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over prior nuclei generation capabilities and offer possibilities for engineering nuclei
with specific and desired properties.
Numerical Modeling Capabilities
Simulations of Seeding Supercooled Orographic Clouds It is now
possible to perform two- and three -dimensional simulations of clouds over indivi-
dual mountains and entire mountain ranges with grid spacing on the order of a
Figure 5 Yield (top panel) and rates (bottom panel) of ice formation by state-of-the-science
pyrotechnic glaciogenic seeding generators, prior to (TB-1) and since about 1990. New-type
pyrotechnics are more efficient in producing ice nuclei on a compositional basis, require
less AgI (as AgIO
3
), and ‘‘react’’ much faster in a water-saturated cloud. The new type
pyrotechnics were developed in the former Yugoslavia but are now manufactured in North
America. Results are from records of the Colorado State University isothermal cloud chamber
(ICC) facility (Figure supplied by Paul DeMott, Colorado State University).
446
WEATHER MODIFICATION
kilometer. For smaller mountain ridges, mixed-phase bin-resolving microphysics
models can be used, while for larger mountain ranges, bulk microphysics is still
needed. Such models can explicitly represent the transport and dispersion of seeding
material, the primary and secondary ice nucleation of natural clouds, as well as
nucleation of seeded material using laboratory-derived activity spect ra.
An example of a numerical simulation of silver iodide transport over the Black
Hills of South Dako ta is given in Figure 6. Five generators were located in the
simulation in the Northern Hills. The north winds spread the seeding material
over the slopes and into the simulated orographic clouds. The AgI is activated in
the clouds and forms snow at an earlier stage than the simulated natural nuclei
(Farley et al., 1997).
Simulations of Seeding Cumulonimbi and Severe Convective Storms
It is now quite common to perform fully three-dimensional time-dependent simula-
tions of cumulonimbi and mesoscale convective systems with grid spacing of about
1 km or even 0.5 km, with mixed-phase bulk microphysics models. For hailstorms, a
hybrid bulk microphysics=bin-microphysics model with continuous accretion
approximations has been implemented in several storm models (Farley et al.,
1996; Johnson et al., 1993). With access to advanced, state-of-the-art computers,
stochastic bin-resolving microphysics models such as Reisin et al. (1996) could be
applied to the simulation of natural and seeded hailstorms. Such a model could be
used to examine in some detail beneficial competition, trajectory lowering (including
hygroscopic seeding), early rainout, and glaciation hail suppression concepts.
Other Advances and General Comments
Research in the past 10 years has shown the close association of the cloud dynamics
with the cloud microphysics and precipitation processes. The total precipitation from
a cloud system is often the product of both the precipitation processes and the
dynamic airflow regime. Consequently, reasoning about seeding effects on precipita-
tion should include the effects of the dynamics of the cloud and cloud system. Cloud
and mesoscale numerical models run on supercomputer systems are needed to keep
track of all of the possible interactions. And efficient field projects are needed to test
the concepts and validate the models. These are tasks that should be taken more
seriously so that the technology can be appropriately developed and applied.
A comment regarding the magnitude of the seeding effect is warranted here.
Cloud seedi ng for weather modification does not cause floods nor does it create
droughts. Floods are caused by natural conditions much more vigorous than those
caused by seeding. The rain processes are fully developed and highly efficient in
flood situations. Likewise, droughts are caused by large-scale weather conditions not
influenced by cloud seeding. The use of cloud seeding to produce more precipitation
is most helpful to store water in reservoirs before droughts occur or to cause as much
precipitation as possible as weather conditions improve. Cloud seeding will not
break a drought, but it may make the drought less severe by extracting as much
of the atmospheric water supply as possible from the existing clouds. This aspect of
3 SOME SCIENTIFIC AND TECHNOLOGICAL ADVANCES IN THE PAST 25 YEARS 447
Figure 6 Seed case. (a) Surface locations where the inert tracer and AgI were released for
day 3. (b) Y–Z cross section of the AgI field at X ¼136 km. The contour inter val is 0.1 g=kg
for (a) and (b) (from Farley et al., 1997).
448
WEATHER MODIFICATION
cloud seeding is still very much in the research stage. The research requires good
cloud climatological data and well-conducted cloud seeding operations in drought
conditions.
A common misconception follows from the fact that the desired resul t of cloud
seeding for precipitation augmentation is to cause more rain or snow than might
have occurred naturally. If the natural rainfall is far below normal, then the seeded
rainfall will also be below nor mal, but not by as much, so the cloud seeding should
not be blamed (but often is) for the below normal rainfall. This is another reason
why more cloud seeding experiments are needed in various weather conditions, in
the field and in computer models, to make more quantitative the effects of the cloud
seeding and to develop methods that will distinguish the seeded rainfall from the
natural component.
Nothing in this review has been said about fog suppression, the suppression of
severe storms, and the reality of inadvertent weather modification, but the AMS
statements give further information on these imp ortant topics (AMS, 1998a,b). Cold
fog suppression is being practiced routinely in several airports around the world. The
suppression of severe storms lacks critical hypotheses and should be tested in
computer models before mounting field efforts. Large-scale land-use changes, wide-
spread burning of forested lands, increased air pollution through industrialization,
and extensive contrails caused by aircraft, have all contributed to inadvertent effects
on the weather.
4 CONCLUDING REMARKS
Of great importance for proof of the efficacy of cloud seeding have been the signifi-
cant strides made in the development of new or improved equipment, the use of more
powerful and convenient compu ter power, and the development of more powerful
statistical methods. Aircraft instrumentation, dual channel microwave radiometers,
Doppler and multiparameter radars , satellite remote sensing, wind profilers, auto-
mated rain gauge networks, and mesoscale network stations have all contributed or
have the potential to contribute to the detection of cloud seeding effects or to the
identification of cloud seedi ng opportunities.
The increased computer power has made possible the greate r use of cloud models
in weather modification. Models have been developed that sim ulate explicitly the
dispensing, transport, and nucleation effects of a seeding agent in convective and
orographic clouds. Prediction and understanding of the seeding effects are products
of the simulations. The results can also be used in evaluation and design of field
projects.
The critical need for clean water in many parts of the world now, and even more
so in the future, makes it imperative that the scientific basis of weather modification
be strengthened. The conditions for increase, decrease, or redistribution of precipita-
tion need to be determined. Stronger scientific and engineering projects in weather
modification by cloud seeding give promise of more rapid progress in this scienti-
fically important and socially relevant topic.
4 CONCLUDING REMARKS 449
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Chai, S. K., W. G. Finnegan, and R. L. Pitter (1993). An interpretation of the mechanisms of ice
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32, 1726–1732.
Changnon, S. A. (1998). In Natural Hazards of North America, National Geographic Maps,
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Cooper, W. A., R. T. Bruintjes, and G. K. Mather (1997). Calculations pertaining to
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DeMott, P. J., W. C. Finnegan, and L. O. Grant (1983). An application of chemical kinetic
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weather modification, J. Climate Appl. Meteor. 22, 1190–1203.
Dennis, A. S. (1980). Weather Modification by Cloud Seeding, New York, Academic.
Dessens, J. (1986). Hail in Southwestern France. II: Results of a 30-year hail prevention project
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modification program, J. Wea. Mod. 31, 91–101.
Farley, R. D., H. Chen, H. D. Orville, and M. R. Hjelmfelt (1996). The numerical simulation of
the effects of cloud seeding on hailstorms, Preprints, AMS 13th Conference on Planned and
Inadvertent Weather Modification, Atlanta, GA, pp. 23–30.
Farley, R. D., D. L. Hjermstad, and H. D. Orville (1997). Numerical simulation of cloud seeding
effects during a four-day storm period, J. Wea. Mod. 24, 49–55.
Feng, D., and W. G. Finnegan (1989). An efficient, fast functioning nucleating agent—
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AgCl–4NaCl, J. Wea. Mod. 21, 41–45.
IBHS (1998). The Insured Cost of Natural Disasters: A Report on the IBHS Paid Loss Data
Base, Institute for Business and Home Safety, Tampa, FL (formerly Boston, MA).
Johnson, D. E., P. K. Wong, and J. M. Straka (1993). Numerical simulations of the 2 August
1981 CCOPE supercell stor m with and without ice microphysics, J. Appl. Meteor. 32,
745–759.
Mather, G. K., M. J. Dixon, and J. M. de Jager (1996). Assessing the potential for rain
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Meteor. 35, 1465–1482.
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Mather, G. K., D. E. Terblanche, F. E. Steffens, and L. E. Fletcher (1997). Results of the South
African cloud-seeding experiments using hygroscopic flares, J. Appl. Meteor. 36,
1433–1447.
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Rev. April issue, 2–6.
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981–986.
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Orville, H. D., R. D. Farley, and J. H. Hirsch (1984). Some surprising results from simulated
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seeding simulations of stratifor m-type clouds, J. Wea. Modif. 19, 57–61.
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Super A. B., and E. W. Holroyd (1997). Some physical evidence of silver iodide and liquid
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452
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CHAPTER 24
ATMOSPHERIC OPTICS
CRAIG F. BOHREN
1 INTRODUCTION
Atmospheric optics is nearly synonymous with light scattering, the only restrictions
being that the scatterers inhabit the atmosphere and the primary source of their
illumination is the sun. Essentially all light we see is scattered light, even that
directly from the sun. When we say that such light is unscattered we really mean
that it is scattered in the forward direction, hence it is as if it were unscattered.
Scattered light is radiation from matter excited by an external source. When the
source vanishes, so does the scattered light, as distinguished from light emitted by
matter, which persists in the absence of exter nal sources.
Atmospheric scatterers are either molecules or par ticles. A particle is an aggrega-
tion of sufficiently many molecules that it can be ascribed macroscopic properties
such as temperature and refractive index. There is no canonical number of molecules
that must unite to form a bona fide particle. Two molecules clearly do not a quorum
make, but what about 10, 100, or 1000? The particle size corresponding to the
largest of these numbers is about 10
3
mm. Particles this small of water substance
would evaporate so rapidly that they could not exist long under conditions normally
found in the atmosphere. As a practical matter, therefore, we need not worry unduly
about hermaphrodite scatterers in the shadow region between molecule and particle.
A property of great relevance to scattering problems is coherence, both of the
array of scatterers and of the incident light. At visible wavelengths air is an array of
incoherent scatterers: The radiant power scattered by N molecules is N times that
scattered by one (except in the forward direction). But when water vapor in air
condenses, an incoherent array is transformed into a coherent array: Uncorrelated
Handbook of Weather, Climate, and Water: Dynamics, Climate, Physical Meteorology, Weather Systems,
and Measurements, Edited by Thomas D. Potter and Bradley R. Colman.
ISBN 0-471-21490-6 # 2003 John Wiley & Sons, Inc.
453
water molecules become part of a single entity. Although a single droplet is a
coherent array, a cloud of droplets taken together is incoherent.
Sunlight is incoherent but not in an absolute sense. Its lateral coherence length is
tens of micrometers, which is why we can observe what are essentially interference
patterns (e.g., coronas and glories) resulting from illumination of cloud droplets by
sunlight.
This chapter begins with the color and brightness of a purely molecular atmo-
sphere, including their variation across the vault of the sky. This naturally leads to
the state of polarization of skylight. Because the atmosphere is rarely, if ever, entirely
free of particles, the general characteristics of scattering by particles follow, setting
the stage for atmospheric visibility.
Atmospheric refraction usually sits by itself, unjustly isolated from all those
atmospheric phenomena embraced by the term scattering. Yet refraction is yet
another manifestation of scattering, although in the forward direction, for which
scattering is coherent.
Scattering by single water droplets and ice crystals, each discussed in turn, yields
feasts for the eye as well as the mind. The curtain closes on the optical properties of
clouds.
2 COLOR AND BRIGHTNESS OF MOLECULAR ATMOSP HERE
Brief History
Edward Nichols began his 1908 presidential address to the New York meeting of the
American Physical Society as follows: ‘‘In asking your attention to-day, even briefly,
to the consideration of the presen t state of our knowledge concerning the color of the
sky it may be truly said that I am inviting you to leave the thronged thoroughfares of
our science for some quiet side street where little is going on and you may even
suspect that I am coaxing you into some blind alley, the inhabitants of which belong
to the dead past.’’
Despite this depreciatory statement, hoary with age, correct and complete expla-
nations of the color of the sky still are hard to find. Indeed, all the faulty explanations
lead active lives: the blue sky is the reflection of the blue sea; it is caused by water,
either vapor or droplets or both; it is caused by dust. The true cause of the blue sky is
not difficult to understand, requiring only a bit of critical thought stimulated by belief
in the inherent fascination of all natural phenomena, even those made familiar by
everyday occurrence.
Our contemplative prehistoric ancestors no doubt speculated on the origin of the
blue sky, their musings having vanished into it. Yet it is curious that Aristotle, the
most prolific speculator of early recorded history, makes no mention of it in his
Meteorologica even though he delivered pronouncements on rainbows, halos, and
mock suns and realized that ‘‘the sun looks red when seen through mist or smoke.’’
Historical discussions of the blue sky sometimes cite Leonardo da Vinci as the first to
comment intelligently on the blue of the sky, although this reflects a European bias.
454 ATMOSPHERIC OPTICS
If history were to be written by a supremely disinterested observer, Arab philoso-
phers would likely be given more credit for having had profound insights into the
workings of nature many centuries before their European counterparts descended
from the trees. Indeed, Mo¨ller (1972) begins his brief history of the blue sky with
Jakub Ibn Ishak Al Kindi (800–870), who explained it as ‘‘a mixture of the darknes s
of the night with the light of the dust and haze particles in the air illuminated by the
sun.’’
Leonardo was a keen observer of light in nature even if his explanations some-
times fell short of the mark. Yet his hypothesis that ‘‘the blueness we see in the
atmosphere is not intrinsic colour, but is caused by warm vapor evaporated in min ute
and insensible atoms on which the solar rays fall, rendering them luminous against
the infinite darkness of the fiery sphere which lies beyond and includes it’’ would,
with minor changes, stand critical scrutiny today. If we set aside Leonardo as
sui generis, scientific attempts to unravel the origins of the blue sky may be said
to have begun with Newton, that towering pioneer of optics, who, in time-honored
fashion, reduced it to what he already had considered: interference colors in
thin films. Almost two centuries elapsed before more pieces in the puzzle were
contributed by the experimental investigations of von Bru¨cke and Tyndall on light
scattering by suspensions of particles. Around the same time Clausius added his bit
in the form of a theory that scattering by minute bubbles causes the blueness of the
sky. A better theory was not long in coming. It is associated with a man known to the
world as Lord Rayleigh even though he was born John William Strutt.
Rayleigh’s paper of 1871 marks the beginning of a satisfactory explanation of the
blue sky. His scattering law, the key to the blue sky, is perhaps the most famous
result ever obtained by dimensional analysis. Rayleigh argued that the field E
s
scattered by a particle small compared with the light illuminating it is proportional
to its volume V and to the incident field E
i
. Radiant energy conservation requires
that the scattered field diminish inversely as the distance r from the particle so that
the scattered power diminishes as the square of r. To make this proportionality
dimensionally homogeneous requires the inverse square of a quantity with the
dimensions of length. The only plausible physical variable at hand is the wavelength
of the incident light, which leads to
E
s
/ E
i
V
rl
2
ð1Þ
When the field is squared to obtain the scattered power, the result is Rayleigh’s
inverse fourth-power law. This law is really only an often—but not always—very
good approximation. Missing from it are dimensionless properties of the particle
such as its refractive index, which itself depends on wavelength. Because of this
dispersion, therefore, nothing scatters exactly as the inverse fourth power.
Rayleigh’s 1871 paper did not give the complete explanation of the color and
polarization of skylight. What he did that was not done by his predecessors was to
give a law of scattering, which could be used to quantitatively test the hypothesis that
selective scattering by atmospheric particles could transform white sunlight into blue
2 COLOR AND BRIGHTNESS OF MOLECULAR ATMOSPHERE 455