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OCR for page 23
Lightning Phenomenology
INTRODUCTION
RICHARD E. ORVILLE
State University of New York atAlbany
Severe weather phenomena that disrupt our lives in-
clude tornadoes, hail, high winds, hurricanes, snow-
storms, and lightning. It is not well known that in most
years, lightning ranks as the number one killer, followed
closely by tornadoes. Much less dramatic than a tornado
passing through an area or a severe snowstorm that par-
alyzes a city, a lightning ground strike can quickly kill
one or two people in less than a second with little or no
warning. Annually in the United States about 100 peo-
ple are killed by lightning strikes, and reliable estimates
for the world would be in the thousands. Lightning on a
global and regional scale is an area of science that brings
together the interests of the atmospheric physicist,
chemist, and meteorologist in an effort to learn its char-
acteristics.
The phenomenology of lightning involves the fre-
quency of lightning observed over large spatial and time
scales. It involves the maximum and average flashing
rate per unit area and the variation of flash characteris-
tics with location and storm type. Studies of lightning
phenomenology can now be discussed in terms of both
satellite and ground-based observations. With the use of
satellites, we obtain data on the global lightning flash
rates and the distribution of lightning with respect to the
continents and oceans. With the extensive use of
23
ground-based observations, we can determine the flash-
ing rates and flash characteristics of individual storms.
In addition, we can monitor the variations of the ground
flashes as a function of location and storm type.
SATELLITE OBSERVATIONS OF LIGHTNING
Optical Detectors
Significant advances in obtaining a better estimate of
global flash rates and distribution have occurred as the
result of satellite lightning observations in the last dec-
ade. Turman (1978, 1979), Turman et al. (1978), and
Turman and Edgar (1982), using optical detectors on
the Defense Meteorological Satellite Program (DMSP)
satellites, showed the distribution of lightning at dawn
and dusk for a period of 1 year. One example of this
recent result is shown in Figure 1.1, where the dusk
lightning distribution for November-December 1977
demonstrates the spatial distribution and the rate. Note
that the lightning is found mostly in the southern hemi-
sphere, but significant activity still occurs in the north-
ern hemisphere.
The latitudinal and seasonal variation of the light-
ning activity is best shown by examining Figure 1.2 (Ko-
walezyk, 1981; Turman and Edgar, 1982~. In this histo-
gram, the lightning rate has been summed over
OCR for page 24
24
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OCR for page 25
LIGHTNING PHENOMENOLOGY
00: ~
SEP-OCT _
_ 0~ ~0 _
-A 20 _ NOV- DEC _
- 15 _ _
10 ~ 1~ -
Z 20F JAN-MAR
10 _
~ O _ _
o 25 - APR
20 _
15 _
10 _
DAWN
I ED
25 - MAY-JUN ~
tool -_,,Wf~ ~
~ ~ · ~ ~. . . .
60~ 4]S US ~ 20~ 40~ ~ ~° US PUS 0° 20
LATITUDE
FIGURE 1.2 The latitudinal variation of the dawn and dusk global
lightning activity as a function of season. Adapted from Kowalczyk
and Bauer (1981) and Turman and Edgar (1982).
longitude at both dawn and dusk and presented as a
function of latitude. The dusk distributions show a
smooth change as the seasons change. But note the sec-
ondary peak at 30-40 ° that persists through all the dusk
distributions except August-September. The dawn dis-
tributions do not change as smoothly, but the seasonal
shift is apparent. The enhanced polar-front activity at
dawn is quite evident in the November-December
southern hemisphere and the April-lune northern hemi-
sphere. At dawn there appears to be an overall mini-
mum for the period January-March. Analysis of the
lightning rates for an entire year show a variation of 10
percent from a global value that is estimated to range
from 40 to 120 flashes per second.
Other studies using the DMSP data from a different
sensor provide a glimpse at the global midnight light-
ning activity with a spatial resolution of approximately
100 km (Orville and Spencer, 1979; Orville 1981~. A
study of the global midnight lightning activity yields a
lightning rate of 96 flashes per second, but this could be
in error by a factor of 2 (Orville and Spencer, 19794.
Orville plotted a series of monthly maps reproduced in
Figure 1.3 for the months of September, October, and
25
November. The progression of the lightning activity to-
ward the southern hemisphere as summer approaches in
that hemisphere is evident. The striking absence of
lightning over the ocean is apparent in all three months
and clearly shows the importance of land in the produc-
tion of thunderstorms.
Radio Detectors
Recent measurements of high-frequency radio noise
by the Ionosphere Sounding Satellite-B have been used
by Kotaki et al. (1981) to estimate a global lightning
flash rate of approximately 300 per second, in contrast
to the optical measurements discussed previously. The
radio measurements may overestimate the lightning fre-
quency since it is assumed that all the emissions are pro-
duced by lightning. But the satellite optical measure-
ments are uncertain in estimating the lightning rate
since the fraction of lightning that is actually detected
depends on a calibration factor that represents a best
estimate.
Despite the availability of satellites to estimate the
global lightning activity, we have made little progress in
obtaining a flash rate with small error bars owing to the
present experimental limitations of sensor sensitivity,
area coverage, and the number of satellite platforms.
Nevertheless, the satellite observation provides us with
the first reliable estimate of the distribution of global
lightning. The resolution of the varying global flash rate
estimates may depend on the close coordination of satel-
lite-based and ground-based observations of lightning
and the availability of larger-coverage-area platforms,
such as geosynchronous satellites.
GROUND OBSERVATIONS OF LIGHTNING
Most of our information on the characteristics of
lightning has come, and will continue to come, from
ground-based observations of the lightning flash. Many
of these studies have focused on the ratio of intracloud to
cloud-to-ground flashes, the lightning ground-flash
density, and the flashing rate of different types of thun-
derstorms.
Intracloud Versus Cloud-to-Ground Lightning
The ratio of intracloud to cloud-to-ground lightning
is of fundamental importance. How does this ratio vary
with latitude and longitude, and how does this ratio
vary in the lifetime of a storm? Are there storms that
have nearly all intracloud flashes and consequently are
less damaging, and are there storms that have almost all
ground flashes and consequently are of greater concern?
OCR for page 26
26
(b)
(c) L
-hem
0NSP SHTELL I TE
HIDNI5HT L16HTNIN6
SEPT 1977 (1813 FLASHES)
~''~'~
· ~
· - · · · -Gus
· ~
~ -A
1 as'
C:,
DEEP SHTELLITE
MIDN16HT L16HTNINE
DCT 1977 (2121 FLASHES)
RICHARD E. ORVILLE
By'
I;
·.'~
id
. ..
N~'-"'¢
..,T, ;._
~ ,
~...
Or
· ·, ~
'vm.2.-... ,,° 'I 4. ~
W..
DH5P SRTELL I TE
HI0N16HT L16HTNIN6
NDV 1977 (2178 ~85HE5)
· ~
.'e''- I- ~.~}'...
FIGURE 1.3 Three maps showing the progression of monthly lightning for (a) September, (b) October, and (c) November. From
Orville (1981), reproduced with permission of the American Meteorological Society.
OCR for page 27
LIGHTNING PHENOMENOLOGY
Studies by Prentice and Mackerras (1977) have sum-
marized much of the available data on the ratio of intra-
cloud to cloud-to-ground flashes (Nt.IN.,). From an anal-
ysis of 29 data sets from 13 countries, they obtain the
following relationship for an average thunderstorm:
Nt.IN~ = (4.16 + 2.16 cos 3~)
(06+ 72-098~' (1.1)
where T. the number of thunder days per year, is less
than or equal to 84 and X, the latitude in the northern
and southern hemispheres, is less than 60°. If the num-
ber of thunder days is unknown, then the ratio can be
estimated from the relation
Nt.IN~ = (4.16 + 2.16 cos 3~. (1.2)
This result is plotted in Figure 1.4. Note that the ratio
has the highest value in the tropics where most of the
lightning was shown to occur by the satellite data. Re-
call, however, that the satellite data were composed of
both intracloud and cloud-to-ground lightning flashes.
There is at the moment no way to distinguish between a
ground flash and an intracloud flash from a satellite.
Lightning Flash Density
The number of lightning strikes per unit time per unit
area, or the flash density, is a fundamental quantity of
interest. Most of the available information has been ob-
tained with lightning flash counters.
Prentice (1977) summarized the values for several ge-
ographical areas and reported 5 flashes per km2 per year
in Queensland, Australia; 0.2 to 3 flashes per km2 per
year in Norway, Sweden, and Finland; and 0.05 to 15
flashes per km2 per year in South Africa depending on
the location.
Piepgrass et al. (1982) reported the results of studying
79 summer storms at the Kennedy Space Center, Flor-
ida, which produced 10 or more discharges, during the
years 1974-1980. Using field mill sites covering an effec-
tive area of 625 km2, they observed an area flash density
for all discharges during June, July, and August to range
from 4 to 27 discharges per km2 per month, with a sys-
tematic uncertainty of perhaps a factor of 2 in the sam-
ple area. The mean and the standard deviation of the
monthly area density over the above years was 12 + 8
discharges per km2. Approximately 38 percent of the dis-
charges were ground flashes. Therefore, they were able
to estimate the ground flash density to be 4.6 + 3.1
flashes per km2 per month.
The most recent estimate of the ground flash density
27
in the United States has been made by Maier and Piotro-
wicz (1983) using thunderstorm hour statistics and is re-
produced as Figure 1.5. They used thunderstorm dura-
tion data from approximately 450 aviation weather
reporting stations, each with an uninterrupted 30 years
of records. The station density available is twice that of
any previous thunderstorm frequency analysis of the
United States. The maximum annual ground flash den-
sities of 18 per km2 are found in the western interior of
Florida. High flash densities greater than 12 per km2 are
found over much of Florida and westward to eastern
Texas. Flash densities greater than 8 per km2 are found
in most of Oklahoma, Kansas, Missouri, Arkansas, Lou-
isiana, Mississippi, and Tennessee. Most western and
northeastern states have flash densities that are less than
4 per km2.
Lightning Flash and Related Characteristics
Data from two summers at the Kennedy Space Cen-
ter, Florida, have been used to estimate the flashing
rates in thunderstorms (Livingston and Krider, 1978~. It
was observed that large storms evolve through an ini-
tial, an active, and a final phase of activity. Most of the
lightning activity was observed to occur in the active
phase with 71 percent of the lightning, although this
phase of the storm occupied only 27 percent of the total
storm duration. During the active phase, 42-52 percent
of all lightning was to ground, while during the final
storm period, only about 20 percent of the lightning was
to ground. The discharge rate for all storms observed in
1975 was approximately 4 flashes per minute with a
maximum flashing rate of 26 discharges per minute dur-
ing any 5-minute period. The highest flashing rate aver-
aged over an entire storm was about 9 discharges per
minute for over 200 minutes. More recent data from a 4-
year interval indicates that the mean rate of flashes is
about 2.4 discharges per minute per storm (Piepgrass et
al., 1982~.
The relationship of rainfall to lightning flash rates has
been investigated by Piepgrass et al. (1982~. They re-
ported that when the meteorological conditions favor
the production of lightning, there is almost a direct pro-
portionality between the total rain volume and the total
number of flashes. Maier et al. (1978) noted in an earlier
paper that the lightning counts were proportional to the
total storm rainfall and that the proportionality in-
creased with the rain volume until the rainfall reached
about 1.2 to 2.7 X 104 m3 per flash. Beyond these vol-
umes, storms that produced more rainfall tended to pro-
duce proportionally less lightning. Piepgrass et al.
(1982) point out that, "Clearly, these problems warrant
further study."
OCR for page 28
28
8
4
2 _
10
.
O 1 1 1 1 1 1 1
0 10 20 30 40 50 60 70
X, DEGREES
FIGURE 1.4 The ratio of intracloud to cloud-to-ground lightning as
function of latitude. From Prentice and Mackerras (1977).
Lightning Location Networks
The study of lightning phenomenology has made a
major advance in the last decade with the introduction
of new magnetic direction-finding techniques (Krider et
al., 1980) that provide the means to monitor ground
strikes over areas exceeding 106 km2. Extensive networks
of lightning direction finders have been established for
forest-fire detection in the western United States, Can-
ada, and Alaska. Figure 1.6 shows the coverage as of the
summer of 1984, and it can be predicted that within the
next few years the entire United States will be covered.
One expanding lightning detection network covers
FIGURE 1.5 Lightning flash density esti-
mates on an annual basis. Adapted from
Maier and Piotrowicz (1983) and MacGor-
manetal. (1984).
RICHARD E. ORVILLE
the East Coast and is approaching the Mississippi River
to provide coverage of the eastern part of the United
States (Orville et al., 1983~. This network is operated by
the State University of New York at Albany in a multi-
drop communication network that links all the direction
finders to one computer. Data are now retrieved on the
time, location, number of strokes in the flash, polarity of
the charge lowered to ground, and amplitude of the
peak magnetic radiation field that can be related to the
maximum current in the first stroke. These data, in
turn, can be analyzed and related to the meteorological
patterns producing the observed phenomena.
To report all initial results would far exceed the space
available in this brief paper; nevertheless, it is interest-
ing to note a few observations. The highest ground flash
rate recorded by the East Coast Network occurred on
June 13, 1984, when 50,836 flashes were detected in a
24-hour period over an area of approximately 250,000
km2. The highest hourly summary was 7800 flashes with
the highest S-minute rate exceeding 10,000 ground
flashes per hour. These results are remarkable when it is
realized that these flash rates were from storms in only
three states- Pennsylvania, New Jersey, and part of
New York and at the time were producing approxi-
mately 3 percent of the entire global lightning activity.
Other results indicate that lightning is recorded in
every week of the year along the East Coast and that the
polarity of the lightning ground strikes shows a change
from negative to positive in the fall and a shift back to
negative in the spring. A discussion of positive lightning
and its characteristics is presented by Rust (Chapter 3,
this volume).
45
4(
35'
30'
25
_
Mean Annuol Ground Flosh Density
(flashes/km2 )
~ I _.
loo105 loon as 90 as 80.
OCR for page 29
LIGHTNING PHENOMENOLOGY
~i~ ~REFERENCES
FIGURE 1.6 Coverage of North America by time of arrival (dashed
lines) and wide-band magnetic direction finders (solid lines) as of the
summer of 1985.
CONCLUSIONS
The past decade has been a period of significant ad-
vances in lightning knowledge. Satellite studies have
provided the first confirmation of the early estimates of
the global lightning flash rates and added new informa-
tion on the distribution of lightning over the land and
over the ocean. The development of widely distributed
ground-based lightning networks provides for the first
time the ability to monitor and calculate lightning char-
acteristics in near real time. Relating these parameters
to the meteorological observations of visible and infra-
red images from space and to radar observations from
the ground poses a major challenge in the near future.
29
Kotaki, M., I. Kuriki, C. Katoh, and H. Sugiuchij (1981). Global dis-
tribution of thunderstorm activity, J. Radio Res. Labs. Japan 66.
Kowalczyk, M., and E. Bauer (1981). Lightning as a source of NOX in
the troposphere, final report FAA-EE-82-4.
Krider, E. P., A. E. Pifer, and D. L. Vance (1980). Lightning direc-
tion-finding systems for forest fire detection, Bull. Am. Meteorol.
Soc. 61, 980-986.
Livingston, J. M., and E. P. Krider (1978~. Electric fields produced by
Florida thunderstorm, J. Geophys. Res. 83, 385-401.
MacGorman, D. R., M. W. Maier, and W. D. Rust (1984). Lightning
strike density for the contiguous United States from thunderstorm
duration records, prepared for Division of Health, Siting and Waste
Management, Office of Nuclear Regulatory Research, U.S. Nuclear
Regulatory Commission, Washington, D.C., 44 pp.
Maier, M. W., and J. M. Piotrowicz (1983~. Improved estimates of the
area density of cloud-to-ground lightning over the United States,
presented at 8th International Aerospace and Ground Conference
on Lightning and Static Electricity, June 21-23, 1983, Forth Worth,
F ~
exas.
Maier, M. W., A. G. Boulanger, and J. Sarlet (1978). Cloud-to-
ground lightning frequency over south Florida, preprint, Confer-
ence on Cloud Physics and Atmospheric Electricity (Issaquah,
Wash.), American Meteorological Society, Boston, Mass., pp. 605-
610.
Orville, R. E. (1981). Global distribution of midnight lightning Sep-
tember to November 1977, Mon. Weather Rev. 109, 391-395.
Orville, R. E., and D. W. Spencer (1979). Global lightning flash fre-
quency Mon. Weather Rev. 107, 934-943.
Orville, R. E., R. W. Henderson, and L. F. Bosart (1983~. An East
Coast lightning detection network, Bull. Am. Meteorol. Soc. 64,
1029-1037.
Piepgrass, M. V., E. P. Krider, and C. B. Moore (1982). Lightning and
surface rainfall during Florida thunderstorms, J. Geophys. Res. 87,
11193-11201.
Prentice, S. A. (1977). Frequencies of lightning discharges, in Physics
of Lightning, R. H. Golde, ea., Academic Press, New York, pp.
465-496.
Prentice, S. A., and D. Mackerras (1977~. The ratio of cloud to cloud-
ground lightning flashes in thunderstorms, J. Appl. Meteorol. 16,
545-549.
Turman, B. N. (1978). Analysis of lightning data from the DMSP satel-
lite, J. Geophys. Res. 83, 5019-5024.
Turman, B. N. (1979~. Lightning detection from space, Am. Scientist
67, 321-329.
Turman, B. N., and B. C. Edgar (1982~. Global lightning distribu-
tions at dawn and dusk, J. Geophys. Res. 87, 1191-1206.
Turman, B. N., B. C. Edgar, and L. N. Friesen (1978~. Global light-
ning distribution at dawn and dusk for August-September 1977,
EOS 59, 285.
Representative terms from entire chapter:
lightning flash
