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OCR for page 81
7
INTRODUCTION
Thunderstorm Origins, Morphology,
and Dynamics
EDWIN KESSLER
NOAA National Severe Storms Laboratory
Thunderstorms involve rapid vertical rearrangement
of deep air layers. Large processes promote and shape
vertical and horizontal air motions, and processes
within storms control development of rain, hail, and
strong local winds. Espy (1841) first presented the con-
cept that thunderstorm circulations are driven largely
by the heat latent in water vapor, released during con-
densation. During 1946-1947, a U.S. federally funded
project investigated thunderstorms in Ohio and in Flor-
ida. Findings of that project (Byers and graham, 1949)
were the first accurate description of thunderstorm de-
tails, and project methods were a principal part of the
foundation of subsequent studies. A modern compre-
hensive and abundantly illustrated textbook was pre-
pared by Ludlam (1980~. Thunderstorm processes are
presented in much detail, with many references, in Vol-
ume 2 of a recent three-volume treatise on thunder-
storms (Kessler, 1985), and the lightning process is de-
tailed elsewhere in this volume.
VERTICAL CONVECTION AND THE
GREENHOUSE EFFECT
Vigorous vertical air currents and thunderstorms are
a consequence of excessive warmth and moisture at low
81
altitudes. In a global sense this stratification of proper-
ties stems largely from the radiative properties of atmo-
spheric constituents. The atmosphere is largely trans-
parent to solar radiation, which is concentrated in the
yellow region of the spectrum. Most of the solar energy
not reflected by clouds passes through the atmosphere
unabsorbed and is converted to heat at the ground or in
surface waters. The absorbing surface also radiates, but
because its temperature is much lower than the Sun's,
terrestrial radiation is concentrated in infrared wave-
lengths. Atmospheric carbon dioxide, water vapor, and
some other trace constituents are significant absorbers
of much of the outgoing infrared emission, so this outgo-
ing radiation is substantially blocked and further heats
the air at low altitudes.
The average temperature at low altitudes provides
long-term equality between the rate at which heat is
carried away and the rate at which it is received. Re-
moval processes include some radiative losses through
spectral windows between absorbing bands of carbon
dioxide and water vapor and transport of heat by air
parcels in motion. This latter process is known as con-
vection. Heat gained at the surface is carried by convec-
tion to altitudes where the radiative process is more ef-
fective because there is less absorbing medium above.
Figure 7.1 shows that, globally, thunderstorms are most
frequent in low latitudes where a larger surplus of heat
OCR for page 82
82
FIGURE 7.1 Annual number of thunder-
storm days. Adapted from World Meteoro-
logical Organization (1953~.
150 120
(due to radiative imbalance) is transported aloft by con-
vection.
The process of vertical convection within an air mass
contrasts with a global process driven by horizontal
temperature differences on a very large scale. Thus,
equatorial regions are warmer than polar regions, and
there are various circulation regimes that function to re-
duce this temperature contrast. These circulations, pri-
marily horizontal, associate with some vertical air mo-
tions as well, and they tend sometimes to enhance and
sometimes to diminish the conditions that favor the in-
tense vertical circulations of thunderstorms.
THE SIZE OF THUNDERSTORMS
About 85 percent of the atmospheric mass is con-
tained in the lowest 13 km this is the layer that gener-
ally participates in the vertical circulation of a typical
thunderstorm. The typical horizontal dimension of a
thunderstorm is similar to its vertical dimension. On the
other hand, the storms of winter, essentially embodying
horizontal air motions, respond to variations of temper-
ature in the horizontal plane extending over hundreds or
even thousands of kilometers; these storms are corre-
spondingly larger horizontally than vertically.
The vertical circulation of air in a thunderstorm is a
result of density differences. When the air is relatively
warm, it is relatively expanded and weighs less than a
parcel of the same size in its surroundings. The warm
parcel is subject to pressure forces; these vary with
height according to the weight of air in the large envi-
ronment, and they are not fully balanced by the warm
parcel weight. The imbalance is represented by a net
upward force, Archimedean buoyancy, which causes
the warm parcel to rise. As it rises, the air ahead of it
EDWIN KESSLER
70p
30
TO
30
_ fi~
70 , , , << 1 ~ ~ ' ' ' ' 1
80 150 120 90 60 30 0 30 60 90 120 150 180
must move out of the way. The greater the horizontal
extent of a rising parcel, the farther the displaced air
must move and the more resistance there is to the par-
cel's motion. Very large buoyant parcels cannot rise rap-
idly because of the large volume of air that must be si-
multaneously displaced horizontally. Thus, vertical
motions are faster in smaller rising parcels. On the other
hand, the smallest buoyant parcels tend to be more
eroded by diffusive processes operative on their bounda-
ries. In the final analysis, more definite conclusions as to
scale rest largely on empirically determined characteris-
tics of diffusion and consideration of equations of three-
dimensional motion.
Figure 7.2 shows a zone of severe thunderstorms ex-
tending hundreds of miles across the central United
States. The large horizontal extent of the zone is related
to the large scale of a disturbance engendered primarily
by horizontal temperature contrasts. Along the zone,
numerous boundaries between individual thunder-
storms are evident. They mark the dimensions of local
storms driven primarily by vertical temperature con-
trasts, locally enhanced by processes intrinsic to the
larger-scale disturbance.
Conditioning the Air for Overturning
A proclivity for overturning is enhanced as the lower
atmosphere is warmed. This occurs daily after sunrise,
and afternoons are often marked by puffy clouds indica-
tive of rising thermal currents. Seasonally, temperature
rises as day length increases, and as the Sun rises higher
in the sky, so spring and early summer usually present
more frequent and more vigorous showers and thunder-
storms than does fall. In other words, the thundery
weather that tends to be frequent around the vernal
OCR for page 83
THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS
FIGURE 7.2 Photograph in visible light from synchronous Meteorological Satellite SMS-1, 6 May 1975, 1405 CST. A major squall line harboring
tornadoes lies across the central United States. Lakes Superior, Huron, and Michigan are in upper right. Photo courtesy of NOAA, National
Environmental Satellite Service.
equinox in mid-latitudes is stimulated by the relative
coolness aloft that remains from winter.
A rising air parcel encounters decreasing air pressure
and expands. This expansion represents work done on
the environment and is accompanied by a decline in
temperature. A rising parcel continues to rise as long as
it remains warmer than its environment-this means
that the ambient temperature must decline with height
as fast as or faster than the temperature of the rising
parcel declines as its altitude increases. A shower or
thunderstorm occurs only when there is a sufficient de-
cline of ambient temperature with height in a deep
layer.
There would be no rain without moisture in the air,
OCR for page 84
84
but moisture represents energy as well a large amount
is latent in water vapor and enters the atmosphere as
surface waters evaporate. Most of the air that harbors
thunderstorms is conditioned during days or weeks over
warm seas. During the westward journey of an air mass
in the North Atlantic trade-wind zone, for example, the
depth of the moist layer increases daily and is often
about 2000 m thick when the air turns northward and
enters the United States. The low-altitude moist layer
may be surmounted by a very dry layer. This stratifica-
tion is produced even as heat and moisture are supplied
from below, while weak subsidence, associated with the
same subtropical high-pressure area that engenders the
trade winds, maintains dry air at middle levels, pro-
tected from incursions of moisture from below by a tem-
perature inversion. Dry air aloft may also be produced
by other processes. For example, precipitation that ac-
companies ascent of air on the windward side of a
mountain range leaves the air drier on the leeward side.
A warm temperature and large quantity of water vapor
in an air mass at low altitudes and only a small amount
of water vapor aloft, is a significant combination
(known as a connectively unstable condition) whose po-
tency is most realized when a large-scale disturbance
causes generalized horizontal convergence and rising air
motion. In such a case, the moist air at low altitudes
cools less rapidly than the air above because the low-
level air gains the heat latent in the water vapor as that
vapor condenses. A result is that the temperature de-
cline with height increases, i.e., the lapse rate becomes
much steeper. Then rising parcels within the air mass
and overturning within the air mass can become a vio-
lent process.
CONDENSATION AND PRECIPITATION IN
RELATION TO AIR MOTION
As an ascending air parcel cools below the tempera-
ture at which its vapor is saturated, condensation occurs
on nuclei, usually motes of sea salt, nitrates, or sulfates,
numbering typically about 1000/cm3 over land. There is
an initial selection or activation process whose details
depend on the nature and number of the nuclei and on
the ascent rate, reflected in extent of vapor supersatura-
tion. Following activation, only the selected nuclei con-
tinue to grow and the balance evaporate. Owing to dif-
ferences in initial size and composition, growth of the
different nuclei proceeds at different rates. If ice coexists
with supercooled liquid water, i.e., liquid at tempera-
ture below the normal freezing point, there is growth of
the ice particles by deposition of vapor diffused from the
liquid particles, which tend to evaporate. Brownian
motion, turbulence, and differential fall speeds among
EDW I N KESS LER
the particles all contribute to coalescence and further
broadening of the size distribution. After some tens of
minutes, unless reduced in size by evaporation in dry air
mixed into the cloud from its environment, some of the
particles have appreciable fall speeds. Whether ice or
liquid, they now begin to sweep out the smaller particles
in their paths and fall ever more rapidly to the ground.
Convective precipitation is typically intermittent. Its
fluctuating character at a place is related not only to the
passage of discrete shower cells overhead but also to a
dual effect of the condensation process. The condensa-
tion process releases latent heat, which tends to reduce
air density and stimulate the updraft, but the condensa-
tion process also burdens the rising air with the weight
of the thousandfold-denser condensate particles, which
may be water or ice. Unless the updraft speed is mark-
edly stronger than the fall speed of precipitation parti-
cles, precipitation accumulates in the updraft until the
positive effects of thermal buoyancy are quite over-
whelmed. Thus an initial updraft usually becomes a
downdraft after about 30 minutes, as indicated in Fig-
ure 7.3. The downdraft can be quite strong, especially
when the weight of precipitation is augmented by a
cooling that accompanies evaporation of cloud and pre-
cipitation into dry air mixed into the cloud at middle
levels. Such downdrafts necessarily become divergent
horizontally as they near the Earth's surface, where they
represent a significant threat to aviation.
In some severe storms, the weight of accumulated
condensation products contributes to a splitting process
that leads to separate storms, moving to the left and
right of the direction of the vertically averaged wind. In
the northern hemisphere, the rightward-moving storms
are more likely to harbor tornadoes; damaging hail is
often found in left movers.
E
-12
c
o
4,, 8
o
D
~ 4
._
I
CU M U L US
MATURE DISSIPATING
~;tftilIT)~\l /:
~ ~,'',//,~D,~;,, ,; '," ~ In,/' ;_1~. Did;_ I at
1 6km
FIGURE 7.3 Three stages in the life cycle of an ordinary thunder-
storm cell, as observed by the Thunderstorm Project. After Byers and
Braham (1949).
OCR for page 85
THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS
STRONG CONVECTION
Development of a severe convective system depends
on a stratification of temperature, moisture, and hori-
zontal wind that produces extreme buoyancy forces
tending to accelerate the motion of both ascending and
descending air parcels. A steepening of the temperature-
lapse rate is not alone sufficient, because there are many
situations in which the energy of overturning can be dis-
sipated in minor events about as fast as it becomes avail-
able through solar heating or other processes. Regimes
of frequent events of weak to moderate intensity charac-
terize air masses that are moist and weakly unstable
through great depths.
Usually a combination of the following conditions is
associated with extreme convection in middle latitudes:
(a) The airmass is connectively unstable. That is,
there is a considerable lapse rate of temperature, and
moisture is abundant only at low altitudes.
(b) A disturbance in the larger-scale flow (e.g., a
short-wave trough in the westerly current aloft, often
having a marked low-pressure system at the Earth's sur-
face) provides generalized lifting, which causes the con-
vective instability to become realized.
(c) The moist lower layer is separated from the dry
zone by a stable layer or even a temperature inversion,
which inhibits early overturning within the airmass and
premature loss of potential energy.
(d) There usually is differential temperature advec-
tion (warm air advection at low altitudes and, occasion-
ally, cold air advection aloft).
(e) A marked increase of the wind with height has a
dual enhancing effect. First, warm air from convective
towers is carried rapidly downstream by the strong
winds aloft, preventing its local accumulation aloft and
permitting longer duration of local convection; second,
variation of the wind with altitude can cause the up-
draft column to slope with height and contribute to an
organization of the flow that enables the intrinsic cool-
ness of the air at middle levels to be manifested in overall
storm energy. In brief, the precipitation formed and
carried to great heights in strong updrafts can descend
into intrinsically cold middle-level air, hastening that
air's descent. In this case, where precipitation does not
descend in the updraft in which it was formed, the storm
may acquire a quasi-steady character.
The conditions described above are often associated
with weather-map features like those shown in Figure
7.4.
Figure 7.5 shows a schematic vertical cross section
through a quasi-steady severe storm that might form un-
der the conditions depicted in Figure 7.4. The more de
85
/ ~N~ \
Upper
Ridge
~km
~ ~ `~-
l
o 200
, ~
FIGURE 7.4 Idealized sketch of a middle-latitude weather situation
especially favorable for development of severe thunderstorms. Thin
lines denote sea-level isobars around a low-pressure center with cold
and warm fronts. Broad arrows represent low-level jetstream (LJ),
polar jet (PJ), and subtropical jet (SJ). The LJ advects moisture-rich air
from subtropical regions to provide the basic fuel for convection. Se-
vere storms (hatched area) are most likely to start near I and gradually
shift toward the east while building southward. Severe thunderstorms
also occur with many variations on this basic theme. From Barnes and
Newton (1985).
tailed plan view of such a storm (Figure 7.6) illustrates
two downdrafts. The forward-flank downdraft in the
rain area is largely an effect of the weight of condensa-
tion products; the rear-flank downdraft is thought to
owe its existence in part to a barrier effect on the ambi-
ent winds produced by ascent of air from low altitudes
in a shearing environment. Another cause of the rear-
flank downdraft is evaporative cooling of air, dry and
intrinsically cold at heights of 3 to 4 km, by precipitation
descending into it from greater altitudes. Finally,
Brandes (1984) proposed a mechanism by which this
rear-flank downdraft would be stimulated by formation
of a tornado or other vortex at low altitudes.
Figures 7.5 and 7.6 show asymmetries that are critical
features of persistent severe-traveling-storm complexes;
Figure 7.3, in contrast, presents more symmetrical fea-
tures, especially in its first and third frames. South of the
severe-storm center in Figure 7.6, where a mesocyclone
and possibly a tornado are located, a line of convective
clouds (the flanking line) marks the intersection of air
descended from middle levels with warm air rushing to
OCR for page 86
86
km mb
16 - 100
14 _
12 ~ 200
10 _
- 300
8 _
6 500
4 _
- 700
2 _
0 _ 1000
EDWIN KESSLER
l
1 ~ ~ . ~
0 10 20 30 40 km
~J~
1~g =,,,
PRECI P ':
`' at. PRECI ~
~ ~ ~ ALOFT ',
- ~ ~ ~ ~
MID-LEV ~: ~Hi_
, ~ ~U_)
PREC`p, ~ INFLow57 \ ~ \)
Em \MDT-7~
FIGURE 7.5 Profile of squall-line thunderstorm (based on series of radar observations, fivefold vertical exaggeration) as it passed Oklahoma City
on May 21, 1961. Winds (full barb, 5 m/see; pennant, 25 m/see) are plotted relative to squall-line orientation; a shaft pointing upward is parallel to
the squall line (azimuth 225°~. In sounding behind storm, balloon passed through anvil outflow, and winds shown are not representative of the
environment. Arrows indicate main branches of airflow relative to squall line that was moving toward right at 11 m/sec. On the right, dashes outline
the supposed air plume; the radar-detected cloud plume at lower elevations consists of small precipitation particles that have partly fallen out of the
air plume while drifting downwind from the storm core. Simplified from Barnes and Newton (1985~.
ward the storm along the ground. At the Earth's sur-
face, the intersection is called the gust front. It can itself
be hazardous to aircraft because of remarkable turbu-
lence and wind shears, and its horizontal winds are
sometimes strong enough to do considerable damage to
trees and buildings.
HAIL
At temperatures below the melting point of ice, solid
and liquid phases can coexist, but the liquid phase is
metastable and starts to freeze with release of latent heat
in the presence of a suitable nucleus or on contact with
r
an ice surface.
Growth of hail to large sizes occurs when strong and
enduring updrafts, with temperature below the melting
point of ice, bear liquid cloud particles in addition to
some icy motes. The liquid particles start to freeze when
contacted by the ice particles, and the latter thereby
grow; they descend to the ground when their fall speed
exceeds the rising speed of the enveloping air current.
The release of latent heat that attends freezing causes
a rise of temperature toward 0°C; a growing hailstone
thus tends to be somewhat warmer than its environ-
ment. (This condition of relative warmth is associated
with an important electrification process treated in
Chapters 9 and 10, this volume.) In the presence of
much liquid water and little supercooling, the hailstone
may enter a stage of wet growth, i.e., one wherein all
the impacting liquid is not frozen immediately but be-
comes absorbed into a previously developed porous
structure or is somewhat shed. In this growth regime,
the frozen material exhibits a clear structure of large
crystals, readily distinguishable from each other under
polarized light. At sufficiently low temperatures and
water contents, liquid cloud and small raindrops im-
pinging on the growing hailstones freeze so quickly that
air bubbles remain entrapped within and between the
globules. The variable growth process is revealed by the
concentric translucent or opaque layers and nearly
transparent zones that appear in hailstone sections (Fig-
ure 7. 7) .
The magnitude and distribution of vertical and hori-
zontal air currents and the associated content of super-
cooled liquid water determine the growth and trajec-
tory of hailstones. If the updraft speed has a maximum
OCR for page 87
THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS
,~
STORM MOTION
N
\
~\ BOUNDARY OF RAIN
EMS< \ \AND REFLECTIVITY
Ad\
FEEDER CELLS ~ ' \\ \ \
~j ~
GUST FRONT ~
\ \\
-
O 5
\
FIGURE 7.6 Schematic plan view of a tornadic thunderstorm at the
surface. The heavy solid line encompasses radar echo. The wavelike
gust front structure Is depicted by a solid line and frontal symbols.
Surface positions of the updraft are finely stippled; forward-flank
downdraft (FED) and rear-flank downdraft (RF13) are coarsely stip-
pled; arrows represent associated streamlines (relative to the ground).
Likely tornado locations are shown by encircled Ts. From Davies-
Jones (1985~.
value of 30 m/see, and the cloud content is 4 g/m3, a
hailstone can grow to about 7.5 cm in diameter before
arriving at the ground, 25 minutes after being selected
for such growth by having fortuitously attained a larger
size than its neighbors.
The rapid ascent of small particles in sufficiently
strong updrafts gives insufficient time for the growth of
any of them to large size. The development of hail in
such strong updrafts may depend on the insertion into
the updraft of hail embryos formed nearby and cycled
into the updraft column by virtue of their descent from
higher levels into horizontally convergent regions be-
low.
Major tornadoes are usually accompanied by large
hailstones. However, we find that tornadoes are usually
absent from the storm class that includes the most dam-
aging hailstorms. Doppler-radar observations of the air-
flows show that the hailstorm updrafts are not so strong
as updrafts in tornadic storms, but the updrafts in major
hailstorms cover a substantially larger area.
87
TORNADOES
Major tornadoes are most often associated with thun-
derstorms of a type illustrated in cross section in Figure
7. 6, and they are identified by a rapidly rotating funnel-
shaped cloud that marks the condensation boundary of
in-spiraling air at low altitudes undergoing adiabatic
expansion and cooling. Tornadoes are most severe and
least uncommon in the United States, but they occur oc-
casionally in India? Australia and New Zealand, South
Africa, Argentina, Japan, and several countries of west-
ern Europe. In Mississippi, the state in the United States
most subject to tornado deaths and damage, statistics on
annual damage and storm frequency suggest that about
1/1000 to 1/300 of the area is affected by tornadoes each
year, with tornado winds of 50 m/see or more and signif-
icant damage to structures. The maximum rate of pres-
sure change may be between 50 and 100 mbars/sec, and
the maximum wind of a major tornado is about 100 m/
sec. The frequency and intensity of these storms and the
areas visited vary from year to year in association with
irregular departures from seasonal norms of other quan-
tities such as temperature and moisture.
Radar data show that tornadoes start at middle levels
(about 5 km), to the rear side of pre-existing cyclonic
circulations about 3 to 10 km in diameter, and develop
downward and upward on a time scale of about half an
hour. At the ground, tornadoes appear on or near a
boundary between rising warm moist air, within which
the release of latent heat of condensation is the storm's
principal source of energy, and air descending from
middle levels where it is intrinsically cold, and cooled
sensibly by the evaporation of precipitation into it.
The most critical dynamical aspect of tornadoes in-
volves the concentration of rotation within them. Vari-
ous investigations during the past 10 years have estab-
lished that two processes have direct importance in this
concentration. The air's angular momentum is a conse-
quence of the Earth's rotation and of various weather
systems. The conservation of angular momentum ac-
companying horizontal convergence and ascent of air
has long been appreciated. Such conservation is mani-
fested, for example, by the increased rotation rate as a
skater's arms are brought in from an outstretched posi-
tion. The second process, more recently detailed, in-
volves the rotation or twisting of horizontal voracity to-
ward the vertical plane. Horizontal vorticity is
represented by the vertical variation of the horizontal
wind (vertical wind shear), already cited as significant
for severe-storm development through its role in facili-
tating removal of condensation products from the up-
draft. The twisting process is commonly effective on an
OCR for page 88
88
FIGURE 7.7 Cross section through
stone. Alternating clear and opaque layers
mark growth under different conditions of
temperature and rate of accretion of liquid
water. Photo courtesy of Charles Knight and
Nancy Knight, National Center for Atmo-
spheric Research.
:''
.. . Aft- At.. ~
i:
ail-;<, ~
~i^ t~ ~r.
.j,:. `,5.!
updraft boundary when the winds at low altitude rela-
tive to the storm motion veer markedly with height.
The properties related to these processes vorticity,
circulation, and angular momentum are interrelated
through considerations of area and distance. Thus the
circulation in a region is the average velocity along the
closed curve that defines the region times the length of
the curve. This is the same as the average component of
vorticity normal to the same region times the area of the
region. The angular momentum of a particle or air par-
cel is measured about a reference point as the distance
from the point times the velocity normal to the line con-
necting the particle and reference point.
CONCLUDING REMARKS
The thunderstorm entity is a result of thermodynami-
cal, microphysical, and electric processes. All processes
interact and must be observed contemporaneously in or-
der to be well understood. The summary presented in
the foregoing pages represents remarkable progress in
understanding, a result of public investment in a fo
EDW I N KES S LER
~. J~ .
I- -:.', A, He ^
Hi..
_.
my.
_ {..
~ ".
-
E
~.-
~s ....
:, .. ..
..
; ^..,
>
cused use of new tools during the past two or three dec-
ades. Meteorological radar, artificial satellites, and
marvelous new technologies for data processing, com-
puting, and communicating have all been critically im-
portant aids and have facilitated impressive new meth-
ods for early identification of storms, dissemination of
information about their location, movement and inten-
sity, significant reduction in death rates from tornadoes,
and marked decline in the rate of weather-related air-
craft accidents. Now there are a host of new methods for
study of the lightning process. As past is prologue, we
confidently expect these new aids to contribute much
toward clarification of the function of electric processes
in severe-storm evolution and toward diminishing the
still significant lightning hazard.
ACKNOWLEDGMENT
The author thanks Evelyn Horwitz, who typed this
paper through several drafts, and Lindsay Murdock,
who improved the paper editorially.
OCR for page 89
THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS
REFERENCES
Barnes, S. L., and C. W. Newton (1985). Thunderstorms in the synop-
tic setting, in Thunderstorm Morphology and Dynamics, E.
Kessler, ea., 2nd edition, Univ. of Oklahoma Press, Norman, pp.
75-112.
Brandes, E. A. (1984). Relationship between radar derived thermody-
namic variables and tornadogenesis, Mon. Weather Rev. 112, 1033-
1052.
Byers, H. R., and R. R. Braham (1949). The Thunderstorm, Report of
The Thunderstorm Project, U.S. Weather Bureau, U.S. Govern-
ment Printing Office, Washington, D.C., 287 pp.
.
89
Davies-Jones, R. P. (1985). Tornado dynamics, in Thunderstorm Mor-
phology and Dynamics, E. Kessler, ea., 2nd edition, Univ. of Okla-
homa Press, Norman, pp. 197-236.
Espy, J. P. (1841). The Philosophy of Storms, Charles C. Little and
James Brown, Boston, Mass., 552 pp.
Kessler, E., ed. (1985). Thunderstorm Morphology and Dynamics,
2nd edition, Univ. of Oklahoma Press, Norman, 415 pp.
Ludlam, F. H. (1980). Clouds and Storms, Pennsylvania State Univer-
sity Press, University Park, Pa., 405 pp.
World Meteorological Organization (1953~. World Distribution of
Thunderstorm Days, WMO no. 21, part 2, Geneva, Switzerland, 71
pp. and maps.
Representative terms from entire chapter:
warm parcel
