rou h place. The winds are this way and that;
cally all control of the airplane. interpretations.- 1Vi~n.d ecZdies.--<Just as eddies and whirls esist in every stream of water, from tiny rills to the great rivers and even the ocean currents,wherever the banks aresucli asgreatly to change the direction of flow, and wher- ever there is a ocket of considerable depth and extent on either side, an tl as similar eddies but with horizontal in- stead of vertical ases occur at the bottom of streams where they flow over ledges that produce ahru t changes in the
reasons, horizontal eddies occur in the atmosphere with rotation proportional,roughly to the strength of the wind. These are most pronounced on the lee sides of cub, cliffs, and stee mountains; but also occur, to a less extent on the win c! ward sides of and above large obstructions. (2)
ton field to the !P acific coast and back, related that on the
enter the Grand Canyon with a plane, but t i a t P
of great speed and, moreover, co n$ ormed with the direc-
now up 100 s eet, then clown 100 feet. One loses practi-
levels of their beds, so too, and for t Yl e same general
--
FIG. S.--Wiiid moasing a ridge. (From rloud movements observed dong the Hudson River, A%. 4, 1918.)
The incrt.ia of the wind crossin the mountains tends
but its drag on the lower air, due to viscosity, deflects it downward. [Fig. 9.1 Because of this deflection a foehn wind often strikes the lower slopes with great violence, from which, and mainly because of its dynamical heatin ,
surface wind may be exceedingly turbulent and violent, while both farther away and also on the higher slopes it is comparativd light. Purthermore, owing to changes in the general Zrection of the crossin current, or m its
mountain or even vanish entirely. - W. Hwmphmys. The air at the top and bottom of wind whirls is moving in diametrically opposite directions-at the top with the parent or prevailing wind, at the bottom against it-and since the are c.lose to the earth they may, therefore, be a
some danger also at the forward side of the eddy where the downward motion is greatest.
close to steep mountains, hills, blufFs, or even large build- ings, for these are the favorable haunts, as just explained, of treacherous vortices. The whirl is best avoided by
to carry it on well above the Val K ey or plain beyond,
it rebounds to higher levels. Alon a belt, therefore, we f 1 clown the mountain, or even alight 19 y beyond its base, the
strength, or both, thewind belt maxs %I ft u or down the
source o f decided danger to aviators. There may be
not be made, if at all avoida Q le, on t i! e lee si .r e of should and When the wind is blowin strong1 landin
528 MONTHLY WEATHER REVIEW. AUGUST, 1919
landinm in an open place some distance from bluffs and large o%structions, or, if the obstruction is a hill; on top of the hill itself. If, however, a landing to one side is
necessary, and the aviator has a choice of sides, other thinp equal, he should take the wi~iu7wurd and not the lee side. Finally, if a landing close to the lee side is com- pulsory, he should if possible head u the hill with n@-
to an eddy current in the same direction. He could, of
couhe, avoid loss of velocity with reference to the air, and therefore loss of su port, by heading along the hill,
be at the expense of the dan ers incident to anding in
ing down the hill, might be correct so far as the direction of the surface wind is concerned, but it probably would entail a long run on the ground and its consequent dan ers. Sddies of a very different ty e, relatively small and
are formed, as is well known, by the flow of strong winds ast the side or corner of a building, steep cliff, and the Eke. In reality such disturbances are, erha s , niore of the breaker type, presently to be es$ainei, than like
a smoothly flowin vortex, and should e avoided when-
iClearly, the sup ort to an aeroplane flying eit,lier with
and may vary between such wide limits t.liatr t.he aviator will find himself in a veritable nest of holes out of
which it is difficult to rise, at least with a slow machine, and sometimes dangerous to try. However, as the tur-
moil due to the horizontal winds rapidly decreases with increase in elevation, and as the aviators safety depends upon steady conhtlons, or upon the velocity of his machine with reference to the atmosphere and not wit.11 reference to the ground, it is obvious t,hat t8he windier it is, the hiTher, in general, the minimum level at which lie should fly. (2). Cf. Mr. Millers account, pp. 525-5%.
E$ct {f gusts.-Aside from the obvious effects of vertical currents, already discussed, the mere clianges of velocit in a horizontal direction that accompany t.hc
in an air lane flying with or against the wind. If the air-
of the wind will momentarily reduce the support of the airplane, thereby causing. i t to drop , wlde any decrease in velocity will momentanly increase the air speed of the machine, tending to make it rise. The reverse is the case if the machme is going against the wind.
This is because in a steady wind an airplane it.self moves as if in a calm. Thus if the wind is unsteady t,he number of gusts encountered in a given time will he the same whether there is a following or a head wind. And
if as the anemometers indicate, usts have no more
less, aviators say the can feel the difference between a
fastest against the wind. Soaring birds have t e same
es erience. This would seem difhcult to ex lain in any
the end. Ap arently, we need more refined observation
Aeron. Joum., (Lmdoie) 22: 2854, 7918. E ect of gu&iness on a turning nirplane.-Probab.ly the
and side slipping-occurs not during straightaway ying
cient velocity to offset any probable I; oss of support due
that is, along the asis o lp the vortex, but this ain would
a side or cross wind. His o 3 y other alternative, head-
so turbulent as to have no well- R efined asis of rot.ation,
ever the wind is a % ove a light breeze.
or against a wind o P this kind is correspondingly errat.ic,
7
passage o s gusts tend to produce an up-and-down mot,ioii
plane is B ying with the wind, any increase in the ve1ocit.y
ahru t onset than end, the 9ffect o f a gust from in front
or o P a lull from behind should be the same. Nevertlie-
head wind and a fo T lowing, one, and that the climb
ot % er way than that gusts begin more su B denly than
to s Tl ow what t K e dxerence is.-Abstract from C. 0. TwiLcr,
chie 4 disturbance due to gusty wind-escesnve ti ping
l?
8
but 8.5 the aviator turns a t low levels from flying a g e s t the gusts to fl ing with them. For example, on Apnl3,
1919, Lieut. &l. E. T. Dickman and Maj. J. W. Butts, were killed in such a disturbance near Americus, Ga.
The t,wo offiwrs * * * had made the last. turn nf the fielil preparn- tory t.o lzniling. Frnm the growid it appeared that Iv1a.j. Butts attempt- ell to turn. The wind ~~~s coming in gusts and apparently iwight. t,he airplane wit.h full for.,e in sur h a way aa t.0 lift the tail vertiidlv into the air. Tl?p mud nose dive followed the jerking of the plane rnto a
vertical position, and it. imslied ti? the earth.-llcrshinglo?i Post, :ipd
.I , 191.9.
Such an accident may be, and presumably is, caused
as follows: The aviator starts tufning, suppose, while in and facing a relative!y slow-niovng.portlon ot the air.
On banking, the plane IS t.ipped wltli 1ts .unclers!de more
or less against the wipcl, yhereupon the hwher wing often runs into, or for bnef Ii?t.ervnls .IS caugxt by a much swifter current t,han that into whlch the lower still clips. Numerical ralues are not afi hand, but the phenomenon of overiunning gust.s is familar from the action of winds on isolated t.ti.11 t,rees. This obvious!y mcreases the tip,
side-slip. On the ot.lier hand, when turning from flying with to flying against t.lie wind the high wing catches the incressecl impact on its upper side, and therefore in this c?se tlie result is merely a temporary clecreme of the tlp-an ent,irely harniless effect. Gusts that envelop the whole of an airplane while turning obviouslv affect thc lift to
some estent,, even when the path of the wind is at right
. angles to t.he course of t,he plane, but seldom sufficiently to be of much importmce. (2)
nnd, in extreme crises, sufficiently to induce a den, -cIous
WINDS OF THE FREE AIR: TURBULENT WIND BOUNDARIES.
The difficuhies of t.he pilot do not cease once he has tiscil
above the turhule1lce of trlie lower air produced either.by local convectionnl curren tu or by the tumbling of trhe wind over the surface of the earth.
hlr. J. 1.:. Edgc-rtrJn, Byins the mail plane 1iet:mai Kashingfon ai!il
Phila~!dlhia. J d y 4 , IPlP, row %1ww tile surfs e I,unil,inessint.nniii: !i
awolcr air :it 3,0(iO or (;,000 feet aiitl liail siiiootli flyixg until f r i m 9,lCh
to 11.3011 f w t lie es )erienceil iip and c l o ~i aineiits. suine of tlirni st,ronp. The logs of his inad flights we full of siniilnr ins tan^ es of 1.iuniliy Iayr:: at various elcvat,ioiis: On June 1, 18 LF, tlrrr air N%S l ~~m p y hetwecii
2,600 and 4.000 feet and again frwn 5.500 to 1;,500 feet between Timil.. iiigton and BJ timare, and I.ietm+een ;.OW) and R:000 feet, near Philaiki- phia. On desceniling, the air was eapecisliy L)umpy at 200 feet. Iktiirning iii tlie afternoon. hemy I louds hall formed t.o a iieiglit oi 10,(10n feet., and Yertical nio~enient.s ?stended lip to this lieighs. 111
t,ho vici1lit.y of rain g ~i ~l l s the air was estrenicly hinipy alii1 *s h :i q ~
cross current.s threw me off my course rqmitrdly. (.hi desi Piidilib:. it w w incremiligly Imnipy down to 2.000 ieet,. -\ioft, a i.olil west. wiiii; i~verru:iiUng a southwest surfat e irinil, ~t s e\-itlentl:,- rr+cBrkl)lv iur tht. vonvecbioiinl currents which mxle the ,.lauds and rain and t.he liumpy conalit,ion of bhe a.ir, first. in two zones nf rnnilerate thickness 3 1 ~1 laier t.llroughout. the loufer 10,000 feet, of the atmwphere, at least.
Holilfs CIII tiis rca.ent. record-bre:%king altit.ut:c Iliglit. to 34.W0 feet.
Heptemlwr IS. 191% eaiil t,hat eveii at 31,000 feet, my niai:hine hit :I
poi.ket mid dropped GOO feet. It rol:kecl f l .i J I i l side t.o side, it t.errilh senaation.-From ficusptiper nrmuiit: Scpkeiber 19! I51 9.
Referring again to the flight of Ctipt. Bmtlett, we find this newspaper statement:
Waiting here [Waco. Tex.] for the etorm t.o ms lie took off a-qin under black clouds nliicli hung as low aa GOO leet ana with a strong south wind dong tlie pound. He climbed to 3,000 feet and there found clear air and a brlsk north wind.
There are numerous accounts by aviators which tell of wind boundaries which are entirely invisible. Says
one :
Near the surface the air was comparatively calm. but whcn aliout. 800
feet waa rrdied the machine w.w noticed to pitch slightly. just a8 a vessel n@it do in a sea with a slight swell. These conditions con-
For esuniple:
-AUGUST, 1919 MONIHLP WEATHER REVIEW. 529
h u e d until about 1.000 feet. On anothm ocmsion. when t,hi. yrount!
temperaturp wm SiOF.. fdloming a swiw froet. emii-mlint eiiiiilnr ron-
ditions prcrsilrd. New the surfwe :.bsolut~~ calm ~r ~w i .i l ~d unt,il nearly noon: but at .SO) trJ 1.000 ftv t B long. uniform  ~wi:11 was I-m-
countered. the waves being probably 50 feet in drpth. <J\*r ~.vIiirli the machine rode grctcd 11). and smoothly. Littit. 8. TP. -4dfZison. (1)
Ordinarily there is not more than 100 feet of turbulence on wind boundaries. The boundary se1)nrat.iii.z t,wo wincls is easily noted by the great disturhan(:e, ani1 ljunl- piness, the violence of which depends on t,he i-elovi tj- id the winds. In many inst,ances the change. o winds vi!! be just, above where the cloud layer is foriiung or slightly above the haze.-E. ill. POILWS.
 Wind lqznycrs.-For one reason or aiio tlier i t of ten 1iq1- pens that adjacent, layers of air differ ahruptly froni tach other in temperature. humidity, and density. nnd there- fore, as esplainecl by Helmholtz, may and oft,eii do glidi. over each other in much trhe same manner thnt nir ~!CJWS
over water, and wit,h the smie general ~~~.n~-e-))nl~ui.in:;.~
effect. These air mwes are seen 0111 when t,hc huiiiidit,y
perature between the crest.s and t>he troughs is suKtc.ieiit to keep the one cloud ca ,ped and trlie ot,hor free ram coii-
just right. Clear1 , then, t,hou@i such clouds oft,eii o i ~u r
eiit velocities and their consequent air hillolj-s iiius t. lie I.)!
far more frequent occurrence. This fact is ahunclantly r e d by d l types c ~f w~u-
logical work, as well as by a t,hose who asrend. iuto t.Iic? air. Kite hnll.ooiis in ascending are ofben seen to rotnt.e. pointing their noses in various directions, incliv.ztivc or
various wind directions a t different levels, yet there i m y
be no cloud layer a t the interface to marii it. Free I d - loonists make use of these layers of air, which iliay hsl:t?
different directions nnd speeds, in order to aid t.hem iii
a given destination. (2)
I U,i:nd b.ilZmm.-Wlien one layer of air runs mer an--
other of different density billows are set up hetvwii tlieni. as is often shown by windrow clouds. Ion-e-, er, t k warning clouds are comparatively seldoiii Ibreseil t, rtiirl
therefore even the cautious aviator maj-, wibli iifo e-c.i- dence of clangor before him, take the i-ery level of the nil.
billows theimelves, and before getting safely a1 jo-i-o CII.
below theni encounter one or iiiore suddcii chnngcs iu wind direction and velocit due, in part, to the etlrly-li!.-e or rolling motion within t. l? e waves, with chnnres iii ewli
case of being de)riyed of a ,ortion of the requisitr RUS-
taining force. h e r e may i e peifect safety in eitlier layer, but, unless headed just right, there netxssnrily is sonie risk in going froin one to the other. Xence, flyin? at the billow level, since it would necessitate Erqueiit. transitions of this nature, shouId he avoided. When the billot\~~ are within 300 nieteiv [l@OO ft..], say,
of the earth (often the case during. winter o:ving bo the prevalence then of cold surface air with \valnier air :h!o\;e)
they are apt to be very turbulent, just as, itnd for ~w h the same reason that, waves in shallow water arc t.urr,u- lent. times is surprisingly rough. Fortunately, hoivej-er,, it is
easy to determine by the aid of a suitihlestation 11nro::rnlh
whether or not billows are prevdent in the ~G W utl>iospliere since they produce fre uent (5 to 12 per hour rouglil~-i pressure changes, usu dh y of 0.1 nini. to 0.3 ~11111. at the
surface. (2).
I-.
a t the interlace is such that the slig T it difference in t e i ~--
densatioii. In short, t 1 le humidity condition niust l.)e
in beautiful pard ? eJ rows, adjacent wind stwta of diffrr-
achieving wliatewr result they seek, such as tli: < t.:incc 01
For this reason, presumably, winter flyin, 0 sollie-
CLOUDS AND RAIN.
Flying in clouds or above them is the choice of the nvia- tor over enemy territory more than over friendly count,ry.
How the aviator in the United States feels toward the clouds is well shown by the classification of the weather conditions under which the postal aviators hetween Wa..;hiagton and New Iork ha.re to fly (8erial Age Weekly, Ju1.v 8, 1918, pp. 816-817). In succcssion these are----ideal, fair, occa~.ional clouds, fret uent clouds, high winds, thick c.louds, t.liick c.louds ai11 I! hisli wi.iu(k, rain
st.orrns, conihination of stornis and heavy fogs. Clouds
nre genera!ly avoided because one can not see where he is going nor keep right side up. Cumulus clouds, p:wticu- 1:irlp those with hard-lookiiig outlines, are avoided became
of t,heir bumpiness, coldness, and fogginess ; also, not infrequent,ly because t-hey have falling rain. (Fig. 3.) Such clouds are usually roughest and xett,est atr their bases. An avin.t.or at4 Wichita Wls, Tes., on appronchino
n. largo cumulus clout1 rose to fly over it; but. it  towere3 to the sky! so he ile-,..- through. Once inside, he said the, cloud wa4 so dense thnt. he could iiot :we t,he t.;d of the nirpl:ine, and t,liat it. required a.11 his flying ability to keep his direct.ion. A full account of I)anger in flying through clouds, by Capt.. 13. C. Huchs, .~va.j presented to the Aeronaiit,ical SDciet,y of Great Britain, and re- printed in Scientific Amehrtn Supplement, Juuo 15, 191S,
page 375.
St.m t.o-cuniulus clouds and cumulus cl~uds with fuzzy eclgc.;: haw wealter ascensionid curren t.s and are less bunip
t o ily through. (Figs. 4 tind5.) iivintolx in flying ni?out,suc cmnulu.; clouds ciii eadv lop off c.orners and even make sriiiill c.louils ernporat,e flying through them. Soine
cumuli hnve cavities 1,000 feet high in t.heir bases. On one of the hottest Tesns clap, an a-.-iator had some trouble
17 it.h an owrheatecl engine, so he flen- in the diwonnected I:;i.t,che; of stmt.o-cumulus c1nud.s at 10,000 feet. These clou~ls were cool ani1 enioot.li. They ma.rkec1, neverthe- le.rs, the t,ops of coiivectional currents, which were pre-
wiit.ed from going higher, app:wently because of an inreixion of temperature at that level. On descending, the amit.or had c.onsiclerahle difficulty 1vit.h bumps in the l!m-est 500 feet.. Strato-cumulus cloucls which grow out of s t,ra t us on suninier mornings are not particularly lmnipy. At. learst, aviat,ors in speaking of flightss up or
tlown through them hn-re mentioned onlv the beauty of tlie tops or tIie vertical thicklieas of the cloud. FOG and str:itus clouils are red lilinclel~ for the aviatois althou li thc use of radio-clirectioiial u 3 mratus i i reducing t. e
danger of get.t.ing 1ost.I In the kbustoiiregioii, the stratus
are oft.en so low t,hat flying below t,heni i-: not at.teniptetl for fear of tress. An aviat.or above low strat.us clouds, or over a fog is in a, very difficult situat.ion if he needs to land.
h i occurrence at Love Field early in September, 191S,
ninlte.j a good 0sa.m le. While 60 aviators were in t.he air, ow-lying cloucL suc1i1enl-y appearetl and gave t i
drizzling rain a t 10 a. ni. Before it cleared a ~a y , 21 of theni hail to come down in wet fielils, and two airplanes were denirdislied hecause the pi1ot.s could not see t.o select t.lirir lnnilirig places. One lit in a pond and the other in
1% tree. Even 111
niid~~iiiiinier in Tesns, ab 7,000 feet in the air, an aviator
flying through R rain cloud felt so cold t1iti.t he described
t.hc rain drops as ice particleu. The impact of rain thops if; sufl-icient to make them feel solitl. On this occaqion, t,liere was a considerahle collection of wat.er 011
bhe airplane. The weight of rain water can hardly affect, the performance of the airplane much, uiileas the drop freeze on. In the spring of 191s an aviator in Tesas hnd such an experience during a shower; he flew up into a
g
Flying in t,he rain is avoided, if possible.
_- . . . - - -. .- .. .
1 Cf. Wireleas navigation lor aircraft, Nature (London), Sept. 11,1919, pp. 24-27.
530 MONTHLY WEATHER REVIEW. AUCIWST, 1919
cloud until at 7,000 feet his airplane had become so covered with ice that he could go no higher. The effect of rain on the propellor is destructive. The propeller
moves a t such a high velocity that unless specially pro- tected the rain drops cut it as if they were bullets. Airmen are unanimous in regard to the danger and un- leasantness of flying through rain and fog. To quote from the Scientific American, July 13, 1918, psge 26, on
The Fog Problem in Aviation:
It is hardly an exaggeration to say that at the present t,ime the only
serious outstanding meteorologiral problem of the iviat.or is fog. There is no means [exce t radio] of keeping ones hearings when fly- ing in or over a fog, au! the same is, of course, true of 1oW-lying clouds. The compass tells which way the machine is pointing at. m y moment, but not t.lie direction in which it is flying, exwpt when traveling exactly with or against the wind. There are no lnirdniarks in the ah. Even more serious is the proldem of landing in a fog. The chances are alyays considerdde of striking dangerous o\mt.ar.les. such as trees, buildings, or telegraph wires, or of alighting in boiliea
of water. avamps, etc. Moreorer. as one veteran flyer has stated: What is distrePsing to the airman in fo is the impotlsibility of knowin whether he be slightly climbing or &glitly descending, with the resuft that he nitty h c l him- sell charging full-tdt: into hills, trees, or houses without time t.n s:ti.e the aituation.
In the recent air race from New York to San Francisco, Lieut. Edward V. Wales was killed by driving his plane into Elk Mountain, Wyo., during a blulcling snow.sl-or?n. Aside from the general unplerlsantness of flying in ram, the water may cause faulty .evgme action by getting into the carburetor and the ignition.system. As to general turbulence within clouds, there is very little definite in- formation from aviators becpise the propeller so violently churns up the air in the neighborhood, and this mixing is often sufficient to mask any real small-scale turbulence that may exist there.
THUNDEBSTOEMS.
In thunderstorms, however, these turbulent conditions are of considerably greater magnitude, and are, in .fact,
BO violent that great danger is entailed m flyinga in or about them. Indeed, few aviators have flown into it
thunderstorm and come out alive. About August
1, 1918, an aviator flew mto a thun d erstorm at Paston,
Ill., and waa found dead, with lightnine burns on his body. Another aviator a year earlier, &inkin a)par-
win$ flew into the storm and was fiilles. Several years ago a flyer named Ehrmann had his mnchine set on fire by lightmng, but he escaped u n h u r t .4 . C. Turner, The Romance of Aeronautics, page 229, Philadelphia, 191.2.
It is osaihle that the actual danger from lightnhg to an aeroplitue flying tEroiigh a thunderstorm may be no more than that incurred bv a pedestrian walking acroea an open common during a storm. A d o t
who was flying above a thunderstorm last summer report.ed that \ox2 sparks were given off by this machine at intervals. It is very likely that this hap ned every time there was a flash of lighhung from t.he cloud below Em.
Aviators in the United States have also experienced such discharges while flying in thunderstorms or through ga between thunderheads.
ge turbulence within a thunderstorm is awful to expe
rience.
Expmiences.-Lightning is one dan er.
entl that the thunderstorm was oin with t i e B l ower
Capt. Cave (12) says:
A French machine was called upon to ascend during a violent thunder and windstorm for important observation work over the German lines. When at a height of several thousand feet the members of the squadron below saw the turret and its mwhine gun stripped from the craft by the gale. The observers seat was next to go, but the occupant, grasping the wing stays, clun to the sailing plane. The craft was whipped about in the sky at wilf and the cloth completely stripped from the fuselage.
Both pilot and ohserver were clinRing to their broken craft when ;.
real hell earth dtpr a seriw of gyraGons rivaliilg the most daring arro- batil:s prwtivcd by the Allied ares. Both 0ccupnnt.s escaped serious
iiijiiry.
It is quite likely that in such cases pilots and observers think very little of thunderstorm structure or the meteor- ological as ects of the situation. An aviator told this
Lielit.. lhiiin goi iiitn a thiuiderstorm once. There was lightning nll around him; hut it vas pit:.h hla~~k ot.hermise. He jerked it!l the con- trols and the ship nppnre!itly did not respond. He went, up and domi and mas ht4uless. One minut,e he monld think he ivas 0. K. and the nest. minute he thought he WVRS . on his kwk.
story of a f rientls experience:
He wts c-ertain he would be killed.
Another aviator, Lieut. Vance, says:
I \ia3 onve imight in an ap1ir;v:hing thunderstorm. I notii.ed that I w a s gniiiing altitude ver:. ra.piillp. so nosed the plane down. but rould not. lose aiiy altitmlt? until I had gained up to ~~,O O O feet. I was in tho
clouds inost of the time. I just. 1;ept the nose down, slid finally came down: then I comnienc*ed to Nettle, and rould gain no altitude even at
the masimiiin climb.
Again :
c.y:t. Bortlett u:is c:iiig!ii, in ii severe st,onn owr hrkadellhia, Ark.. niid 1;eld iit tha.t :ituntioii for :<?I miiiiiteR. His plme set.tled from ail Jtiturlc oL ii,Oi)O ic-rt to 3,iion feet. and drifter1 akJOtd ixw i d e s side-
\i v a > s, when he iiiixlly lrroke through t.he Rt,orin a!id cmne out niiles off hiscmurse lint in dr>- weather."
Lieut. 17. Davis is reported to have fallen to within 300 feet of the ground in trying to fly through a thunderstorm ne= Memphis, Tmn.
An Australian aviator, Lieut. H. W. Ellis, reports thGt he ran into an isolated storm cloud and rec.eivec1 two
~lowii~vartl buiiips, caused by clescending currents. Thc-s,. bumps were rery sewre, and the speed gauge registervII well over 90 iiiiles pcr hour. (I) Also he states that  011
one occasion (about September, 19151, a machinp gut into a black or thunder cloucl, and wits turned com- ktely round, a t the same time receiving a clownward %ump.J, As an esaiaple of the dnn prs encountered in t h
vicinity ol thuntlerstorms, tshe 9 ollowing is quo tecl from the account of Lieut,. G. S. Mason, U. S. N.:
On Jul 33, 1919, near Pensacola, Fla., machine No. W74 was in horizontal ai, -lit a >proaching a squall on the rbht. As we passed a small black-fringei cloud, a t about 1,500 feet ahtude, I felt bumps affecting the plane emanating from the clourl. I started to glide loner, thinking t o ?void thcsca apparently local hmps, when the plane eud- denly noso tliwd, so suddenly, in fact, that the ,vasoline waa thrown out of the ventliole in the carburctor. flooding everything and stalling thc- motor. I waa thrown up under the yoke of the controls and for some little time was unable to recover my seat, although I pushed up h s d to finwe myself down. The macliinc was thm in vertical nose diw, with no pressure on the flippers. I kept the flip ers neutral, realizing that when I obtained preseure on them it.woulfbe sudden andl very possibly COllalJSC the wngs. Trying the flippers occasionally, I finally got them to take hold after losing between six and eight huntlrcd fect altitude.
An aviator related that one evening in the vicinity
of Sail Antonio, Tes., local thunderstornis forced a
landing, nnd that later the flying was the bumpiest he ever esperienced. He said there was a thnilerstorm every 3 or 4 miles.
I ex erienced rough riding between Wichita Falls and Gainesville, Tex. ffy ship was tossed up and down like a leaf, falling as low aa 500 feet and rising suddenly to 2,500 or 3,000 feet.
Lieut. E. M. Powers wrote:
I have found that flying just in front of a thunderstorm is the most trying ex erience an aviator can have. The bumps are very violent and rapi$ though they are usuallv not very deep, 200 feet being the greatest. Sometimes the ship will fill with such ra idity that the mater will spurt from the vent in the radiator cap and ?have even had the aedine spurt out. However, i t has been my experienre that when the gravy rain is encountered the bumpiness decreases. On one occa-
IN. 8. Forrest, New York Tribune report, July 14,lSlS. ZFrom newspaper account October, 1018.
Lieut. Weddington reported:
-_
AUQUST, 1010 MONTHLY WEATHER REVIEW. 531
mon, June, 1918, I flew just in front of a good-sized s t o h It maa the only cloud, with the exception of a em white ones that were small, in the sky. The wind started to blow and the clouds werr rolling one over the other when I started across the face of the storm. My altimcter mid 2,800 feet and the clouds were a proximately 1,000 feet above me. It finally got so rough that I canie $own to 1,500 feet and found tlint @e air waa lots more smooth. Lster I passed just to the rear of the stoym, through some of the rain, and found that the air was pcrfecty quiet and remained so until I pmed from over the danipenedarea, when I ran into the usual hot-dsy bumpiness.
Many aviators have esperienced the great up:c.urren t which occurs on the front of the squall wind issuing from the base of a thunderstorm. Ascents of several thousand feet have been reported not only in Tesas but also in Florida. Two of the most striking ones, both from Tesas, will be cited. On one occasion, near Fort Worth, the aviators be an to return to Carruthers Field on the
for a turn while just over the squall front was suddenly lifted from 2,000 to an elevation of 7,000 feet, a rise of 5,000 feet in almost no time. He thought that his alti-
Other aviators were lifted similarly by 2,000 to 4,000 feet. Those that landed after the 50-mile squall began had to land with their propellers going full speed.
approach of a t % undeistorm. Lieut. Morgan on banking
crazy. On descending immediately hc just before the squall struck it.
-s----- -s-----
0
f
PIG. lO.-Winda experienced on thnmierstorm &ont mu Love Field, Ttx., frolu dv.
scription hg Lieut. Cobb.
In the other instance, Lieut. Cobb was near Love Field at about 3,500 feet altitude during t e approach of a thunderstorm. After cloin t h e e evolu-
ained some 3,500 feet in altitude, instead of losin 1,500 feet, as was usual. Sometiniea the lifting woulf be of the order of 500 feet in a minute. This indicates that there was a wind with an avera e vertical !ompoilent u ward a t about 5 miles per hour f 2 m/s) blowmg toward t%estorm, an upward rate quite to be espected under such circumstances. At the end, the aviators noticed shar
below them. They seemed to be a t an altitude about midway between them, thou h they were some 5 miles
tered extremely bumpy air and were able to land only by diving into the wind with the engine on (Fig. 10). The rain began immediately after that. Landing in thunder squalls presents considerable diffioulty. Once an avi- ator above such a squall, in the opposite return current, headed into the wind and made R descent to the field. He clid not observe that the surface wind was o posite he
noticed that for some reason he was not losing speed, and when only 5 feet from the ground saw that he was going wlth the wind. He was moving so fast that he
tiom, taking about 15 minutes, he foun f that he had
mammato-cumulus above and s trato-cumulus clou (P s
away from the storm front. 8 n descending they soon en-
in direction. As he approached the landing pace F
went 4wles before gettin sufficient elevation to turn and
careful observation of dust, smoke, wlridmills or trees would show the surface wind direction. The main diffi- culty of landing in a s uall wind seems to be due to its
n smooth landing impossible. Very frequently, abo, just as a landing is being made, or later, a gust will overturn the ship, even if the propeller is run a t full speed. Intwpretutioiu.-The a r movements about thunder- storms, in the lower level a t least, seem to be relatively The heavy fall of rain and the coolness of the a1p
spreads laterally in the form of a s uall. The s uall wind
is a considerable rate of ascent on the front; and the cold squall wedge also forces up the warm general wind. The air which goes down from a thunderstorm is supplied
by R return flow of low velocity above the squall. (Fig. lo.) This return takes place above 1,000 or 2,000 feet and is robablystrongest a t 3,000 feet or higher. In the
iacrease the rrtte of ascent of the air, especially when suchobstructions are in the leeof a flatarea. Theextraor- dinary up-current near Fort Worth, referred to above, occurred just in the lee of Lake Worth. An airplane in the rain under the thunderstorm is likelv to be carried down not only by the weight of the rain but also by the domnflow of the air. An airplane in the squall itself is likely to be disturbed by eddies. An aviator who wants to go around a thunderstorm will find the fastest going a t a hei h t of about 2,000 feet, between the outflowing
come back. Others have a ad similar esperiences, though
variable sustaining qua 7 ities, which nearly always make
sini?e- um er a thunderstorm produce a down-flow of air which
is stronger than the forward rate o 1 advance, an 8 so there
front o P the squall the presenceof obstructions will locally
wind be 7 ow and the inflowing one above.
AIR DENSITY C W Q E S AS AFFECTINQ SUPPORT.
On cold days with high atmospheric,pressure an air-
lane has little difficulty in taking off. For example, Papt. H. H. Storrer (1) cites an instance when with the barometer at 30.67 inches and the tern erature 30F.
the position usually employed in horizontal flights; this, of course, in calm air. 
On hot days or at altitudes of a few thousand feet, on the contrar the rarefied air often makes it difficult to
and Fort Sill, Okla., have noticed repeatedly that on very hot days there was difficulty in taking off, due to the rarefied ah. More strikin than this, however, is
rare air of higher elevations. Lieut. Nutt, of Ellington lWd, Tex., accustomed to the distance required to take off in low elevations, failed to take into account the fact that a greater distance would be re uired at a hi her
into a fence in consequence. Not only are these difficulties notired in taking off, but also tue they troublesome in landing, for the plane continues to roll along the ground for an unusual distance before coming to a stop. Landing fields at relatively high elevations should be lar er than those near sea level
like telephone and power lines, which might be permitted at lower levels.
one could ascend at a fair rate with t 1 e elevators in
rise froni t t e ground. Aviators at Kelly Field, Tex.,
the case of failure of the propel K er to take hold in the
elevation, and at Denver, Colo., Octo B er, 1918, cras B ed
and also at their edges shou f d be free from obstructions
ACmOWLEDQMENTS.
Appreciative acknowledgments are due especially to
the following Signal Corps meteorologists, formerly in
AUGUST, 1919 53 2 MONTHLY WEATHER REVIEW.
c.har e of aero1ogic.d stations at flyin fields, who did so muc % to collect information: P. W. Btkes. E. 13. Scott, I. R. Tannehill, and P. S. Wagner. Also to the Post
Office De artnient for lacin at our clisposltl copies of
sumnier o 1918; to the Navy Department for ais-
counts of some unusual esperiences of n a r d aviators; and to Dr. Griffith Taylor, of Australia, who kindly for- warded copies of his material. Prof. W. .T. Huniplwqs generously allo~-ed esteiisive estracts from his revised menustri t. Winds adverse to aviation. et#*.. to be
used. Tie photographs were sent by Signal Corpe .meteor010 ists, R. D. Rusk, S. N. Gaines, nntl H. P. Parker. fh. C!. IAeRoy Meisiiiger assistd in preparing the discussion.
Mtws~s. 2~ d ertons an %% Web 8 mail-plane logs of the
CONCLZTSION.
From the accounts of tho numeroyqesciting experie!icp aviators have because of the condltlons of t.he air, i t IS
obvious t-hat t.he more. meteorology +n aviator knows !lie
better he can handle hlmself in the illy, other thing? bemg equal. Furthermore, it is evident t.hat the ttvplcme has opened to tlie professi!mal meteorologist a new aid
ot.ent mettiis of investagating the phenonlenn of the air. kuch can be surniived from careful observstlons of cloud movements tnkcii from the grouncl; but how niucli mow
satisfectory i t is to be ith1.e to fly up nncl invsstig:%tc. personally, what is lia.ppenmg!
The Weather Bureau would be glad to receive accounts
of unusual flying experienccs mcribable to weather or air
conditions.
BIBLIOGRAPHY.
(Including publications not referred to in t,hia paper.)
(1) Taylor. Griffith. compiler: 3Ieteorological mapects of aviation, sccoiints of flying eq~eri~nces by .dudelit aviators. Austrdiaii Mont.hlv Wenther RqIOrt, July and Aiig., 1913 (published. 191.7;),
pp. 369k374: 430433.
(2) Humphrey. W. J.: Wincls adverse to aviation, etc., manii-
script eularged from paper published as Chapter IV, in A Maniial of Aero:rapliy for the United St.%tcs Navy. 1918.
(3) Flarcl, R. DeC. : bIrtewolo.gJ- and War Flying, some prii.ctica1 suggestions. MoNTriLY WEATHER REVIEW. December, 1917. pp.
5917600; also. Annals iiasoc. Am. CieoLpphere, vol. Y. 191% pp. 3-33.
(Thw is an exi:ellent clummsry fJf meteorology in its interest to aviators. and cont,ains axt.ensive IdJliogriqdiy. J Further notes by ianie author have been published in Geogt. Rw. (New Tork). 1913, vol. 5. TI. 337 and vol. 6, pp. 175-176.
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(5) Rawson. El. E.: Atmosphenc Waves, Eddies. ancl Vortices.
AerO)~uii.tirvil Jotirnnl (London), October, 1913, pp. 245-356. [Diecusses vertical movements of air, due Po topogmph y, pressure and temperature change.]
(6) T,ey: (7. H.: Vertical Mot.ion in Atmospheric Waves. dcro. Jow. ( Loudon). October. 1913, pp. 35i-25S. [On the causm of waves in the st!nosphere.]
(7) Cir;lh.tiiie-dl;hite. C . : Coriiniercial and Plmiire Flying, Atro.
Jwtr. i Loiiiloii), May, 1919. pp. 231-256. [Discusses flying in fog and abuve clouLls and the use of aerisl buoys aa giicles.]
(5) Hanimell, Gustav. : Flying: Some practical osperiences. London, 1916.
(9) Dince. W. H.: Meteorology in relat,ion to aeronautics. dcro. Jour. (London I . January, 1918, pp. 17-29; 33-35.
(IO) Linlie. Fraiiz.: hIct.rorologiache Xusbildung dcs Pliegers. Munich and Berlin. 1913. pp. 31-li7. Review. 84 A.m. Supp., Aiig. 30. 1913, 1). 144. [Discusucs effect of topography, radiation, con- vection. clouds, and niore violent disturlxmcwi, such as thunc1erst.orms. torna$ofis. etc.1
( I 1 ) Walliden. S. I,.: A?roplanea in Gusts. London, 1913. [Chap- tprs 1. 11. nntl I\ t.re.it nieaaiirr~ment ni gusts, th? erffert upon aeroplanP flight. of giiat.a, and tlie strwtiirw of gust.s.1 (131 t 'rive. (.!. J. P.: Some meteorologicd coudit,ions which increase the clnugrr of dying. Bc.lo. Jortr. (LonJon). 1917, vol. 21, p. 301.
/&-]?riiltc?d in d il~iIn11d of -hwqq?hy for the zi. d. Navy. C,hap.
sv. pp. 1-I:<-1.51.1
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( 151 (:!liui.itology :inJ :LII :!L,znrlrincd Flying School. Nature. May
:{I). 191 8. I.III. 9 li-243. [!;lying scliool :dmic\rJned because natural air ronditions wer~ I?rfJhihiti\rf?.l
i 16) 1Tiwks. Ilapt.. E. L!.: The Effects of High Flying. Aero. SOC. of Cir. Brit.. I Keiwiiitixl in yart 8 c i . Am. S u p /)., Apr. 13, 1918, 1. 227.1
(,17! IIlickS. CqJt.. 3. L: h n p r in flyiny through clou&. Aero.
SI:IP. Gr. E d . [i?lmbract in 8 c i . Am. bl~pp., June 15, 1915, p. 375.1
f IS) 1)o:iglss. ( I.
K. 31.: 3Ioteoroloqical ol:~si?rvat.ions by an aeronaut,
RPVIEW. Nli, 4S:9:<.]
1.19) 1)ou;Ias. t.. K. bi.: The IilJper air: Anine iniprcesions gained by flvlnp. Sroitkk .1k!E 8nc. h t o . .. 1918, No. 35, pp. 3-12. [Nature, 103,
Fkh. 13. 1919, lip. 47:3-47G: S e i . d h s . 3Jd, lBltl,] (201 Ihrglas. (2. K. hl.: Th!. 1:1ps~ lins ancl its relxtion to cloud for- mation. S e k d i d f c t  l 8 :.x Joitr.. 1917, No. 34, pj). 123-1-17.
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(24) Huniphrcys, W. J.: P1iysii.s nf the air. Jour. FrutiXZin Inst.. 1917, f!/f8, 1!)1!1. I All-rowit1 c1inci:ssiun of meteorolwgicl physics.]
ayrOf\iSk i&// .J.W/., 1916, KO. 3% [hljStritCt IlOXTHLS \vEATHElt
[C+enerd clisciiasion .
[To face p. 532.1