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components of the parent discharge. The charge moment (in C-km), not the peak
current as measured by the NLDN, is the key parameter in the basic QE conventional
breakdown mechanism first proposed by Wilson. The key physics of the problem
appear to involve the altitude and magnitude of the removed charge and the time
scale on which this occurs—parameters about which little agreement exists. Many
theorists note that even with an assumed tall þCG channel (10 km) this still
requires extremely large (100 C) charge transfers, typically 10 times larger than
in ‘‘conventional’’ lightning. Some models yield a 1000-fold enhancement in opti cal
intensity at 75 km for a doubling of the altitude of lightning charge removal from 5
to 10 km. The use of shorter channels to ground, say 5 km, would imply truly large
charge transfers. Yet evidence is accumulating that indeed such may be the case.
While the various models simulate optical emissions bearing some (though in
many cases rather minimal) resemblance to the observations, such wide ranges in the
lightning source term parameters do not appear physically realistic. If, in fact, such a
range of lightning characteristics could produce sprites, why does only a very small
subset of þCGs (<1:20 even in active storms) actually produce observable TLEs
(with current sensors)? It appears that most models have made assumptions about
the lightning to produce something resembling a TLE—rather than starting with
hard physical constraints on the source term. The reason, of course, is that there are
very little data on the actual CGs that generate specific TLE occurrences.
TABLE 2 Current Speculations as to Characteristics of TLEs and Their Parent
Lightning (None for Blue Jets)
Sprite Elve Blue Jet
Color of emission Red top=blue base Red? Deep blue
Polarity of parent CG Positive (almost all) Positive (mostly) None
þCG peak current > 50 kA > 100 kA None
Charge transferred (C) Largest Large N=A
Charge moment (C-km) Largest (>300) Large (<300) N=A
Parent CG location Stratiform area Stratiform area (?) N=A
Parent CG vertical
channel height
5–7 km (?) 10 km (?) N=A
di=dt value Moderate Very large (?) N=A
Total flash duration Very long Short (?) N=A
Spider involved Yes (?) No (?) N=A
Continuing current
duration
Very long (?) Short in any (?) N=A
VLF=ELF slow tail Distinct Yes N=A
ELF spectral color Red White None
VLF audio character Low freq. Higher freq. None
Duration of TLE 1–150 ms 0.5 ms 100–200 ms
Altitude range of TLE 25–95 km 75–105 km Cloud-40 km
Onset delay after CG 1–100 ms 0.3 ms N=A
Brightness 50–35,000 kR 1000 kR 1000 kR
Horizontal size
of emission
100 m–100 km 100–400 km 1–2 km
426
ATMOSPHERIC ELECTRICITY AND LIGHTNING
During the summer of 2000, an extensive field research effort was conducted over
the High Plains of Colorado, Kansas, and Nebraska. Several radars, research aircraft,
and mobile storm monitor teams were deployed, along with a three-dimensional
lightning-mapping array. As sprite-bearing storms passed over the LMA, LLTV
systems at Yucca Ridge (som e 300 km to the northwest) was able to document
the sprites and other TLEs coincident with a variety of detailed measurements of
the parent lightning discharges. Data analyses have just begun, but initial findings
suggest that þCGs with very large charge moments (>300 C-km) are indeed asso-
ciated with sprite events above their parent storms.
10 WHY STUDY TLEs?
Aside from their intrinsic scientific interest, there may be some rather practical
reasons to explore TLEs in more depth. It has been suggested that there may be
significant production of NO
x
in the middle atmosphere by sprites. This becomes
even more interesting in light of recent observations that regional smoke palls from
biomass burns radically enhance the percentage of þCGs within storms, and thus
increase sprite counts (and middle atmosphere NO
x
production?). Cur rently, no
global chemical model accounts for any potential effects of TLEs (Lyons and
Armstrong, 1997). Once a better estimate of NO
x
production per sprite is obtained,
it will be necessary to know the global frequency and distribution of sprites. It has
been demonstrated that several Schumann resonance monitoring sites working in
tandem are capable of obtaining a worldwide TLE census.
There is growing interest in determining the sources of unusual infrasound emis-
sions detected above sprite-capable MCSs as determined by the National Oceanic
and Atmospheric Administration’s (NOAA’s) Environmental Technology Laboratory
near Boulder, Colorado. TLEs thus produce optical, radio frequency (RF), and
acoustic emissions that have the potential of mimicking or masking signatures
from clandestine nuclear tests. Such findings may have important implications for
global monitoring efforts supporting the Comprehensive Test Ban Treaty.
Transient luminous events may contribute in ways not yet understood to the
maintenance of the global electrical circuit. To quantify the impacts of TLEs, we
require information on their global freque ncy (now roughly estimated between 1 and
10 per minute) and their geographic distribution. It has been proposed (Williams,
1992) that the Schumann resonance can be used in the manner of worldwide tropical
thermometer on the assumption that as global warming occurs, the amoun t of light-
ning may rise rapidly. The implications for sprite production of any global warming
are uncertain .
ACKNOWLEDGMENTS
This material is based upon work supported in part with funds from the National
Aeronautics and Space Administration (Office of Space Sciences), the U.S. Depar t-
10 WHY STUDY TLEs? 427
ment of Energy, and the U.S. National Science Foundation (Contract number ATM-
0000569).
11 SOME WEATHER AND LIGHTNING-RELATED WEBSITES
National Lightning Safety Institute: http:==www.lightningsafety.com
Global Atmospherics, Inc.—National Lightning Detection Network: http:==www.
LightningStorm.com
Sprites and Elves: http:==www.FMA-Research.com
Lightning Injury Research: http:==tigger.uic.edu= macooper=cindex.htm
Kennedy Space Center=Patrick Air Force Base: www.patrick.af.mil=45og=45ws=
ws1.htm
The National Severe Storms Laboratory—Norman, OK: http:==www.nssl.
noaa.gov
U.S. National Oceanic and Atmospheric Administration: http:==weather.noaa.gov
and http:==www.srh.noaa.gov=ftproot=ssd=html=lightnin.htm
The World Meteorological Organization: http:==www.wmo.ch
National Center for Atmospheric Research: http:==www.rap.ucar.edu=weather
Federal Emergency Management Agency: http:==www.fema.gov
NASA Marshall Space Fligh t Center: http:==thunder.msfc.nasa.gov
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Andrews, C. J., M. A. Cooper, M. Darveniza, and D. Mackerras (1992). Lightning Injuries:
Electrical, Medical and Legal Aspects, Boca Raton, FL, CRC Press.
Armstrong, R. A., D. M. Suszcynsky, W. A. Lyons, and T. E. Nelson (2000). Multi-color
photometric measurements of ionization and energies in sprites, Geophys. Res. Lett. 27,
653–656.
Boccippio, D. J., E. R. Williams, S. J. Heckman, W. A. Lyons, I. T. Baker, and R. Boldi (1995).
Sprites, ELF transients, and positive ground strokes, Science 269, 1088–1091.
Boeck, W. L., O. H. Vaughan, Jr., R. Blakeslee, B. Vonnegut, and M. Brook (1998). The role of
the space shuttle videotapes in the discovery of sprites, jets and elves, J. Atmos. Solar-Terr.
Phys. 60, 669–677.
Changery, M. J. (1981). National thunderstorm frequencies for the contiguous United States.
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Cooper, M. A., (1995). Myths, miracles, and mirages, Seminars in Neurology 15, 358–361.
Cummins, K. L., M. J. Murphy, E. A. Bardo, W. L. Hiscox, R. B. Pyle, and A. E. Pifer (1998).
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Network, J. Geophys. Res. 103, D8, 9035–9044.
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electrical discharge above a thunderstor m system, Science 249, 48–51.
Fukunishi, H., Y. Takahashi, M. Kubota, K. Sakanoi, U. S. Inan, and W. A. Lyons (1996).
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Lett. 23, 2157–2160.
Fuquay, D. M., A. R. Taylor, R. G. Hawe, and C. W. Schmid, Jr. (1972). Lightning discharges
that caused forest fires, J. Geophys. Res. 77, 2156–2158.
Golde, R. H. (1977). Lightning, Vol. 1, Physics of Lightning, London, Academic.
Goodman, S. J., and D. R. MacGor man (1986). Cloud-to-ground lightning activity in
mesoscale convective complexes, Mon. Wea. Rev. 114, 2320–2328.
Hampton, D. L., M. J. Heavner, E. M. Wescott, and D. D. Sentman (1996). Optical spectra
characteristics of sprites. Geophys. Res. Lett. 23, 89–92.
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their meteorological uses, NOAA Technical Memorandum ERL NSSL-102, National
Severe Storms Laboratory.
Holle, R. L., R. E. Lopez, R. Ortiz, A. I. Watson, D. L. Smith, D. M. Decker, and C. H. Paxton
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Holle, R. L., R. E. Lopez, L. J. Arnold, and J. Endres (1996). Insured lightning-caused property
damage in three western states, J. Appl. Meteor. 35, 1344–1351.
Holle, R. L., R. E. Lopez, and C. Zimmermann (1999). Updated recommendations for
lightning safety—1998, Bull. Am. Meteor. Soc. 80, 2035–2041.
Huang, E., E. Williams, R. Boldi, S. Heckman, W. Lyons, M. Taylor, T. Nelson, and C. Wong
(1999). Criteria for sprites and elves based on Schumann resonance observations, J.
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Inan, U. S., C. Barrington-Lee, S. Hansen, V. S. Glukhov, T. F. Bell, and R. Rairden (1997).
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referred to as ‘‘elves,’’ Geophys. Res. Lett. 24, 583–586.
Krider, E. P., R. C. Noggle, A. E. Pifer, and D. L. Vance, 1980: Lightning direction-finding
systems for forest fire detection, Bull. Am. Meteor. Soc. 61, 980–986.
Lopez, R. E., R. L. Holle, T. A. Heitkamp, M. Boyson, M. Cherington, and K. Langford (1993).
The underreporting of lightning injuries and deaths, Preprints, 17th Conf. on Severe Local
Storms, Conf. on Atmospheric Electricity, St. Louis, American Meteorological Society,
pp 775–778.
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CHAPTER 23
WEATHER MODIFICATION
HAROLD D. ORVILLE
1 INTRODUCTION
Although much remains to be learned about the seeding of clouds, scientists and
engineers have accomplished much in the 55 or so years since the discovery that dry
ice and silver iodide are efficient ice nucleants, able to influenc e the natural preci-
pitation processes. In addition, wi thin the past 10 years renewed efforts using hygro-
scopic seeding mat erials have yielded very promising results in producing more rain
from convective clouds. This chapter will discuss the basic physics and chemistry of
the precipitation and hail processes and explain the primary methods for changing
the various atmospheric precipitation components. Some of the key advances in
weather modification in the past 25 years will be highlighted. The primary source
material comes from a workshop report to the Board on Atmospheric Sciences and
Climate (BASC) of the National Research Council (NRC) published in December
2000 (BASC, 2000). In addition, the most recent statement by the American
Meteorological Society (AMS, 1998a) and the scientific background for the
policy statement (AMS, 1998b) has much good information concerning the current
status of weather modification.
Basic Physics and Chemistry of the Precipitation Processes
Only a brief review of these complex processes will be given here. Detailed treat-
ments can be found in the textbooks or scientific studies listed in the references at the
end of this chapter (e.g., Dennis, 1980; Rogers and Yau, 1989; Young, 1993).
Concerning rain and snow, the basic problem of precipitation physics is: How do
one million cloud droplets combine to form one raindrop or large snowflake in a
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.
433
period of 10 to 20 min? Two primary processes are involved after the clouds have
formed.
Atmospheric aerosols are important for the formation of clouds, the precursors of
rain. The chemical content and distribution of the aerosol plus the cloud updraft
speed determine the initial distribution and number concentration of the droplets.
Normally there are ample suitable hygroscopic aerosols, called cloud condensation
nuclei (CCN), for clouds to form at relative humi dities very near 100% (at super-
saturations of a few tenths of a percent only). The relative humidity with respect to a
liquid or ice surface is defined as the ratio of the environmental vapor pressure to the
saturation vapor pressure at the liquid or ice surface, respectively. Maritime and
continental regions have very different aerosol populations, leading to characteristi-
cally different cloud droplet distributio ns.
For clouds with all liquid particles (certainly all those clouds warmer than 0
C) a
process known as collection (a combination of collision and coalescence)—the
warm rain process—is thought to be operative. This process depends on the
coexistence of cloud droplets of different sizes and, consequently, of different fall
velocities. Sizes range from a few micrometers in diameter to 30 or 40 mm. The
larger droplets fall through a population of smaller droplets, collecting them as they
fall. The larger particles become raindrop embryos. If conditions are right (depth of
cloud, breadth of droplet distribution, cloud updraft, etc.), the embryos can form
raindrops by this process in the necessary time period. Raindrops range in size from
200 mm to 5 mm diameter (the upper limit occurs because of raind rop breakup).
One might think that with billions and billions of cloud droplets in a cloud that it
would be relatively easy for the droplets to collide, coalesce, and grow to raindrop
size. However, even though very numerous, the droplets occupy less than one
millionth of the volume of the cloud. Consequen tly, the droplets are relatively far
apart and require special conditions to interact and grow. Indeed, even large thunder-
storms and massive raining nimbostratus clouds are more than 99.99% clear space
(dry air and vapor), but by the time precipitation particles have formed in such
clouds the collection proces s is well developed and relatively efficient.
The second precipitation process is called the cold rain process and depends on
the formation of ice in the cloud. This process also depends on nuclei (ice nuclei,
very different particles from the CCN mentioned above), but in this case there is a
shortage of ice-forming nuclei in the atmosphere so that liquid water does not freeze
at 0
C. The water is then called supercooled. To understand this cold rain process,
one other important fact is needed: For the same subzero temperature, the relative
humidity will be much above 100% for the ice particle while staying at 100% over
the liquid surface, due to the fact that the saturation vapor pressure is higher over the
liquid surface than over the ice surface. The growth of the particles depends on the
vapor pressure difference between the cloudy air and the particles’ surface. The
liquid dropl ets are generally much more numerous than ice crystals in a cloud
and control the relative humidity (the vapor pressure in the cloudy air). The stage
is then set for rapid growth of an ice particle if it is introduced into a cloud of
supercooled liquid droplets. The ice cr ystal grows, depleting the water vapor in the
cloudy environment, which is then replenished by water vapor from evaporating
434 WEATHER MODIFICATION
droplets trying to maintain 100% relative humidity with respect to liquid. The crystal
rapidly increases in mass and falls through the cloud droplets, collecting them and
further increasing the ice particle’s mass. Snow or rain will appear at the ground
depending on the tem perature conditions of the lower atmosphere. Bergeron and
Findeisen first described this process in the 1930s.
The formation of the ice particle by the ice nucleus can occ ur in at least four
ways: (1) Deposition of water vapor molecules may occur directly on the nucleus.
(2) Condensation may occur on a portion of the nucleus, followed by freezing of the
liquid. (3) A drop or droplet may come in contact with a nucleus and freeze. (4) And
finally a drop or droplet may have formed including a nucleus in its bulk water
content, which then freezes the water when a low enough temperature is reached.
How low? All of the processes mentioned above require a supercooling of from 15 to
20
C, that is, temperatures of 15 to 20
C in continental-type clouds (those
clouds with a narrow drop size distribution and large number concentrations).
Clouds with extremely strong updrafts may delay their primary ice formation to
temperatures near 40
C (Rosenfeld and Woodley, 2000). There is some evidence
accumulating that the ice nucleating temperature may depend on the type of cloud;
maritime clouds with large and fewer droplets and a broad droplet distribution
produce ice particles at temperatures as warm as 10
C. The more freque nt colli-
sions among large and small droplets may contr ibute to the early formation of ice in
those clouds.
As mentioned above, in the absence of any ice nuclei, the water will remain in the
liquid state until a temperature of 40
C is reached. The most effective nuclei are
insoluble soil particles, kaolinites, and montmorillonites, but industrial and natural
combustion products may also contribute to the ice-forming characteristics of
clouds.
These ice processes refer to primary ice formation in a cloud. Secondary
processes may also contribute (Mossop, 1985). Ice crystals may fracture or graupel,
and droplets may interact to form additional ice particles, or ice particles may be
transported from one cloud cell to another. A stormy, turbulent cloud situation has
many ways to create and distribute ice among the clouds.
Basic Hail Processes
Hailstorms are normally large thunderstorms that produce hail. Hailstones are irre-
gular ice particles larger than 5 mm in diameter (about one quarter of an inch). Large
hailstones may grow to several centimeters in diameter and fall at many tens of
meters per second. Hailstorms are characterized by large updrafts, usually greater
than 15 m=s (30 mph) and sometimes as great as 50 m=s (100 mph). The large
updrafts in a storm are closely associated with the largest hailstones in a storm.
Such storms produce more than a billion dollars of crop damage and a like amount
of property damage per year in the United States.
In addition to large updrafts to suspend the growing hailstones, a two-stage
process is thought needed to produce hailstones. The first stage requires the forma-
tion of hailstone embryos. Either frozen raindrops or graupel particles may serve as
1 INTRODUCTION 435