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S.
. fores
OVERVIEW*
It is clear that nuclear explosions can ignite large-scale fires
(Broido, 1960. In addition, it has been estimated that the smoke
emissions from nuclear-initiated fires could produce major atmospheric
perturbations (Lewis, 1979; Crutzen and Birks, 1982; Turco et al.,
1983a,b). Only two nuclear explosions have ever occurred over
populated areas (Hiroshima. Auoust 6. 1945. and Nagasaki. Auoust 9 .
_ _ _ _ _ ~ ~ , , _ , _ _ , ~ , _ _ , ,
1945); In each case, a clty-slzea conflagration resulted. At
Hiroshima, a ~12-kt weapon caused a mass fire over an area of ~13
km2, essentially the entire central city (Ishikawa and Swain, 1981~.
At Nagasaki, where high terrain shadowed large regions of the city from
direct irradiation by bomb light, a ~20-kt device burned ~7 km2
(Ishikawa and Swain, 1981~. It is difficult to extrapolate the effects
of these two isolated events, which involved <40-kt total yield, to
the possible effects of a global nuclear exchange involving 6500 Mt.
Nevertheless, a logical sequence of steps can be taken to obtain
estimates of the areal extent and particulate emissions of fires
initiated in a full-scale nuclear war:
1. Review historical fire experience to assess the probability of
ignition and spread of large fires.
2. Define the effectiveness of nuclear explosions for initiating
fires in urban and forest settings.
*In the text, the following symbols are used: a, approximately equal
to; ~, of the order of; ,
greater than or of the order of.
tFor the purposes of this report, large-scale fires can be
classified as "mass fires, n in which many individual fires burn
simultaneously over a large area, "conflagrations, n in which the fire
is most intense along a line of propagation, "firestorms, n in which the
entire area of the fire burns intensely and strong winds blow inward
from all directions, and "fire whirls, n in which a firestorm plume
develops an unusually strong vorticity.
36
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37
3. Determine the burdens and distributions of combustible
materials around potential nuclear targets.
4. Evaluate data on the quantity and physical properties of smoke
generated by common fuels.
5. Consider mass fire dynamics to determine the likely heights and
rates of injection of the smoke.
6. Describe a scenario for the locations, yields, and heights of
nuclear detonations (See Chapter 3, The Baseline Nuclear Exchangers.
7. Combine the foregoing information to estimate the total
quantity and optical characteristics of nuclear war smoke emissions.
These topics are discussed in subsequent sections of this chapter. On
the basis of such an analysis, an approximate equation can be written
that emphasizes the important factors that enter into the estimation
process,
E = YfA0m0fbe x 101°,
where E is the total smoke emission (in grams), Yf is the total
explosion yield (in megatons) in air bursts that effectively ignite
fires, AD is the average area ignited by each megaton of yield (in
square kilometers per megaton), my is the average loading of
flammable materials (in grams per square centimeter), fb is the
fraction of my burned, and £ is the mean smoke emission factor
(grams of smoke per gram of material burned). The factor of 101°
converts square kilometers (Ao) to square centimeters.
The key parameter values that apply to the baseline nuclear war
scenario are given in Table 5.1. The total smoke emission calculated
for the baseline case is ~180 Tg (1 Tg = 1012 g ~ 106 metric
tons), or ~0.7 g/m2 averaged over the northern hemisphere. Since
the specific extinction (scattering plus absorption) coefficient of
many smokes at visible wavelengths is ~5.5 m2/g, the hemispherical
average optical depth* in this case is ~4. Of course, if the smoke
were confined to the northern mid-latitude zone, the optical depth
would be ~2 to 3 times larger, or ~8 to 12. A more detailed
discussion of these estimates follows. The optical and climatic
effects of the smoke are discussed in Chapter 7.
PRESENT-DAY SMOKE EMISSION AND REMOVAL
It is estimated that the current global smoke emission to the
atmosphere is ~200 Tg/yr (Seller and Crutzen, 1980; Turco et al.,
1983a,c). The graphitic carbon fraction is about 5 to 10 percent by
*The optical depth is a dimensionless quantity that determines the
light transmission properties of a layer of gas or aerosols. If the
layer has an optical depth T. e ~ is the fraction of a beam of
light perpendicularly incident on the layer that suffers no scattering
or absorption in passing through the layer. The total light transmitted
consists of the direct light plus a scattered (diffuse) component.
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TABLE 5.1 Baseline Nuclear War Fire and Smoke Parametersa
Parameterb Urban Fires Forest Fires
Yf (Mt)
10001000
AD tkm2/Mt)250250
me (g/cm2)42
fb
g/g)C
0.75 0.20
0.02 0.03
aExcursions from the baseline case, and uncertainties in the baseline
parameters, are discussed in the text.
bYf is the effective ignition yield in megatons, An is the average
ignition area per megaton, my is the burden of combustibles per unit
area, fb is the fraction of the combustibles burned, and ~ is the
net smoke emission factor per unit of fuel, assuming in the case of
urban fires that 50 percent of the smoke is promptly scavenged and
removed from the plumes mainly as black rain. n
CThe smoke consists of 20 percent graphitic carbon (soot) by mass,
and 80 percent transparent oily compounds.
NOTE: Urban fire smoke emission: Eu = 150 x loll g
Forest fire smoke emission: Ef = 30 x 1012 g
Total smoke emission: Et = 180 x 1012 g
mass. The primary sources of smoke are agricultural burning, fossil
fuel combustion, and wildfires. The important characteristics of
background smoke emissions that distinguish them from "nuclear" fire
emissions are as follows:
1. The smoke emission factors are low in relation to the quantity
of fuel burned, because most of the burning takes place under
controlled conditions.
2. The overall graphitic carbon component is low, because most of
the smoke is generated during the prescribed combustion of natural
cellulosic materials.
3. Almost all of the smoke is injected into the lowest 1 km of the
atmosphere, because the sources are small in horizontal scale and/or
total power.
4. The smoke emissions occur in diverse locations throughout the
course of a year, which prevents significant concentrations from
building up.
5. The average atmospheric lifetime of the smoke is < 10 days
(Ogren, 19821.
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39
As a result of these factors, the average background concentration
of airborne graphitic carbon is typically only ~ 0.1 ug/m3, and
its integrated vertical absorption optical depth is <0.01 (Charlson
and Ogr en, 1982; Turco et al., 1983c). Over a period of about 1 month,
background smoke emissions would be negligible in comparison with the
estimated smoke emissions of a nuclear war (Turco et al., 1983a,b;
Crutzen et al., 1984~.
Removal of smoke and soot from the atmosphere occurs mainly through
precipitation scavenging. Smoke particles have sizes of about 0.1- to
0.5-pm radius, at which sedimentation is negligible and dry
deposition is very inefficient (Slinn, 1977; Sehmel, 1980~. In the
background atmosphere, soot is usually found as a minor component of
hydroscopic sulfate aerosols. This suggests removal by efficient
scavenging of the hydroscopic aerosols in and below clouds (Radke et
al., 1980; Ogren, 1982; Turco et al., 1983c).
The arctic haze that forms in winter and spring is known to contain
soot (Rosen and Novakov, 1983~. The haze is (relatively) highly
absorbing because of the soot (Patterson et al., 19821. The seasonal
conditions that lead to the formation of the winter polar vortex create
a stable air mass with low precipitation in which carbon emissions
produced by combustion can remain suspended for several months. This
demonstrates that under some meteorological conditions, particularly
with the suppression of precipitation, smoke and soot can have an
extended atmospheric lifetime.
Generally speaking, it is expected that smoke from nuclear-initiated
fires would have a longer atmospheric lifetime than background smoke
(notwithstanding prompt scavenging in the fire plumes), because of its
greater heights of injection. This point is expanded in subsequent
sections.
HISTORICAL FIRE EXPERIENCE
Human experience with mass fires and firestorms includes urban
conflagrations triggered by natural disasters (e.g., earthquakes),
wartime city fires initiated by incendiary and nuclear bombing, massive
wildfires and forest fires, and field experiments with large-scale fuel
beds (Carrier et al., 1982~. Although few of these experiences are
directly applicable to the nuclear war problem, all contribute to a
general understanding of the properties and behavior of large-scale
fires.
Earthquakes
Earthquakes have started urban conflagrations by breaking gas lines,
exposing stored fuels, shorting electrical circuits, breaching open
fires, and hampering effective firefighting. Particularly striking
examples of large fires induced by earthquakes occurred in San
Francisco in 1906 and Tokyo in 1923. A nuclear blast wave would have
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similar impact and, in combination with the thermal light pulse, would
represent a much greater fire threat than an earthquake.
World War II
The World War II saturation bombing of German and Japanese cities
provided ample evidence that mass fires can be readily ignited in urban
settings. The nuclear explosions over Hiroshima and Nagasaki are
discussed later. The conventional bombing of cities such as Hamburg,
Dresden, Darmstadt, and Tokyo produced intense fires over many square
kilometers and, in some instances, triggered firestorms. From
anecdotal evidence, it is known that thick, dark plumes rose from these
fires to altitudes of 6 to 12 km. Within the fire zones, almost all
the buildings were gutted and all combustible materials consumed. Such
experiences show that, when many simultaneous fire ignitions occur
among closely spaced structures and firefighting capability is
suppressed, mass fires are likely to develop.
Occasionally, massive urban conflagrations, such as the Great
Chicago Fire of 1871, are touched off by single ignitions (Kerr,
1971~. Although such fires are not typical, they are symptomatic of
the hazardous fire conditions that exist in many crowded urban centers.
Forest Fires
Plummer (1912), Ayers (1965), and F.E. Fendell (in Appendix 5-1), among
others, have reviewed the largest forest fires of the past 160 years in
which areas up to 20,000 km2 were blackened. The conditions under
which these catastrophic fires developed included long drought, low
humidity, and high winds (e.g., Plummer, 1912~. Clearly, such
conditions are not common over large areas of the northern hemisphere
during most of the year (Chandler et al., 19631. However, for the
analysis of nuclear-induced fires, three general types of fire danger
conditions should be distinguished: (I) fires are difficult to ignite
and do not spread if ignited; (II) fires are readily ignited, but their
spread is limited by factors such as humidity, moisture, topography,
winds, and firebreaks; and (III) fires readily ignite and spread
uncontrollably over large areas.
Historical catastrophic forest fires are exclusively of type III.
By contrast, most nuclear forest fires would probably be of type II.
Historical fires are characterized by a limited number of ignition
points, perhaps one ignition for each 50 to 500 km2 burned (Ayers,
1965~. Nuclear explosions, by contrast, can ignite forest debris
instantly over a large area, with numerous ignition points developing
into moderate size fires (although the probability of extensive fire
spread outside of the original burning zone would be much lower--see
below).
The great Tunguska meteor, which fell over Siberia on June 30,
1908, provides a very rough indication of the effects that might be
produced by a high-yield nuclear explosion over a forest. The Tunguska
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event was equivalent, in terms of the blast wave, to a ~10-Mt
detonation at 8-km altitude (Krinov, 1966~. (As noted below,
high-yield nuclear bursts have smaller incendiary efficiencies than
low-yield bursts.) Roughly 16QO km2 of Siberian forest was
flattened. Eyewitness accounts describe burning falling trees" and
widespread fires. A series of Russian scientific expeditions to the
fall site concluded that several major fires had broken out in the
central zone of devastation and burned for 5 days. From the
description of the charred remains, it appears that bark and many small
branches were stripped from the trees and burned, to an extent not
usually observed in natural fires of that area.
Experimental Fires
Experimental large-scale fires have been used to study fire development
and plume dynamics. Among these experiments are the Flambeau series
(Martin, 1974; Palmer, 1981), the Euroka fires (Williams et al., 1970),
and the Meteotron events (Desserts, 1962; Church et al., 19803.
However, because the extent of these fires was only about 103 to
105 m2, extrapolation of the results to city-size fires is
difficult. Of particular interest here is the height of the smoke
plume in a large fire. In the experiments noted above, the plume
aspect ratio (i.e., the plume height divided by the fire diameter) was
always >>1, and the plumes often formed vortices penetrating to
heights >1 km. (The plume aspect ratio cannot be simply scaled to
larger fires. The dependence of plume height on fire size and
intensity, and extrapolations to city-sized fires are discussed in
later section.)
The Flambeau experiments also led to the definition of a set of
conditions for firestorm genesis that has been widely accepted (FEMA,
19821. The conditions include a fuel loading of >4 g/cm2, a
building density of >20 to 30 percent, a fire area of >3 km2,
initial fires in >20 percent of the buildings, and ambient winds of
<10 km/in (Baldwin, 1968; Martin, 1974~. However, these conditions
are still controversial, as they have never been tested on an
appropriate scale. Moreover, in view of the atmospheric effects being
considered here, it is not clear that firestorms and very intense mass
fires need to be differentiated, except perhaps to refine the
estimation of smoke injection altitudes (see below}.
IGNITION OF NUCLEAR FIRES
Thermal Phenomena
In a nuclear air burst at low altitude (<10 km), about 30 to 40
percent of the energy is released as an intense pulse of visible light;
about 45 to 55 percent of the energy is converted to blast pressure
waves; and about 15 percent is contained in prompt and delayed nuclear
radiation (Glasstone and Dolan, 1977, hereafter GD77~. Most of the
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100
N
-
C~
-
LL
~ 10
CD
o
X
LL
Ad
U]
N
0.
\ ~ ~ \
2
~2
_ 1 1
1 1
, t1 1 \1 1 i 1 ~1 1\
5 10 20 50 100
HORIZONTAL DISTANCE (km)
FIGURE 5.1 Maximum radiant exposures versus ground range from a 1-Mt
air burst (detonated below several kilometers altitude) as a function
of the ground level visibility. The radiant exposures scale roughly
with the yield in megatons. (From Kerr et al., 1971)
bomb light is emitted within a few seconds for megaton yield
explosions, and in less than a second for kiloton-size bursts (GD777.
For a 1-Mt low air burst, Figure 5.1 shows the thermal fluences (in
calories per square centimeter incident on a surface normal to the
line-of-sight through the burst point) as a function of distance from
ground zero, and for various atmospheric Risibilities. With a 1-Mt
explosion and normal Risibilities (>10 km), the 20-cal/cm2 thermal
fluence contour lies about 7 km from the explosion hypocenter, versus 9
km in a perfectly transparent atmosphere. With a 100-kt explosion,
atmospheric transmission, for Risibilities of >5 km, has little
effect on radiant exposures where fluences exceed 20 cal/cm2. Lower
Risibilities restrict the range at which nuclear thermal effects are
important. Oblique incidence of the bomb light on exposed surfaces
also reduces the effective fluence. On the other hand, cloud and
surface reflections enhance the radiant fluxes in localized regions.
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As sunlight, focused by a lens, can ignite flammable materials, so
can the thermal emissions of a nuclear explosion (Glasstone, 1957;
Miller, 19621. Ignition data obtained during atmospheric nuclear test
detonations and by laboratory experimentation are summarized in Table
5.2. At a specific thermal fluence, small nuclear explosions are
generally more efficient at igniting fires than large explosions
because the thermal pulse has a shorter duration and larger peak
intensity (in addition, there is a lower probability of significant
atmospheric attenuation over the shorter ranges involved). Newspaper,
brown paper, cotton cloth, and dried plant material can be ignited by
10 cal/cm2 from a <1-Mt explosion. The perimeter of the Hiroshima
fire zone roughly coincided with the 10-cal/cm2 contour. At
Nagasaki, in directions unobscured by hills, the conflagration zone
also extended roughly to the 10-cal/cm2 limit.
In the application of nuclear weapons against ~soft. targets (e.g.,
urban and industrial targets), peak overpressures* of >5 psi (pounds
per square inch) are often used to define the zone of assured
destruction (GD77~. The 5-psi contour circumscribes an area of ~1.4
km2/kt for a 1-kt explosion (at the optimum height of burst), ~0.30
km2/kt for a 100-kt explosion, and ~0.14 km2/kt for a 1-Mt
explosion. The corresponding areas enclosed within the 20-cal/cm2
thermal irradiance contours (GD77) are ~0.30 km2/kt, ~0.30
km2/kt, and ~0.25 km2/kt, respectively (in the 1-Mt case, the
atmospheric visibility is assumed to be 20 km). In estimating the
potential fire areas for nuclear air bursts, the committee has chosen
an average ignition area of 0.25 km2/kt (250 km2/Mt) for individual
explosions, which is roughly consistent with 5-psi overpressures and 20
cal/cm2 thermal fluences at the limits of the ignition region, under
normal conditions of atmospheric transmission. These areas are quite
conservative in relation to the areas burned at Hiroshima (~1
km2/kt) and Nagasaki (~0.35 km2/kt).
The question of overlap of ignition zones for closely spaced
detonations, and the total potential fire area in a full exchange, are
discussed in a separate section of this chapter.
Close to the hypocenter of a nuclear explosion, the thermal
energies are much larder than 20 cal/cm2. Within the 30-cal/cm2
contour (about 150 km for a 1-Mt explosion), substantial quantities
of natural and synthetic organic and cellulosic materials would be
instantly pyrolized, and the combustible vapors ignited in a massive
"flashover" fire. The rising fireball would then draw the flames and
smoke toward the stem of the nuclear cloud, establishing the conditions
for accelerated burning and, in some cases, the core of an incipient
firestorm.
For surface and subsurface nuclear detonations, the potential
thermal effects are greatly reduced (although the dust and prompt
radioactive fallout effects are increased). The bomb light from a
*The term "overpressure" refers to the incremental static pressure
above ambient atmospheric pressure (about 14.7 pounds per square inch
at sea level) caused by the passage of the explosion wave.
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TABLE 5.2 Approximate Radiant Exposures for Ignition of Various Flammable
Materials for Low Air Bursts
Radiation
Exposurea
(cal/cm2)
Effect on35 1.4 20
Material Color Materialkt Mt Mt
Household Tinder Materials
Newspaper, shredded Ignites4 6 11
Newspaper, dark Ignites5 7 12
picture area
Newspaper, printed Ignites6 ~15
text area
Crepe paper Green Ignites6 9 16
Kraft paper Tan Ignites10 13 20
Bristol board, 3 ply Dark Ignites16 20 40
Kraft paper carton, Brown Ignites16 20 40
used (flat side)
New bond typing paper White Ignites24b 30b 50b
Cotton rags Black Ignites10 15 20
Rayon rags Black Ignites9 14 21
Cotton str ing Gray IgnitesLob 15b 21 b
scrubbing mop (used)
Cotton string Cream Igniteslob lob 26b
scrubbing mop
(weathered)
Paper book matches, Ignites11b 14b 2ob
blue head exposed
Excelsior, ponderosa Light Ignites__c 23b 23b
pine yellow
Outdoor Tinder Materialsd
Dry rotted wood Ignites4b 6b 8b
punk (fir)
Deciduous leaves Ignites4 6 8
(beech)
Fine grass (cheat) Ignites5 8 10
Coarse grass (sedge) Ignites6 9 11
Pine needles, brown Ignites10 16 21
(ponderosa)
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TABLE 5.2 (continued)
Radiation
Explosurea
(cal/cm2 ~
Effect on 35 1.4 20
Material Color Material kt Mt Mt
Construction Materials
Roll roofing, mineral
surface
Ignites c >34 >116
Roll roofing, smooth Ignites _ c 3077
surface
Plywood, Douglas fir Flaming 9 1620
during
exposure
Rubber, pale latex Ignites 50 80110
Rubber, black Ignites 10 2025
Other Materials
~-
Aluminum aircraft Blisters 15 3040
skin (0.020 in.
thick) coated with
0.002 in. of standard
white aircraft paint
Cotton canvas
sandbags, dry filled
Coral sand
Siliceous sand
Failure 10 18 32
Explodes
(popcorning)
Explodes
(popcorning)
15 27 47
11 19 35
aRadiant exposures for the indicated responses (except values marked with a
superscript b, see footnote b) are estimated to be valid to +25 percent
under standard laboratory conditions. Under typical field conditions, the
values are estimated to be valid within +50 percent with a greater
likelihood of higher rather than lower values.
bIgnition levels are estimated to be valid within +50 percent under
laboratory conditions and within +100 percent under field conditions.
CData not available or appropriate scaling not known.
dRadiant exposures for ignition of these substances are highly dependent on
the moisture content.
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surface detonation is more effectively shadowed by buildings, terrain,
and other obstructions than is the light from an air burst (Miller
19621. The crater ejecta may also cover nearby fuel and smother
incipient fires. In a subsurface explosion (where an armored
penetrating warhead is used) the thermal pulse is substantially
attenuated (GD77~. Moreover, the base surge (caused by ejected
material falling back upon the crater) could snuff out small fires and
cover the fuel near the explosion site. Nevertheless, in a surface
burst, it is still likely that primary thermal (and in cities,
secondary blast-induced) fires would occur out to the -2-psi
overpressure contour (i.e., over an area of about 150 km2 for a 1-Mt
detonation; GD77~. In buried explosions the situation is more
complicated because both ground shock and air blast could contribute to
secondary fire ignitions in cities. In any case, the present baseline
scenario specifies air bursts against all urban and industrial targets,
with only 30 percent (1500 Mt) of the remaining bursts detonated on the
surface.
The fire effects of multiple nuclear detonations over cities and
forests are complex and undetermined. Smoke from the fires of initial
bursts could block subsequent thermal flash effects in some cases.
Delayed bursts would probably spread existing fires, however,
particularly by generating strong surface winds and convective plume
activity. Closely spaced explosions over forests could greatly enhance
the probability of fire ignition and spread. The problem of multiburst
phenomena has not yet been adequately treated in the nuclear effects
literature.
Urban Ignition
Some evidence that nuclear explosions are unique in their ability to
ignite mass fires is offered by the Hiroshima and Nagasaki
experiences. One crude estimate of the average energy release rate
places the Hiroshima fire among the least intense of the mass fires of
World War II (Martin, 1974~. Nevertheless, centripetal winds
characteristic of a firestorm apparently developed, and the fuel
consumption within the fire zone was nearly complete (GD77; Ishikawa
and Swain, 1981~.
Some of the factors that affect nuclear fire genesis in cities are
summarized in Table 5.3. Even though the blast wave that follows the
thermal pulse could extinguish many of the primary thermal radiation
fires, a substantial number of these ignitions would continue to burn.
Idealized field tests to determine the efficiency of fire extinction by
pressure waves are contradictory, and often little or no effect is
observed (Wiersma and Martin, 1973; OTA, 1979; Backovsky et al.,
19821. In fact, in one study, the blast dispersal of burning curtain
fragments through a room was a major factor in fire development
(Goodale, 19711. In addition, the blast ignites many secondary fires
and creates conditions (Table 5.3) that strongly favor the growth and
spread of the surviving fires. Overall, blast would appear to
encourage mass fire development. The evidence from Hiroshima and
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96
Wiersma, S.J., and S.B. Martin (1973) Evaluation of the Nuclear Fire
Threat to Urban Areas. Report AD779-340. Menlo Park, Calif.:
Stanford Research Institute. 131 pp.
Williams, D.W., J.S. Adams, J.J. Batten, G.F. Whitty, and G.T.
Richardson (1970) Operation Euroka: An Australian Mass Fire
Experiment. Report 386. Maribyrnor, Victoria, Australia: Defense
Standards Laboratory.
Wolff, G.T., and R.L. Klimisch feds.) (1982) Particulate Carbon:
Atmospheric Life Cycle. New York: Plenum Press. 411 pp.
Woodie, W.L., D. Remetch, and R.D. Small (1983) Fire spread from
tactical nuclear weapons in battlefield environments. PSR Note 566
Santa Monica, Calif.: Pacific Sierra Research Corp. 53 pp.
Wright, H.A., and A.W. Bailey (1982) Fire Ecology, United States and
Southern Canada. New York: John Wiley and Sons.
.
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97
APPENDIX 5-1: OBSERVATION OF PLUME HEIGHTS AND
ASH TRANSPORT IN LARGE FIRES, by F.E. Fendell
Plume Heights
The altitude achieved by a plume over a maintained source of buoyancy
depends largely on the strength of the source (heat released per unit
time), the stratification and humidity of the ambient air, the strength
of the crosswind (if any), and the size of the region of
exothermicity. Rarely are all the desired inputs known for a single
event.
As a reference, one of the more dramatic persistent plumes of the
last quarter century was that associated with the creation of Surtsey
off Iceland. An effective heat source estimated at 1011 J/s (with
upflow at the base of roughly 120 m/s) was initiated at 7 A.M. on
November 14, 1963 (the energy release rate was equivalent to about 250
kt every 3 h). By 10:30 A.M. the plume was at 3.5 km; by 3 P.M., at
about 6.3 km; and by the next day, at over 9.3 km (i.e., to the height
of the tropopause near Iceland). Vapor columns rose from neighboring
sites on the sea to 2.5 km, and ash-laden steam burst upward to 0.6 km
in a gigantic, ink-black column (Bourne, 1964; Thorarinsson and
Vonnegut, 1964; Thorarinsson, 19661.
As another reference, the series of artificial convection
experiments conducted at the Centre de Recherches Atmospheriques Henri
Dessens, on the Lannemezan plateau in the French Pyrenees, entailed 105
fuel oil burners deployed in a three-arm spiral within a 140 m x 140 m
square (the Meteotron). The heat release rate was about 109 J/s for
20 to 30 min (a total energy release of about 0.5 kt), and the plume
reached 1 to 2 km {Benech, 1976; Church et al., 19801.
Plumes of most small-scale fires reach only a few kilometers into
the troposphere. The black plume of a 101° J/s oil fire that
persisted for days near Long Beach, California, rose to 4 km (Henna and
Gifford, 1975~. The convection column associated with the bombing of
Leipzig in World War II, an event severe enough to give 15 m/s
ground-level radial inflow at 4 km from the center and 34 m/s closer
in, rose to only 3.9 km (Broido, 1960~. The first thousand-bomber raid
by the British in World War II (on Cologne, on May 30-31, 1942)
produced a column of smoke that rose to 4.5 km (and hung as a huge pall
at daybreak) (Barker, 19651. Taylor et al. (1973) reported a brushfire
near Darwin River, Australia, on September 10, 1971, in which the
ambient temperature fell almost linearly from 301 K at ground level to
268 K at 6 km. Whereas the plume rose to 3 to 4 km for a heat release
rate of 1011 J/s, during a 10- to 15-min interval the plume advanced
to 5.8 km when the heat release rate doubled. A small cloud above the
plume was sucked down into it 10 min after this rapid additional
ascent. However, the fuel loading for this case was about one-tenth
that in portions of the American Pacific Northwest, which has the
highest loadings in the continental United States. Thus one is
motivated to examine severe burning events more closely.
Of the acreage burned in the United States annually, 95 percent
comes from 2 to 3 percent of the total number of fires; these
exceptional fires tend to occur in dry, hot, windy weather, can jump
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98
rivers and lakes, and decay only with wind shifts, the arrival of
precipitation, and/or the exhaustion of fuel. Thirteen fire complexes
in the recorded history of North America have each taken 4000 km or
more. Twelve thousand square kilometers were burned by the Maramichi
and Maine fires of 1825, the North Carolina fire of 1898, and the Idaho
and Montana fires of 1910; the Alaskan fires of 1957 consumed 20,000
km2. Fire complexes in Michigan in 1871, in Wisconsin in 1894, and
in Washington and Oregon in 1910 each burned 8000 km2. Southern
states lead the national fire statistics annually in both frequency and
area burned; however, natural decomposition is slower in the North and
fuel loads accumulate, so while the number of fires is fewer, with
droughts come holocausts. As for extremes in spread rate, an 1887
Texas grass fire spread 26 km in 2 h, and crown fires propagating at 16
km/in have been recorded (Pyne, 1982~.
At one time the August 1933 fire in Tillamook County, Oregon, was
regarded as the most intense in recorded American experience. On
August 24, 1933, hurricane-like winds arose, and 800 km2 were burned
in 20 h. The plume, which had reached 3 km (Holbrook, 19431, pierced
an inversion, and the smoke column reached 11.1 to 12 km,
near-tropopause-level altitude (Pyne, 19821.
In recent years, several events perhaps comparable in intensity to
the Tillamook fire have been recorded. The Sundance fire in the
northern Idaho area of Pack River and McCormick Creek advanced 14.5 km
and burned 200 km2 from 2 to 11 P.M. on September 1, 1967. The
energy release rate is estimated at 5 x 105i J/s, and the convection
column reached 10.7 km, even though a 32- to 80-km/in wind was blowing
(Anderson, 1968~. The peak rate was achieved during saturation
spotting in a valley somewhat sheltered from the wind.
A fire at an Air Force bombing range in North Carolina in 1971 was
characterized by a crosswind of 32 km/in, a heat release rate of 1.2 x
1011 J/s, and a plume height of 4.6 km. A fire in the Sierra
National Forest on July 16, 1961, burned 20 km2 in 5 h, and a
convective column rose to 6 to 9 km. The so-called Mack Lake fire in
the Huron National Forest, Michigan, on May 5, 1980, burned 100 km2
in 6 h; though the highly bent plume rose to only 4.6 km in the intense
crosswind, the heat release rate has been estimated at 1.6 x 1011 J/s.
However, the highest free-burning-fire heat release rates are
associated with firestorms, the exceptional heat-cyclone consequences
of massive incendiary air raids on urban targets during World War II.
The rareness of these events is evidenced by the fact that the U.S.
Strategic Bombing Survey characterizes only four firestorms (Hamburg,
Kassel, Darmstadt, and Dresden) arising from the 49 major German cities
subjected to incendiary bombing (SPRI, 1975~. No firestorm arose as a
consequence of fifteen massive incendiary raids from March to June 1945
on Osaka, Kobe, Nagoya, Tokyo, or Yokohama, although the atomic bombing
of Hiroshima produced a firestorm.
At Hamburg, during the raid on July 27-28, 1943, a cumulonimbus-
cloud-like plume with an anvil top, of 3-km thickness, rose to 10 km
(Ebert, 1963; Morton, 1970) in a near-adiabatic lapse rate in the
lowest few kilometers of the troposphere; this altitude was ascribed by
a meteorologist 6 km away, although Brunswig (1982, page 245) ascribes
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99
a height of only 7 km. Thick black smoke reached 6.9 km in half an
hour after the onset of bombing; later-arriving crews reported severe
turbulence, and some aircraft returned to base soot-covered
(Middlebrook, 1981; Musgrove, 1981~. Large black greasy raindrops fell
along the outskirts of the fire (Caidin, 1960~. Smoke and dust blotted
out the sky for 30 h after the attack; the sun was not seen by Hamburg
residents the next day (Rumpf, 19631.
Dresden was subjected to two massive raids on February 13-14, 1945,
though stratocumulus clouds caused a total overcast above 3 km for most
of the night, and strong winds persisted. In these raids, 12.4 km2
were 75 percent destroyed, and an additional 4 km2 were 25 percent
destroyed, by fires that persisted 7 days and 8 nights. A firestorm
occurred in a quarter circle of 2.2-km radius around the time of the
raid. At daybreak on February 14, the city was obscured by a column of
yellow-brown smoke filled with lifted flotsam; this column appeared
particularly dark up to 4.8 km. Sooty ash showered downwind as far as
29 km for several days (Irving, 19651.
Smoke Obscuration
There are accounts of smoke so thick from Pacific Northwest forest
fires that navigation on the Columbia River and other inland waterways
was brought to a standstill in 1849 and 1868.
An instance of sun obscuration is given by the Peshtigo fires
(October 8-9, 1871), in which 5000 km2 were burned along both banks
of the Green Bay. The sun was obscured for 320 km, and gloom
persisted, even at noontime, for a week {Holbrook, 19431. Paper lofted
from Michigan crossed Lake Huron and landed in Canada. On August 20,
1910, some 1750 separate fires in Idaho and Montana blew up and 12,000
km2 were burned, such that the sun was blotted out (Holbrook, 1943~.
However, the time scale for reduced daytime visibility was days, not
weeks.
References
Anderson, H.E. (1968) Sundance Fire: An Analysis of Fire Phenomena.
Research Paper INT-56. Ogden, Utah: Intermountain Forest and Range
Experiment Station, Forest Service, U.S. Dept. of Agriculture.
Barker, R. (1965) The Thousand Plan. London: Chatto and Windus.
Benech, B. (1976) Experimental study of an artificial convection plume
initiated from the ground. J. Appl. Meteorol. 15:127-137.
Bourne, A.G. (1964) Birth of an island. Discovery 25 (Aprill:16-19.
Broido, A. (1960) Mass fires following nuclear attack. Bull. Atmos.
Sci. 16~10~:409-413.
Brunswig, H. (1982) Feuerstrum uber Hamburg. Stuttgart: Motorbuch
Verlag.
Caidin, M. (1960) The Night Hamburg Died. New York: Ballantine.
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100
Church, C.R., J.T. Snow, and J. Dessens (1980) Intense atmospheric
vortices associated with a 1000 MW fire. Bull. Am. Meteorol. Soc.
61(7):682-694.
Ebert, C.H.V. (1963) The meteorological factor in the Hamburg fire
storm. Weatherwise 16~2~:70-75.
Hanna, S.R., and F.A. Gifford (1975) Meteorological effects of energy
dissipation at large power parks. Bull. Am. Meteorol. Soc.
56~1~:1069-1076.
Holbrook, S.H. (1943) Burning an Empire. New York: Macmillan.
Irving, D. (1965) The Destruction of Dresden. New York: Ballantine.
Middlebrook, M. {1971) The Battle of Hamburg. New York: Charles
Scribner's Sons.
(1970) The physics of fire whirls. Fire Res. Abstr. Rev.
Morton, B.R.
12:1-19.
Musgrove, G. (1981) Operation Gomorrah--The Hamburg Firestorm Raids.
New York: Jane's.
Pyne, S.J. (1982) Fire in America--A Cultural History of Wildland and
Rural Fire. Princeton, N.J.: Princeton University Press.
Rumpf, H. (1963) The Bombing of Germany. New York: Holt, Rinehart,
and Winston.
Stockholm Peace Research Institute (1975)
Cambridge, Mass.: MIT Press.
Incendiary Weapons.
Taylor, R.J., S.T. Evans, N.K. King, E.T. Stephens, D.K. Packham, and
R.G. Vines (1973) Convective activity over a large-scale bushfire.
J. Appl. Meteorol. 12:1144-1150.
Thorarinsson, S. (1966) Surtsey, the New Island in the North Atlantic.
Reykjavik, Iceland: Almenna Bokafelagio.
Thor arinsson, S., and B. Vonnegut (1964) Whirlwinds produced by the
eruption of Surtsey volcano. Bull. Am. Meteorol. Soc. 45~8~:440-444.
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101
APPENDIX 5-2: WATER IN NUCLEAR CLOUDS
Clouds produced by nuclear explosions and by the fires they ignite can
hold large quantities of water. The injection of this water into the
upper air layers, and the consequences of the injection, are discussed
in this appendix.
Explosion Clouds
Nuclear explosion clouds hold water that is vaporized and engulfed by
the fireball. Surface bursts over deep water are expected to be
relatively rare in a nuclear exchange and will be neglected (based on
Pacific test data <3 x 106 tons of condensed water per megaton of
yield are expected in the stabilized clouds (Gutmacher et al., 1983~.
Subsurface ocean bursts do not generate high-altitude clouds (Glasstone
and Dolan, 1977~. Surface bursts over land can raise about 3 x 105
tons/Ml of soil to the stabilized cloud height. About an equal amount
of groundwater and mineralized water of hydration might be assumed.
The fireball also entrains ambient water vapor as it rises through the
lower troposphere. Adopting a fireball expansion rate such that dR/dz
~ 0.2 (that is, the increase in the fireball radius is about
one-fifth of the height traversed), and a U.S. Standard (1976)
mid-latitude water vapor profile, the entrained water vapor could vary
from <1 x 105 to about 1 x 106 tons/Ml for a surface burst,
depending on surface humidity. Accordingly, an average stabilized-
cloud-water content of 1 x 106 tons H2O Mt is generous. The water
concentration in stabilized nuclear clouds would be <1 g/m3, which
is generally too small to cause precipitation but large enough to form
an optically thick (ice) condensation cloud. As the nuclear cloud
disperses, the ice particles would either settle out or evaporate. Air
bursts above about 2 to 3 km would hold <1 x 105 tons of H2O per
megaton.
The total water injected by explosion clouds in the baseline
exchange would almost certainly be less than 6000 Tg. Most of the
water would be deposited in the troposphere.
Fire Plumes
There are three sources of moisture for fire plumes: water of
combustion, evaporated surface water, and entrained water vapor. Most
combustible materials generate <1 g-H2O/g-burned. Thus, in the
present baseline exchange, up to 8500 Tg of H2O would be produced
directly by fires, and could disperse with the plumes. Even if 1 cm of
water were evaporated over the entire fire area in the baseline
scenario, only 5000 Tg of additional water would enter the plumes; the
actual amount would be much less, of course. Entrainment of ambient
humidity into the plume, particularly at ground level where air is
often efficiently sucked into the fire, could add >1 g-H2O/g-burned.
At Hiroshima, a crude estimate suggests that about 10 g-H2O/g-burned
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102
were entrained due to the high humidity at the time of the fire (R.P.
Turco, private communication, 1984~. However, most of this water fell
as precipitation (the "black rainy.
Due to condensation and precipitation, only a limited quantity of
water can remain suspended in the fire plumes and be carried long
distances. This quantity is assumed to be S g H2O/g-burned, which
consists primarily of moisture drawn into the fire near the ground.
The total fire plume water injection in the baseline exchange may then
be estimated as 40,000 Tg. The water is injected uniformly between 0
and 9 km (as is the smoke in the baseline case), or about 4000 Tg/km of
altitude. Note that the injection represents primarily a
redistribution of water vapor from the boundary layer into the free
troposphere--as occurs during natural convection--because very little
"new. water vapor is introduced by the combustion process.
The water concentration (condensate plus vapor) in the stabilized
high-altitude plumes of large fires is expected to be about 1 g/m3,
based on the analysis of the water budget of a fire plume discussed
above, air inflow rates obtained from plume theory, and direct
measurements in fire cumulus cap clouds (L. Radke, private
communication, 1984~. The onset of condensation in the convective
column of a fire may occur above the level expected for condensation in
surface air lifted adiabatically, due to the added heat of combustion
and the entrainment of dry ambient air aloft (Taylor et al., 1973~.
Low surface humidity, induced precipitation and entrainment of dry air
can all limit the water concentration in fire plumes.
The column abundances of water in fire clouds could be 1000 to 5000
g/m2, compared to about 10,000 g/m2 in natural cumulus clouds and
10 to 100 g/m2 in cirrus clouds.
An upper limit to the water injection by fires in a nuclear
conflict is in the vicinity of 500,000 Tg. This figure assumes that
the initial fire plumes occupy a volume of 1017 m3 (about one-tenth
of the volume of the northern hemisphere mid-latitude troposphere), all
of the air in the plumes originates in the surface layer and holds an
average of 5 g H2O/m3, and no rainout occurs. Obviously, these
circumstances are highly unlikely.
Water Perturbation
Table 5.2-1 gives the average ambient profile of water vapor at
mid-latitudes. The global troposphere holds roughly 107 Tg of water
vapor and the stratosphere, about 3000 Tg. If all of the water in
nuclear explosion clouds were confined to the mid-latitude
stratosphere, H2O concentrations could increase by a factor of <10
there. Because the stratosphere normally is very dry, with a relative
humidity of only 1 to 5 percent, and injected smoke and dust clouds can
be heated by solar and infrared radiation, any condensed water would
soon evaporate as the individual explosion clouds dispersed. A factor
of 10 increase in stratospheric H2O would affect ozone photochemistry
and the infrared radiation balance of the stratosphere. The
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103
TABLE 5 . 2-1 Amb lent Atmospher ic Water Vapor a
Equivalent Cumulative H2O
Altitude Water Vapor Air Global H2O Mass up to the
Interval Mixing Ratiob Density2 in the Layer Top of Layer
(km) (ppmm) (kg/m3) (Tg) (Tg)
0-0.5
0.5-1.5
1.5-3.0
3.0-5.0
5.0-7.0
7.0-9.0
9.0-11.
11.-13.
13.-15
15.-17.
4686
3700
2843
1268
554
216
43.2
11.3
3.3
3.3
1.225
1.112
1.007
0.8194
0.6601
0.5258
0.4135
0.3119
0.2279
0.1665
1.4x106
2.1x106
2.1x106
l.Ox106
3.7x105
1.1x105
1.8x104
3500
750
550
1.4x106
3.5x106
5.6x106
6.6x106
7.0x106
7.1x106
7.1x106
7.1x106
7.1x106
7.1x106
he
aCondensed water, which may reach concentrations of 10 g/m3
(5 x 106 Tg globally in a 1-km-thick layer), is neglected.
U.S. Standard Atmosphere (1976) Midlatitude Mean Model. The water
vapor mixing ratio is given in parts per million by mass (ppmm).
Local water vapor fluctuations typically exceed 10 percent.
photochemical effect of the H2O, however, would probably not be any
more important than the photochemical effect of the explosion-generated
NOX. The radiation perturbations are discussed below.
The fire plume water injection of about 4000 Tg/km up to 9 km is
typically <1 percent of the ambient water vapor at any level in this
height interval. The total fire H2O injection is <0.5 percent of
the global water vapor burden, and represents about 45 min of the
normal global atmospheric water budget. The maximum perturbation could
occur in the 7- to 9-km layer, where the average mid-latitude water
vapor burden could increase by about 20 percent. If all of the fire
water were put into this layer at northern mid-latitudes, the water
burden would increase by about 0.20 g/m3, or about 400 g/m2.
However, most of this water originates in lower regions of the
atmosphere; the redistribution of water is likely to be less
significant than an increase in the total water burden of the
atmosphere.
The improbable "upper limits water injections discussed in the
previous section would lead to more substantial effects. Nevertheless,
in view of the large ambient quantities of water vapor in the
atmosphere, and the indirect water vapor perturbations to be discussed
below, even the maximum credible water injections by fires could turn
out to be of secondary interest.
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104
CO2 Perturbation
Carbon dioxide injections by nuclear fires are much less important than
water injections. Because CO2 is uniformly mixed throughout the
troposphere and stratosphere (at about 340 parts per million by
volume), the transfer of air between different altitude levels by
nuclear explosions and fires has little effect on the CO2
distribution. The global atmosphere holds about 3 x 106 Tg of
CO2. Nuclear fires could generate about 1 x 104 Tg of CO2,
roughly the amount produced in 1 year from fossil fuel combustion.
Carbon dioxide is transparent in the visible spectrum, does not
condense, and has only a limited infrared opacity (Liou, 1980~. On the
other hand, CO2 perturbations could result from indirect disturbances
in the global biospheric carbon cycle in the aftermath of a nuclear war
(a subject that is not pursued in this report).
Effects of Water Injections
The water injected into the upper atmosphere with dust and smoke can
have a number of important effects:
1. Modification of the photochemistry of ozone (see the previous
discussion and Chapter 6~.
2. Scavenging and washout of dust and smoke particles (see the
discussion in Chapter 5~.
3. Perturbation of the visible and infrared radiation balance by
the condensed and vapor states of water.
During the first week after the start of a nuclear war, the
localized explosion clouds and fire plumes could hold significant
quantities of condensed water. The visible and infrared opacities of
these clouds could be very large (>>1~. Light levels below the
clouds would be very low, particularly when heavy soot loadings are
present. The infrared energy balance of the clouds would be complex,
and some degree of thermal blanketing could result. Nevertheless,
without solar insolation, the ground should still tend to cool. A
strong greenhouse effect is not likely (at least in the case of smoke
plumes) because solar absorption and heating would occur above most of
the infrared opacity of the clouds (see Chapter 7~.
In daylight, the smoke clouds would warm up rapidly, possibly
inducing strong vertical and horizontal mixing of the cloud tops and
edges, and perhaps causing some of the condensed water to evaporate.
At night the clouds would cool by infrared emission, and subsidence
might occur. The turbulence created by these heating and cooling
cycles would be confined primarily to the upper cloud layers where
precipitation is less probable. The major effect might therefore be to
accelerate the dispersion of the smoke clouds. Some of the extended
fire plumes would hold sufficient water to form thick cirrus anvils.
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105
These cirrus could greatly increase the albedo above the smoke plumes,
but would also hold in upwelling infrared radiation.
The large-sca~e advection and spreading of smoke clouds by
self-induced heating has been studied on different size scales. Chen
and Orville (1977) investigated cumulus-scale convection of
carbon-black clouds. R. Haberle et al. (private communication, 1983)
and M. MacCracken (private communication, 1984) simulated the motions
of hemispherical-scale soot clouds. In each case, the same general
behavior was predicted. The clouds tended to rise and spread
horizontally at a faster rate than would be expected if only ambient
air motions were acting. Direct observations of large sooty smoke
clouds reveal the same behavior (Davies, 1959~.
Thus it is expected that some of the energy absorbed in the dust
and smoke clouds would be converted into the kinetic energy of winds,
which eventually dissipates as frictional heat.
Within about 2 weeks, the nuclear dust and smoke clouds would be
sufficiently dispersed that their infrared opacities would be quite
small (<1~. The atmosphere could then approach the radiative regime
analyzed by Turco et al. (1983a,b), Crutzen et al. (1984), and others,
in which the infrared properties of the injected nuclear debris are
less important than the visible properties.
Water vapor, particularly in the stratosphere, can affect the
infrared radiation balance of the atmosphere. It has been estimated,
for example, that a five-fold increase in stratospheric H2O (with all
other factors unchanged) would eventually lead to a 2°C surface warming
(e.g., Manabe and Wetherald, 1967~. However, in the perturbed
atmosphere, even this modest effect is unlikely to occur, because the
surface temperatures and infrared radiation fluxes of the lower
atmosphere would already be greatly reduced.
Indirect Water Perturbations
Changes in surface air temperatures, winds, and atmospheric stability
would disturb the "normal" hydrological cycle. Such disturbances could
be more important than the primary water injections of the explosions
and fires. Among the hydrological perturbations that might develop:
1. Increased low-level storminess and precipitation near
ocean-continent margins, induced by exaggerated sea-land temperature
contrasts.
2. Formation of widespread ground fogs over continents due to
rapid radiative cooling of surface air.
3. Suppression of deep convection and upper-level precipitation
caused by soot-induced heating of the upper troposphere.
4. Decrease in general cloudiness above several kilometers
altitude as a result of warming and reduced relative humidity.
5. Reduction in the global water vapor burden associated with a
general decrease in surface air temperatures.
6. Increase in water vapor concentrations above several kilometers
altitude due to the enhanced moisture capacity of the heated air.
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106
It is not likely that all of these effects would occur. A partial
discussion of the possibilities is given in Chapter 7. Further
research into these problems will be necessary to determine their
importance.
References
Chen, C.-S., and H.D. Orville (1977) The effects of carbon black dust
on cumulus-scale convection. J. Appl. Meteorol. 16:401-412.
Crutzen, P.J., C. Brahl, and I.E. Galbally (1984) Atmospheric effects
from post-nuclear fires. Climatic Change, in press.
Davies, R.W. (1959) Large-scale diffusion from an oil fire. Pages
413-415 In Atmospheric Diffusion and Air Pollution, edited by F.N.
Frenkiel and P.A. Sheppard. New York: Academic Press.
Glasstone, S., and P.J. Dolan (eds.) (1977) The Effects of Nuclear
Weapons. Washington, D.C.: U.S. Department of Defense. 653 pp.
Gutmacher, R.G., G.H. Higgins, and H.A. Tewes (1983) Total mass and
concentration of particles in dust clouds. Rep. UCRL-14397.
Livermore, Calif.: Lawrence Livermore Laboratory. 22 pp.
Liou, K.-N. (1980) An Introduction to Atmospheric Radiation. New York:
Academic Press.
Manabe, S., and R.T. Wetherald (1967) Thermal equilibrium of the
atmosphere with a given distribution of relative humidity. J.
Atmos. Sci. 24:241-259.
Taylor, R.J., S.T. Evans, N.K. King, E.T. Stephens, D.R. Packham, and
R.G. Vines (1973) Convective activity above a large-scale
brushfire. J. Appl. Meteorol. 12:1144-1150.
Turco, R.P., O.B. Toon, T.P. Ackerman, J.B. Pollack, and C. Sagan
(1983a) Nuclear winter: Global consequences of multiple nuclear
explosions. Science 222:1283-1292.
Turco, R.P., O.B. Toon, T.P. Ackerman, J.B. Pollack, and C. Sagan
(1983b) Global Atmospheric Consequences of Nuclear War. Interim
Report. Marina del Rey, Calif.: R&D Associates. 144 pp.
U.S. Standard Atmosphere (1976) Washington, D.C.: U.S. Government
Printing Office.
Representative terms from entire chapter:
forest fires
