October 1970 745
UDC 651.&€ 19.6’3:551.576.1:629.135.2(798)
ON THE POSSIBILITY OF WEATHER MODIFICATION BY AIRCRAFT CONTRAILS
WALLACE B. MURCRAY
Geophysical Institute, University of Alaska, College, Alaska
ABSTRACT
The possible effect of contrails in modifying the weather is reconsidered in the light of information obtained from
ground-level contrails in Alaska. It appears likely that inadvertent cloud seeding b y jet aircraft may be of the same
order of magnitude as that attained in commercial cloud seeding operations. Further investigation is needed; but in
the meantime, the possibility of contrail contamination should be kept in mind when evaluating the results of seeding
operations.
1. INTRODUCTION
Aircraft contrails first attracted public attention during
World War 11; but as air traffic has built up to its present
level, they have come to be accepted as part of the environ-
ment. Even during World War 11, it kas difficult to watch
the cloud cover laid down by a large bomber formation
without wondering what it might be doing to the weather;
at present, there is widespread belief among the general
public and some feeling among scientists (Fletcher 1969,
Reinking 1968, Livingston 1969, and Schaefer 1969) that
contrails are increasing cloudiness, if nothing more, in
some regions. The writer himself has seen instances in
which a single contrail seemed to grow until it became an
overcast covering the whole sky. If the con trail were indeed
responsible, which is by no means certain, this would
constitute definite proof that contrails are capable of a
significant effect on local weather, and even possibly on
global climate, if such occurrences are widespread and
frequent. It seems worthwhile, in view of all this, to
consider quantitatively whether OT not there is reason to
believe contrails are capable of exerting a significant influ-
ence on weather.
2. OBSERVATIONS ON THE COMPOSITION
OF THE CONTRAIL
Any quantitative discussion should start with the
contrail itself. A medium-sized jet airliner-the Boeing 727
to be specific-burns over 3100 kg of fuel per hour while
cruising, producing over 1.2 times as much water (more
than a kilogram per second). It is this mater that forms the
contrail, and a discussion of the basic processes by which it
does so was given by Appleman (1 953). His discussion also
gave a fairly accurate description of the conditions under
which a contrail will be formed, but did ndt discuss the
nature of the contrail particles further than to specify that
they are ice crystals. Some attempts have been made to
sample the contrail particles in flight, but there is always
skepticism as to how well the particles collected under such
circumstances represent those in the contrail. During the
winters of interior Alaska, however, conditions are
frequently such that a contrail is left while the aircraft is
taxiing and taking off or landing so that it is easily
accessible for study. (On the ground, the contrail is called
ice fog, and it can become a serious problem in flight
operations.) This discussion is based on observations made
under the direction of Ohtako (1967; see also Huffman
1968) during the course of investigations of ice fog from
this and other sources. As a result of these observations,
enough is known about the way the contrail is formed to
make it safe to state that the contrail formed at cruising
altitude is very unlikely to differ greatly from that formed
on the ground.
Ephemeral contrails are of no particular interest here,
although a great deal can be learned about conditions in
the atmosphere by observing their formation and disap-
pearance ; the discussion will therefore be limited to those
formed while the aircraft is cruising in atmosphere at least
approximately saturabed with respect to ice and at a
temperature of approximately -40°C or lower. When the
jet exhaust enters such an atmosphere, it is very rapidly
cooled by expansion and turbulent mixing, and the water
vapor condenses in the form of very small droplets. Al-
though the cooling is so rapid that levels of supersaturation
at which homogeneous nucleation could take place should
be reached, electron microscope photographs of the
crystals show no evidence that it ever happens. The
droplets all appear to form on nuclei, which are abundantly
provided by the exhaust products, even if not otherwise
present. The liquid droplet phase is very short-lived-of
the order of a second-since the ambient atmosphere is
below the spontaneous freezing threshold. The droplets
freeze, and the result is a cloud composed mainly of the
small, roughly spherical crystals called “droxtals” by the
Stanford group who first reported them (Thuman and
Robinson 1954). The time before the cloud is completely
frozen is much too short to allow mass exchange to take
place between liquid droplets or between liquid drops and
ice crystals, but there may be some crystal growth while
the environment is between water and ice saturation. The
end result seems to be dependent mainly upon ambient
temperature and little upon details of the way the exhaust
746 MONTHLY WEATHER REVIEW vola 98, No. 10
enters the atmosphere, the ice fog produced by an auto-
mobile exhaust being practically identical to that from a
jet engine. One therefore feels quite confident in using the
results of the ground observations to ,discuss the contrail
formed a t cruise altitude.
Nearly 80 percent of. the crystals collected near the
airport runway had a diameter near 4 I.(. All collection
methods are open to the suspicion of weighting in favor
of larger particles; furthermore, very small particles
would not have been detected a t all. Many of the crystals
collected showed evidence of having ruptured and emitted
a spurt of water as they froze from the outside in (Bally-
Dorsey effect, Blaflchard 1951) ; this may have produced
crystals too small to be detected, though there is no evi-
dence of this effect. At any rate, assuming spherical
shape and 2 p radius gives an average reciprocal mass of
approximately 3 X 1O1O droxt'als gm-', which is believed
to be a very conservative lower limit,.
Auk CRYSTALS AS FREEZING NUCLEU
A cruising Boeing 727 then produces 3X10I3 crystals
sec-I if all of the water vapor produced by the fuel com-
bustion goes into crystal formation, as probably happens
under the stated conditions, or if only a third of it
does. (The writer feels that the actual crystal production
rate is probably at least 1014 and possibly ioi5 sec-l.
This seems to be confirmed by the area and optical depth
of the contrail.) Except for hail suppression operations,
large-scale weather modification attempts currently in
progress employ ground-based generators to introduce
freezing nuclei into the atmosphere. Under these condi-
tions, silver iodide, the nucleating material most generally
used, is considered to deliver 1013 freezing nuclei gm-I
of silver iodide on the average. Under optimum conditions,
it may deliver as many as 1015 nuclei gm-'; under adverse
conditions, it may deliver none at all, since silver-iodide
nuclei deteriorate rapidly in effectiveness after entering
the atmosphere. I t can be seen, then, that a medium-
sized jet forming a persistent contrail is introducing
nuclei in numbers equivalent to ground-based generators
burning more than a gram per second of silver iodide.
Since silver iodide is introduced with the object of forming
ice crystals from supercooled water droplets, it seems
obvious that the contrail ice crystals can produce any
effect of which the silver-iodide nuclei are capable, and
in addition can do things of which silver iodide is not
likely to be capable, such as extracting water from air
supersaturated over ice but under water saturation.
(Silver iodide apparently can be effective in the absence
of water droplets by acting first as a condensation and
later as a freezing nucleus, but there seems to be some
doubt about how efficient it is in this.) It is then evident
that, whenever and wherever persistent contrails are
being produced, the airlines are conducting a seeding
operation in which each passing aircraft deposits nuclei
in numbers a t least comparable to a ground-based seeding
attempt. The coverage is global in extent, and while
random in the sense that airline flights are not scheduled
with weather modification in mind, it is not really random
with respect to meteorological conditions. Airline opera-
tions are affected by the weather, and such things as
pressure pattern navigation for instance could well intro-
duce a significant bias. The formation of a persistent
contrail is itself a function of ambient conditions that
would seem to lead to the crystals being deposited pref-
erentially in situations favorable to modifications of the
weather being produced.
ODlFlCAUBON BY PROCESSES
OTHER UWAN SEEDING
The contrail crystals are so small that they fall very
slowly in still air, even at j e t cruising altitudes, and in the
presence of even slight updrafts will not fall at all. Under
some conditions, it seems likely that they can accumulate
sufficiently to cause some high cloudiness along a busy
airway even if they remain essentially unchanged in size
and numbers after they are first introduced. This, how-
ever, seems to be marginal, and it is unlikely that simple
accumulation, even if accompanied by modest growth, is
causing much effect at present. There is, however, good
reason to believe that some mechanism exists that results
in crystal multiplication.
Careful observation of contrails shows that, in general,
their behavior is consistent with the higher rates of crystal
production given in the discussion above; but in many
instances, the contrails attained dimensions that are very
difficult to explain by simple growth of the existing crystals
unless the numbers given here are very gross under-
estimates. The crystals in the contrails that exhibited
this behavior were obviously growing very rapidly, since
well-defined fall streaks developed from them in less than
half an hour; so they were evidently in air highly super-
saturated with respect to ice. (Natural air can become
very much more highly supersaturated with respect to
ice than it ever does with respect to water because of the
very low content of nuclei capable of removing water
vapor from the air once it is below water saturation.) It is
also obvious that, if the contrail crystals really did produce
the complete sky coverage in the instances mentioned in
the introduction, some rapid and efficient process of
crystal production must have been triggered. (It is quitc
evident that systematic and quantitative observation of
contrails can lead to a considerable increase in our knowl-
edge of the meteorology of the 200- to 300-mb region and
of cloud physics in general, in addition to the value of
such observations in connection with the subject of this
paper.)
5. GENERAL DISCUSSION AND INFERENCES
It has been shown above that there is good reason to
believe that aircraft contrails may be capable of con-
siderable weather modification by producing cirrus clouds
a t altitudes close to those a t which the aircraft is
cruising. It is obvious that the contrail crystals may in-
Ocfober 1970 Wallace B. Murcray 747
fluence cumulus development by getting into clouds that
extend to and beyond normal jet cruising levels. A little
reflection will show that there is also good reason to believe
that contrail crystal seeding may be producing all the
effects of which deliberate seeding with freezing nuclei is
capable. Crystals falling from cirrus clouds through what
appears to have been a t least moderately dry air have been
observed to survive for astonishing distances (Braham and
Spyers-Duran 1967). The contrail crystals must grow consid-
erably before they fall, but they are frequently seen to develop
respectable fall velocities within a few. minutes after they
are formed. Whether or not they are capable of surviving
a long fall through dry air, it is likely that they are often
deposited in air that is saturated with respect to ice all
the way from the altitude of formation to the cloud bases.
Under such conditions, they will of course at least survive
to the melting level, and it is likely that they will grow
and possibly multiply. Under such conditions, they can
enter clouds anywhere above the freezing level and
possibly for some distance below. (It is difficult to see
how they could be injected into a cloud in a manner
that would exactly duplicate the conditions of a seeding
flight through it or a silver-iodide shell exploded inside,
but not so difficult to see how the contrail crystals might
produce essentially the same results.) I t is not intended
here to undertake an exhaustive discussion of the possible
effects of cloud seeding by contrail crystals, but one or
two possibilities seem worth mentioning.
The orographic cloud situation in which a wet airmass
is being driven upward over a mountain range is con-
sidered to be one of the more favorable for increasing
precipitation by ground-based silver-iodide seeding. This
situation, however, leads to an airmass that is rising and
cooling over a region that extends for a considerable
distance upwind and to altitudes well beyond those at
which jet airliners normally cruise. It is likely, therefore,
that the air will be at or near saturation, at least over ice,
from the cloud base up, and that a persistent contrail
will be formed that can and will seed the clouds. When
one considers the number of airline flights over an area
such as that west of Denver, Colo., one immediately
wonders what the airlines may be doing to the statistics
used in evaluating the results of commercial seeding.
(It should be noted that the writer is not contending that
either or both operations actually do increase precipita-
tion, he is merely pointing out that both should be
considered.)
The plains region of the United States is traversed by
heavy air traffic; in the summer, it is also heavily populated
by towering cumulus so that a jet aircraft is often flying
among them. The flight path rarely goes through the
clouds because of the turbulence and the likelihood of
eiicountering hail, but the whole region is likely to be
saturated with respect to ice. The air is also likely to con-
tain considerable numbers of natural ice crystals, so it is
not immediately obvious that the introduction of the con-
trail crystals can have any effect. However, while riding
an aircraft through this region, one sees numerous con-
trails left by other flights, so it is evident that the natural
ice crystal concentrations are much exceeded along the
flight path and that this condition persists for some time.
Schaefcr (1969) mentions this in discussing inadvertent
weather modifications. He implies the seeding capabilities
but does not discuss them specifically. The probability
that a developing thunderhead will entrain such a high
ice crystal concentration is thus increased; and a little
calculation shows that, if the Russians and the French are
right, it could lead to suppression of destructive hail.
There seems little doubt that contrail crystal seeding
sometimes disperses clouds in the same manner as seeding
with Dry Ice' does. Over the years, many instances of
cloud dispersal by aircraft have been reported in Weather,
and many very good photographs have been published.
In most instances, not enough data are available to form a
firm opinion of how the dispersal was accomplished, but
the resemblance of the photographs and the descriptions of
the occurrences to those of the results of seeding super-
cooled water clouds with Dry Ice is so striking that it
seems safe to conclude that most, if not all, of the reports
are examples of the seeding of clouds by contrail ice
crystals. The possible consequences of this are consider-
able; in fact, i t seems probable that one of the projects for
modifying the global climate discussed by Fletcher (1965),
namely modification of the cloud cover over the North
Polar Basin by cloud seeding, is already underway, al-
though the scale is still more modest than he envisioned.
6. CONCLUSIONS
The conclusions rcached as a result of the above dis-
cussion are that nearly all results that can be produced by
seeding with ice crystals arc in fact being produced as a
result of routine airline operations; that the effectiveness
of ice crystals as nuclei over such a wide range of metero-
logical conditions and the scale on which they arc being
deposited make it likely that they are affecting precip-
itation to a much greater extent than are present delib-
erate seeding operations (although tho deliberate op-
erations may still show more net results because of the
selection of situations suitable for seeding).
It cannot be concluded that any net effocts are neces-
sarily being produced on even local climates, although it
seems certain that the seeding is taking place, because tho
possible results of any kind of seeding are not well under-
stood at present and because the very massiveness and
frequency of the seeding makes it likely that such a pro-
fusion of effects in contradictory directions is being pro-
duced that the net result may bo small. The actual
magnitude and direction of the effect on the weather can
only be established by analysis of the records. The purpose
of this paper is to show that there is sound basis for the
1 Mention of commercial products does not constitute an endorsement.
403-235 0-70-4
748 MONTHLY WEATHER REVIEW Vol. 98, No. 10
suspicion that contrsjls may he influencing the weather
and that i t is important to find out if they are.
affecting the weather it is not necessarily for the worse,
although if there is any considerable change i t is sure to
make Someone unhappy. The Russians might well be
pleased with an ice-free Arctic Ocean; but if i t leads to R
major glaciation in central Canada, i t is unlikely that the
canadians and ~~~r i ~~~~ would regard it as favorable,
Blanchard, D. C., “A Verification of the Bally-Dorsey Theory of
Spicule Formation on Sleet Pellets,” Jouinal of Meleorology,
Vol. 8, No. 4, Aug. 1951, pp. 268-269.
Cirrus Crystals in Clear Air,,, Journal of Applied MMeteoro20gy,
vel. 6, N ~. 6, D ~~. 1967, pp, 1053-1061.
Fletcher, J. O., “The Heat Budget of thc Arctic Basin and Its Rela-
tion to Climate,” Rcport R-444-PR, Thc Rand Corporation,
Santa Monica, OCt. 1965, 179 PP.
Fletcher, J. O., “Managing Climatic Resources,” Paper P-4000-1,
The Rand Cornoration. Santa Monica. Calif.. Mar. 1969. 27 DD.
In conclusionr it be stated that if are Braham, Roscoe R,, Jr., and Spyers-Duran, Paul A., CCSurvival of
ACKNOWLEDGMENTS
The writer wishes to acknowledge many disoussions with Ilr.
Sue Ann Bowling whose contributions have been so significant a8
to nearly merit listing as a coauthor. The work upon which key
points in this presentation are based was supported by Grants
APOO 449-01, APOO 440-02, and APO 00449-03 fmm tho National
Center of Air Pollution Control, Public Health Service, Department
of Health, Education, and Welfare. This work was partially sup-
ported by Grant GA-10475, Atmospheric Sciences Section, Na-
tional Science Foundation.
REFERENCES
I
.. Huffman, Paul J., “Size Distribution of Ice Fog Particles,” M.S.
thesis, University of Alaska, College, Alaska, 1968, 93 pp.
Livingston, W. C., “Aircraft and Cloud Formation,” Wealher,
Val. 24, No. 2, Feb. 1969, pp. 56-61.
Ohtake, Takeshi, “Alaskan Ice Fog,” Proceedings of lhe Inlerna-
lional Conference on Low Temperalure, Sapporo, Japan, August 14-
19, 1966, ITakkaido University, Sapporo, Japan, 1967, pp.
105-118.
Reinking, Roger F., “Insolation Reduction by Contrails,” Weather,
vol. 23, No. 4, Apr. 1968, pp. 171-173.
Schaefer, Vincent J., “The Inadvertent Modification of the Atrnos-
phere by Air Pollution,” Bulletin of the American Meleorologieal
Society, Vol. 50, No. 4, Apr. 1969, pp. 199-206. ~.
Appleman, Herbert S., “The Formation of Exhaust Condensation
Trails by Jet Aircraft,” Bulletin of the American Meteorological
Soeiely, Vol. 34, No. I , Jan. 1953, pp. 14-20.
Thuman, William C., and Robinson, Elmer, “Studies of Alaskan
Iee-Fog Particles,” Journal of Meteorology, V d . 11, No. 2, Apr.
1954, pp. 1.51-156.
[Received December 8.92, 1969; revised March 83, 19701
CORRESPONDENCE
UDC 651.501.81:551.508.8:G21.3QG.QG~.~1.3
Effect of Range on Apparent Height and Frequency of
High-Altitude Radar Precipitation Echoes
A. J. KANTOR and D. D. G R A N T H A M
Air Force Cambridge Research Laboratories, Bedford, Mars
As coauthors of the captioned Monthly Weather Review
article (Vol. 97, No. 6, June 1969, pp. 429-431), we
welcome this opportunity to restate some of our findings
so that possiblc misinterpretation by interested readers
may be avoided.
Our conclusion concerning echo altitude and frequency
of occurrence due to range effects should state that the
net or resulting errors due to range are insignificant
rather than the errors are insignificant. The individual
errors can be substantial, as mentioned in our paper, and
as pointed out in a private communication from Dr.
D. Atlas, Director of the Laboratory for Atmospheric
Probing at the University of Chicago. We agree, for
example, that there is a balancing effect between the
increasing width of the beam with range that tends to
increase the altitude of reported tops and thc beam-width
averaging or filling effect that decreases the average
reflectivity of the top region with range. Also, height
errors obviously depend on reflectivity and its height
profile, so that, as Dr. Atlas emphasizes, serious over-
estimates of precipitation echo tops may occur when the
edges or side lobes of the beam detect sufficient reflectivity
from below. That such false echoes may occasionally
enter the records despite operational procedures and
professional manpower as used in thc WSR-57 network
underscores the fact that reflectivity is a vital factor in
radar climatology of storm tops.