JUNE, 1988 MONTHLY WEATHER REVIEW . 221
TEMPERATURE INVERSIONS AT SAN DIEGO, AS DEDUCED FROM AEROGRAPHICAL
OBSERVATIONS BY AIRPLANE
By DEAN BLAKE
[Weather Bureau Omce, San Diego, Calif., May, l9%]
With the development of apparatus for the explorat,ion of the atmosphere, some of the older conc.ept,ions of tem-
perature and wind c.onditions in t,he upper air have been
altered or greatly modified. It will be recalled that the
theory of a constant temperature decrease in the region of the upper air now known as the stratosphere, has been mqterially c.hanged sinc.e Teisserenc de Bort in 1S98 car-
ried on his epoc.ha1 soundings. Wind veloc.it,ies and direc-
tions also have been found to depart mat'erially from the
expected, and in the realm of met'eorology there is no field more fertile for investigat,ion bhan t,hat of the upper air.
Since observations were begun by means of airp1ane.s and pilot balloons in San D i e p and vicinity, unlooked for
tem erature and wind variat,ions have been found. Eary P in the history of aviation, pilot,s were at, a loss to
explain the causes of the remarkable t,emperature inver-
sions encountered aloft, and as the number of pilot balloon soundings increased the existence of unexpected
and inexplicable air cmrents was discovered. Besides the desirability of iniprovirig forectLst,s for
aviation, and widening the scope of our knowledge of aerology, other reasons have prompted the preparat,ion
of this paper. In the first place there has been a spec.ific request for the information it contains. Airplane maneuvers a t the
naval air station last summer met with such high teni- peratures a t elevations bordering the 1,600 meter level
that this year it has been proposed that they be held at a level where the air-cooled motors would not overheat.
Again, temperature and humidity are vital factors in the
handling of dirigible balloons, and accurate d a h are
necessaiy, Especially is this true in t,he vicinit,y of San Diego, as the Navy Department has begun det,ailed
ground observations in the county for the selecbion of the
most suitable site for the establishment of a base for light-
er-than-air c,raft.
TOPOGRAPHY AND C L I U T E OF SAN DIEGO COUNTY
San Diego County lies at t.he extreme southern end of California with Lower California adjoining on the sout,h. To the west the Pacific Coast stret,ches from the nort,hern
to the southern boundary. Exce,pt for a narrow ship
alon the coast, it is rough and mountainous with iso- late f valleys and mesas of limited extent. The nioun-
tains, however, are only moderately high, few peaks ris-
ing over 6,000 feet, and their eastern slopes abruptly
drop into the desert beyond. Climatically, the littoral strip is mild and equable
with a light rainfall that is confined t'o the winter season. Its infrequent high temperatures are the produck of
dynamically heated air currents, and therefore are
accompanied by low humidities. The most dominmt
characteristic is the night and morning strata of "high
fog," which persists from May to October with remark- able regularity, and is to be found less frequently during
the rest of the year. As this proteching veil extends but a few miles inland, the mountrains, mesas, and valleys are much warmer in summer and cooler in winter, with large
daily ranges in temperatures most of the year and increas- ing amounts of preci itation with elevation. Beyond,
that is, extremely hi h temperatures in summer and mod-
erate in winter, and fittle or no rain a t any time.
the great interior Val f eys have typical desert climates;
AEROGRAPHICAL FLIGHTS AT NAVAL AIR STATION
Observations of temperature, humidity, and pressure
in the upper air have been made by airplane at the naval
air stmation on North Island, San Diego Bay, since Jan- uary 1923. An aero-met,eorograph of the Friez type is
in use. To keep the data comparable each flight is made along the same general rout8e over San Diego and it,s
environs, and elevation is made slowly so that the ele-
ments will hare hime to bec.ome properly adjusted to t8he changing conditions. The climb is maintained unt,il the
temperature decrease bec.onies regular or the inversion
ceases, which is usually bet,ween 2,100 and 2,500 meters. Until January, 1928, the aerograph was carried by the
0bserve.r in the c.ockpit of the airplane. At pre.sent it is exposed bet,ween the upper and lower right wings, and
fastened between the two outer struts in such a manner that the vibration is no gre,ater than before the change.
In order that an intimation of the variations between
the two exposures might be obtained, the writer on May 2
made a fli h t with the two aerographs used a t the air station. 'fhese particular records showed a slight time
lag in both the hygrograph and the thermograph by the
instrument in the cockpit, and a noticeable tendency not to register the extremes. Undoubtedly, the wing exposure, which is away from heat from the engines, is
much the best. All records used in this paper have been made avail-
able through the kindness of the commanding officer of the naval air station, Capt. F. R. McCrary; the aero-
logical officer, Lieut. W. K. Berner; and the chief aerog-
rapher, A. A. Stotts and the personnel of his offic.e, who have done everything possible to assist in its preparatjon.
The writer wishes to express his appreciation of the many
courtesies extended, and the opportunities present'ed for obtaining firs t-h and information.
It is unfortunate that a more continuous record could not have been made. There are many reasons why
regular observations have not been practicable, the chief being the lack of available airplanes, unfavorable flying conditions, and the closing of the ste.tion on Sundays and
holidays. To April 1, 1928, 250 aerographkal flights were made. Their distribution by years, months, fore-
noons, and afternoons is shown in Table 1. At present
the record is being taken in a regular and systematic
TABLE I.-Distribution of aerographic Jights
222 MONTHLY WEATHER REVIEW JUNE, 1928
way at 10 a. m., but in the earlier flights no set hour was consistently used. Efforts are made to obtain pilot balloon soundings at or near the time of the aerological
“hops,” as they are known on the island, so that syn- chronous wind velocity and direction data are available.
In the sumniarization of the data, the temperature gradients found aloft have been divided into three classes;
those with a regular decsease with increase in elevation,
marked inversions, and slight inversions. For con-
venience of classification, a continuous rise in tempera-
ture of more than 5’ C. (9’ F.) has been arbitrarily designated a marked inversion, and when below this
range, a slight inversion. The distribution of the three groups by months is given in Table 2.
TABLE 2.-Distribution of flights showing marked inversions, slight inversions, and regular decreasing temperatures
Marked inversions _-----__-- Slight inversions. -. -.-. - - - - . Decreasine temoeratures -... .=
Total ._______._._______
1 22 1 32 1 2 4 1 R ~Z (/P P ~2 1 ~P ~1 7 1 2 i )/1 8 /1 R /2 5 0
Owing to their paucity and lack of continuity, no at- tempt has been made to segregate the date into forenoon
and afternoon readings.
It is a t once apparent from the table that the tempera- ture inversions encountered aloft are slight during the winter months and well-marked during the summer
months. In fact, every observation from June 1 to
October 1, save one, showed an increase a t some of the
levels.
CAUSES OF WINTER INVERSIONS
r h e inversions during the colder months are readily
explained by (1) the presence of clear skies and still air causing a net loss of surface heat by radiation; (2) the
importation of hot, dry air above the surface layers; (3)
the occasional overspreading of the land areas by a stratum of relatively warm, nioist air drawn in from the
ocean.
The first type cited occurs during periods of clear, cold anticyclonic weather when low temperatures prevail a t
the surface, due to rapid loss of heat by radiation. Under thebe conditions a slight increase in temperature some-
times is found as high as 500 meters. Inversions from this source give us in southern California a better under-
standing why the so-called “frostless belts” and many of
our flourishing and unprotected citrus groves are located
at elevations between 500 and 1,500 feet.
Contrary to expectations, the importation of dynam- ically heated air appears to be a minor cause of inversions, as it operates only when the warm winds have not reached the surface, which, consequently, is relatively cool. Even
then the increases shown are not pronounced, and there
is normally little change in temperature to great heights. Air currents of this nature are produced when high pres-
sure overlies the Plateau States, and low barometer is centered over extreme southwestern California or is in-
dicated in the Pacific Ocean off the coast.
When the distribution of barometric pressure is such as t,o cause
an indraught of ocean air to overspread the land, a rise in temperature is generally experienced in this layer.
For example, the only marked inversion that was recorded
Regarding the third type of winter inversions.
in any February occurred on the 25th, in 1927, under these conditions. The aerograph trace sheet on that date showed a steady rise of 7’ C. (13’ F.), with an accom-
panying increase in relative humidity of 12 per cent to
500 meters, after which it decreased gradually. Some- times, however, a decrease is encountered for several
hundred meters, which is followed by rising temperature
until the top of the strata of ocean air is reached, after which the lapse rate becomes nearly normal.
SUMMER INVERSIONS AND THEIR CAUSES
But a more complex problem is presented by the in-
versions encountered during the summer flights. Their very regularity is puzzling. The explanation that has
been advanced generally has been expressed by Chief
Aerographer J. W. Thomas in the following:
At that season (the summer months) the southwestern semiper manent “Low” is at its greatest intensity and it seems to be the concensus of opinion that the inversion is due to the outflow of heated air from the valleys of the interior * * *. The above is a tentative conclusion deduced from observations and subject
to modificat.ion. We have found that although the greater num- ber of pilot balloon soundings made in connection mth the aero- graphic flight do show a layer of wind with an easterly component at practically the same elevatiou as the inversion, nevertheless there are several records showing an inversion while the pilot balloon ascension made at the same time shows a solid west and northwest wind from the surface t,o an altitude considerably above that attained in t.he aerographic flight.
In Figure 1, the temperature lapse rate, the humidity
curve, and the prevailing wind directions for every 100 meters up to 2,500 meters, derived from 35 afternoon flights during July and August, 1924, are plotted. As
anticipated, the lowest temperatures were reached a t the average level of the top of the clouds, the decrease for
the first 500 meters averaging 0.6’ C. per 100 meters, or 0.4’ C. less than the adiabatic rate for dry air. From
500 to 1,250 meters a rise of 7.2’ C. is shown, and from
1,250 meters on the temperature fall is a t the same rate
as the inibial decrease, or 0.6’ C. for each 100 meters. Surface temperatures corresponded generally with those
found a t 1,800 or 1,900 meters.
~
I Thomas J. W.: Aerological work at the naval air station at San Diego. COnImn@3 on the Phy’8ieal Omnography and Marine Meteorology of the Northeast Pacific and
the Climate o l the Western Part of the UmM States (p. 23).
JUNE, 1928
~~________
N ..................
NE ................ E .................. SE ................. 8 ...................
sw ................
w .................
NW ...............
MONTHLY WEATHER' REVIEW
~~_____~.. -
12,ixlo 1 3 ,m i4.m) 1s. MKI 16. no0
-~~____~~~
.................................
..........................................
..................
4 3 1 1 1
..................
................. ..............................
.......... ........................
223
Naturally, the relrttive hyniidit,v and tempera.ture
curves parallel en.ch ot,her rat,her closely but, in an in- verse sense. With a fall in temperature aiid a consequent decrease in the capacity of the space for water rapor,
the relative humidity would increase uiit,il t,he hen-ipera-
ture began to rise, when it would decmase. Although summer averages aloft are comparahle to
temperatures reported at stations back from t,he Goast
at intermediate elevations, lit,tle or no clnily siinilnrity
exists betw-een the tivo, and, except in cases of wide- spread warm waves over t,lie district, t>emperatures in t,he free air seem independent, of surface condit,ions eit.lier in
San Diego County or in the great valleys beyond. It may be of interest to not that the niasimuin inversion
encountered was 16'C. (29O F. j on August 3s 1924, and the highest temperature registrered a t any elevation was 35.6'
C. (96' F.) at, an approximate height, of 1,300 meters. There is one relation, however, hetween t.he free-air
and surface teniperat,ures that st,nnds out prominently, i. e., marked inversions are almost cert,ain to be followed
by fog or low clouds every ni.ont,li in the year. This
relation, though, is not confined to this locnlity, hut l i ~s
been found to obtain over other parts of the Californitt
littoral. Wright in ft study of t,he fog conditions n.t Mount Tamalpais observed t,hat " * * * the t,ein-
perature of the upper air must be higher than bhat of the lower to produce proper conditions for fog forintition,"
and hlcAdie in Bulletin L writes that, "fogs seein to occur a t times of steep inverted gradients." As no figures showing the percent,ages of winds from
the land and sea a t the various levels a.re available, and no discussion of the subject is complete without, them,
Table 3 was prepared from t8he 1 p. m. pilot balloon
ascensions during June, July, and August,, 1924-1937,
the same summer periods covered by the aerological data.
TABLE 3.-Summary of pilot soundings during J w e , July, and August, 1924-1927 (in iiictersj
face 500 1. GIN 1. 500 2, OOo 2, 500 3. M)o 3, 50fJ 4, OOo 4, GO1):5, NHl
I s u r -!/ 1 1 I I I I I I
N.: ....................... NE .............................
E ................................ SE ........................
s .......................... SW ....................... W ........................ NW ...................... Calm ............................... ......... .....
Total ................... Sea, per cent ........
...... Land, per cent 40
Total ............ 54 Sea. Land, perwn per cent. t...l 611 39 43 E! 31 50
$1 38 7 i i 25 ;$ ,5( 20 141 1 4 1 ~O Z 0
Even the most c.ursory e,xamination forces us to draw
several obvious conclusions, namely : ((I .) That at rirtu- ally every level, winds were from the ocean tlie larger
percentage of the time; (b ) that, betwe,en 2,000 and 10,000
meters the percentages from hhe land and ocean remained
2 Wright, Herbert H. Fog in Relition to Wind Direction on hlonnt Tnmalpais,
a MbAdie, Alexander Q., Climatology oi California, Bull. L., pp. 241. Calif. Monthly Weather Revww vol. 44 pp. 342-314.
5021-28-2
fairly constant; (c) t'hat, beyond 10,000 meters the few
soundings obtainable showed an increasing frequency from the, ocean; ((7) t,liat the prevailing direction at all
levels WR,S southwest; (E ) that the northwest currents believed to predominate at t,he higher elevations, summer
as well as winter, we,re not in evidence. Altliough a chart has not been prepared for velocities
during t,he same period, it was further observed that at the
soundings under 2,500 meters they were rarely other than light, and whe,n from the eastern or land quarter were more
in the nature of a drift than a carrent.
If we can make our deductions from four year's record,
then the,re must be otlier causes for the steep inverted
gradients besides an overflow of hot air bo the c,oast from convectional ac.tion in the Iniperinl and Colorado Valleys,
as inversions oc.currec1 at every observation regardless of
wind direchion. Hot weather in sout,liern California, in summer as well
as in bhe ot,her seasons, is almost always caused by dyna-
mically heated winds that have their origin in anticy-
clonic areas over tlie Plat,eau States. A relationship
be,tween easterly winds at high elevations a t Los Angeles
and unusually high surface temperatures on the following
day has occasionally bwii noted and referred to in several of tlie monthly free-air summaries. (See Aerological
Observations, Monthly Weather Review, p. 244, May,
1927.) Occasionally, when the pressure gradients are unusually steep, these winds extend to the o:ean, but
normally the sea bre.eze modifies the temperature near t'he coast.
When such pressure condit'ions prevail, winds are from the eastern qudrttnt a t most levels irrespective of the siirfa.ce direction. Thus there appears to be a circula-
bion between land aiid sea which has been confused with
the daily land aiid sea breeze that is so prevalent in many
regions. In summer no well-defined interchange has been discovered, but,, its Hann expresses it:
The .sea breeze npon the coast of sout.hern California is, however, a wind which partakes rnt,her of the charact.er of the monsoon, because it 'is an effect of the prevailingly higher temperature in t,he inferior of California as compared with t.he ocean.
Ham's conteiit8ion is further proved by the fact that all
mountain stat,ions show prevailing winds from the west during t,he ma.rnier inon t,hs of the year.
It is highly probable that air movement from the east
is given impetus at times by currents originating in the desert, regions, but the. generally accepted idea that con-
vection is the cause of winds, from that direction is not
ten able.
Lit,toral California enjoys it,s remarkably c,ool summer
climate because of its prosiniity to the ocean. B s the inland regions he,conie he.at>e,d an indraught of cool, moist
air sets in which is m:iintainecl all summer. Consider-
able cloudiness results that further screens the land from
the sun, and it does this so well that the mean tempera- tuws are lower than any in t'he entire State, except those
report)ed from the highest, elevations. A distance of only
n few miles froin the shore line makes a great difference, ,
and we find monthly averages between 10 and 20 degrees
higher at stations a dozen or so miles inland.
There are re,asons for believing that .this stratum is
wedge shaped, dee,pest over the ocean and thinning out over the land. Optical haze, due to sharply defined tem-
perature layeis, and the observations of aviators seem to just,ify such a conclusion.
Unquestionably, then, the explanation of the inversion
lies in the presence of an abnorinally cold stratum at and
~~ ~
4 FIann, J. Handbook of C'limatology. translated by R. De C. Ward, pp. 16&157.
224 MONTHLY WEATHER REVIEW JUNE, 1928
immediately above the surface extending to about 800
meters, and not in an anomalous air current aloft. Wyatt
and Lawing have stated that inversions were not en-
countered in the few ncrologicd trips t81iey niade over tlie ocean and the Imperial Valley.5 This is in arcordance with my conclusions; they shoultl bo found only ovcr the
narrow strip para1le.lmg t'he coast'.
CONCLUSIONS
Temperatures aloft in other than the summer months
normally decrease with elevation. or, occasionally, there is a sniall inversion from onc of three causes-radiation
and conduction in the layers just above the ground; importation of warm, dry air; or an indraft of relatively
warm, moist air from the ocean. Inversions during the suiiinier are of regular occur-
rence, usually following a drop in temperature from tlie
surface to 500 meters. The highest temperatures occur near the 1,250-meter level, where they average about 4' C. higher than a t the surface, and temperatures comparable
to those a t the surface are reached at 1,800 or 1,900
meters. That the suninier inversions are not caused by a n over-
flow from ascending air currents in the desert valleys to
the east is evident. Prevailing wind directions a t all
heights are from the ocean. Occasionally a drift from the east is observed when temperatures in the interior are unusurtlly high, but as these high temperatures are
caused by anticyclones over the Plateau States, then the winds from this direction are also the result of this same
pressure distribution. Bs already stated there is a stratuni of relatively cool
air of oceanic origin over the littoral districts. This stratum is overtopped by warmer air of continental
origin that slowly drifts oceanward in the hot si5ason.
These conditions are brought anout by the broader. rela-
tions of marine and continental climates and may be found in their fullest development in the border zone
between the two climates.
DISCUSSION
Chairman Lastreto felt the paper was too technical for a layman like himself to discuss, though he confessed to
I Lieut. B. H. Wyatt and M. R. Lawing. Discussionof Pspersin Bulletin Qni. Aletl. Society, Nov., 1923. Pp. 154-157.
a deep emotion a t the ability with which many old theo- ries had been blasted.
hlr. Gordon woiiderecl where the rising hot air goes from Tunia; i t goes up and should come down some-
where, but where?
hlr. Blnlie stated that Sonorti storins are caused by a
meeting of the sea and land winds over the niountains; but just how far westward tlie convectional currents out
of the Imperial Valley extend is unknown and is a prob- lem that needs stiidy.
hZr. Gordon said there should be currents aloft out of tlie valley in practically all directionr.
Mr. Blake replied that strong convection over the Val-
ley had not been experienced by flyers whom he had in-
terviewed. No extreme bunipiness was reported a t the elet ations where i t is usually found. hlr. Young asked if different directions of wind were
found at diflerent times of the clqv from tlie various
pilot-balloon obsermtiona, to which Mr. Blake replied that while lie used the 1 p. 111. observations, the aerog- raphers at North Isliind beliex-ed that there was little
difference cluring the 24 hours. Nocturnal data, however, were not avltildde and therefore the answer was in doubt.
hlajor Bowie explained that by means of numerous
pilot-balloon reports received in the San Francisco office from stations in the southwestern quarter of the United
States i t is a siniple matter to construct the isobars for various elevations up to 4 or 5 kilometers. Thus the
changes in wind direction at various levels, ranging from
an inflowing circulation a t the surface to an outflow-
ing circulation aloft, indicate that as we strip off the isobaric surfaces over the interior, one by one, the LOW
a t the surface gradually gives way to a HIGH in the upper air.
At the higher elevations the barometric gradient is actually outward instead of inward. We are in such a
case dealing with it theoretical tliernial cyclone as de-
scribed by Ferrell. The explanation of the wind circu- lation aloft a t San Diego is very easy, it, seems to me;
the air is moving in a wide circle, and while it may orig- inate in the region to the southward of the Imperial
Valley i t passes seaward in a circuitous route aloft, and when observed a t San Diego is moving in a clock-
wise direction, becau'se a t high levels i t constitutes the
outflow of air from the anticyclone capping the low- pressure area, which exists only in the lowest atmos- pheric strata.
THE MEASUREMENT OF SKY COLORING
By FRANZ LINKE
(Frankfort on the Main, Germany]
In the year 1922 I approached Prof. W. Ostwald, of
Grossbothen, wit>h a request for the making in his
laboratory of a bechnical, well-defined, and certainly reproducible blue scale for the estimation of t>he color of the sky. With a well-known obliging int,erest in all
applications of his color lore Professor Ostwalcl under-
took the task and put a t my disposal a rablier large. number of copies of a blue scale in seven parts, which showed logarithmic t'ransitions from pure whit8e to ultra-
marine blue. I then numbered the pure whibe 0 and the ult,rarnarine 14, so that t'he even numbers indicated
the several color st,eps of the scale and the odd numbers
interpolations. Sinc.e that time the scale has been em- ployed a t many places for the e,stimation of the coloring
of the sky. Professor Ostwald himself reports on the
color-technical principles of this blue scale, so that I
have only t,o malie statenien6s on the method and purpose
of the observations ancl bhe results to date.
Hethod of oherrat ion..-The observer places himself with his hack to the sun and observes for a t least 30
seconds t,he bluest point in the sky, which is 70° to 90'
diqtant from the sun in the direct)ion of its meridian.
Wit'hout removing his eye,s from t'he sky the observer arbitrarily opens t,he scale a t a tone and quickly brings it, into the range of the eye so that i t comes into the light
of the sun. After solile pract'ice, even when the exact coloring of the sky does not cont'ain white and blue only,
the 0bserve.r fornis an opinion whether the blue tone of
t'he scde is lighter or deeper as c.ompared with the blue
of the sky. The scale made up in book form is then turned until the observer either finds a color tone in sufficient agreement with the coloring of the sky or is