Bull Volcanol
DOI 10.1007/s00445-010-0393-4
RESEARCH ARTICLE
Volcanic lightning: global observations and constraints
on source mechanisms
Stephen R. McNutt & Earle R. Williams
Received: 2 June 2008 / Accepted: 4 June 2010
# Springer-Verlag 2010
Abstract Lightning and electrification at volcanoes are
important because they represent a hazard in their own
right, they are a component of the global electrical circuit,
and because they contribute to ash particle aggregation and
modification within ash plumes. The role of water
substance (water in all forms) in particular has not been
well studied. Here data are presented from a comprehensive
global database of volcanic lightning. Lightning has been
documented at 80 volcanoes in association with 212
eruptions. The Volcanic Explosivity Index (VEI) could be
determined for 177 eruptions. Eight percent of VEI=3–5
eruptions have reported lightning, and 10% of VEI=6, but
less than 2% of those with VEI=1–2. These findings
suggest consistent reporting for larger eruptions but either
less lightning or possible under-reporting for small eruptions. Ash plume heights (142 observations) show a
bimodal distribution with main peaks at 7–12 km and 1–
4 km. The former are similar to heights of typical thunderstorms and suggest involvement of water substance,
whereas the latter suggest other factors contributing to
electrical behavior closer to the vent. Reporting of lightning
is more common at night (56%) and less common in
daylight (44%). Reporting also varied substantially from
year to year, suggesting that a more systematic observational strategy is needed. Several weak trends in lightning
occurrence based on magma composition were found. The
bimodal ash plume heights are obvious only for andesite to
dacite; basalt and basaltic-andesite evenly span the range of
heights; and rhyolites are poorly represented. The distributions of the latitudes of volcanoes with lightning and
eruptions with lightning roughly mimic the distribution of
all volcanoes, which is generally flat with latitude.
Meteorological lightning, on the other hand, is common in
the tropics and decreases markedly with increasing latitude
as the ability of the atmosphere to hold water decreases
poleward. This finding supports the idea that if lightning in
large (deep) eruptions depends on water substance, then the
origin of the water is primarily magma and not entrainment
from the surrounding atmosphere. Seasonal effects show
that more eruptions with lightning were reported in winter
(bounded by the respective autumnal and vernal equinoxes)
than in summer. This result also runs counter to the
expectations based on entrainment of local water vapor.
Editorial responsibility: JC Phillips
Keywords Volcanoes . Lightning . Electrification .
Ash plumes . Water
Electronic supplementary material The online version of this article
(doi:10.1007/s00445-010-0393-4) contains supplementary material,
which is available to authorized users.
S. R. McNutt (*)
Geophysical Institute, University of Alaska Fairbanks,
903 Koyukuk Drive, P.O. Box 757320,
Fairbanks, AK 99775, USA
e-mail: [email protected]
E. R. Williams
Massachusetts Institute of Technology 48-211,
Parsons Laboratory,
Cambridge, MA 02139, USA
Introduction
Electrical discharge in volcanic eruptions (i.e., volcanic
lightning) is fairly common yet relatively understudied.
Lightning is reported at only a few of the 55–70 volcanoes
that erupt each year (Simkin 1993), so it appears that either
there are systematic problems with reporting, or some
special set of circumstances must prevail to favor the
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production of lightning, or both. Lightning and electrification at volcanoes are important because they represent a
hazard in their own right (people were killed by volcanic
lightning at Paricutin and Rabaul; see McNutt and Davis
2000, p. 45–47), they are a component of the global
electrical circuit, and because they contribute to ash particle
aggregation and modification within ash columns (Lane
and Gilbert 1992; Gilbert and Lane 1994a, b; James et al.
2003; Textor et al. 2005a, b; Mather and Harrison 2006;
James et al. 2008). Conventional thinking has been that
interactions between dry silicate ash particles, such as
collisions and fractures, have been dominant processes
causing electrification, while the role of water is secondary.
This is in contrast to early studies of Surtsey, Iceland, in
which seawater played a prominent role (Blanchard 1964;
Anderson et al. 1965; Blanchard and Björnsson 1967).
Further, the existence and role of ice particles in ash
plumes, now recognized as a fundamental aspect of
thundercloud electrification, has been noted in only a few
studies at volcanoes (e.g. Thorarinsson 1966; Rose et al.
1995a, b, 2000; Hoblitt 2000; Thompson 2000). In this
report, the expansion of steam and its subsequent condensation and freezing are emphasized as primary processes
whose magnitude and effects have been understudied with
respect to volcanic lightning.
Despite the frequent occurrence of volcanic lightning,
and many spectacular photographs, only two systematic
compilations of basic lightning facts have been published
(McNutt and Davis 2000; Mather and Harrison 2006). Here
we present results of a more comprehensive literature
search on the occurrence of volcanic lightning and attempt
to summarize the effects of observed parameters such as
height of plume, volume of tephra and gases (mainly water
vapor), and magma composition. We also discuss reporting
parameters because these affect the historical record of
lightning occurrence. Few instrumental studies of volcanic
lightning have been undertaken (Hoblitt 1994; McNutt and
Davis 2000; Paskievitch et al. 1995; Thomas et al. 2007,
2010), but this study is complementary to all of them.
Accordingly, some recommendations are included regarding desirable electrical studies that will be necessarily
aimed at constraining mechanisms for volcanic lightning.
Data and methods
The main information source for this study is a comprehensive literature compilation that was carried out over a
period of about 10 years. A standard bibliography keyword
search yielded only a handful of studies, so instead the
literature was searched systematically, colleagues were
polled, photographs were collected, and information from
newspapers and other sources, such as poster displays and
videos, was also compiled. The bulk of this work was
completed before modern computer search engines, such as
Google™, were in common use.
Key references included the Bulletin of Volcanology, the
Journal of Volcanology and Geothermal Research, the
Smithsonian Institution’s Scientific Event Alert Network
and Global Volcanism Network Bulletins, and the books by
McClelland et al. (1989), Taylor (1958), and Newhall and
Punongbayan (1996), amongst others. We note that books
published after about 1984 had better indexes than older
works, probably a result of the increased use of electronic word
processing and the ease of making searches by keywords.
Information that was collected included the name of the
volcano, the date (and time, where noted), the Volcanic
Explosivity Index (VEI; Newhall and Self 1982; Simkin
and Siebert 1994) of the eruption, the plume height
maximum, the magma composition, whether it was day or
night, any significant ancillary observations, and references
(Appendix). Latitudes and longitudes for volcanoes are
available from Volcanoes of the World (Simkin and Siebert
1994). VEI values were generally taken from Simkin and
Siebert (1994) after checking that the date of the lightning
observations coincided with the date of an eruption that was
assigned a VEI. We also estimated many VEI values
ourselves based mainly on reports of tephra volumes and
plume heights.
Analyses and results
Reporting problems
We first discuss reporting problems because these must be
understood before other features of the data can be
interpreted. Considerations of the detectability of volcanic
lightning over the full range of sunlight conditions must be
determined. Of the 131 eruptions for which the time of day
was known, we found that 44% occurred during daylight
hours and 56% at night (Appendix). This suggests that
lightning is more easily seen against a dark background and
may be missed during bright daytime conditions. As a
specific example of this effect, the eruption of Mount Spurr,
Alaska, on September 17, 1992, occurred at night and had
dramatic displays of lightning readily noted by people on
the ground. However, the amount of lightning for this
eruption (7 flashes recorded instrumentally) was nearly an
order of magnitude less than for the other two 1992
eruptions (61 and 57 flashes, also recorded instrumentally),
which were of similar size but occurred during the day,
when ground observations of lightning were lacking
(McNutt and Davis 2000). All three Spurr eruptions used
the same instrumental data to determine the number of
flashes; details are given in McNutt and Davis (2000).
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An additional reporting problem with regard to daytime
observations has to do with the fact that many volcanoes
are located in tropical areas where lightning is common
from thunderstorms. Thus volcanic lightning may not be
noticed if an observer is not aware of an eruption, because
the lightning would be dismissed as being due to normal
weather patterns. Conversely, thunderstorm lightning that
happens to occur near an erupting volcano might be
associated with the eruption even if it is not caused by the
eruption, depending on the interpretation of the observer.
For some regions, such as the Alaska/Aleutian arc,
lightning is not common (J. Painter, National Weather
Service, pers. comm., 1997; D. Dissing, pers. comm.,
2003). During a rare thunderstorm near Augustine Volcano
in 1998, several local citizens called the Alaska Volcano
Observatory to ask if Augustine was erupting, because they
remembered that the 1976 and 1986 eruptions had been
accompanied by vigorous lightning (Kienle and Swanson
1985; J. Kienle pers. comm. 1992). The volcano was not
erupting at the time of the 1998 thunderstorm. In contrast,
Augustine’s 2006 eruptions were accompanied by abundant
lightning (Thomas et al. 2007, 2010).
We determined the number of eruptions with reported
lightning versus year (Table 1). Considerable variation is
noted, with values ranging from 0 to 19. The peak year was
1979. Given that between 55 and 70 volcanoes erupt per
year (Simkin and Siebert 1994) this suggests that up to 27–
35% of erupting volcanoes can produce lightning. Another
way to view this is that volcanic lightning is generally
under-reported. It is not clear why the reporting varies so
much from year to year. There is a rough tendency for the
number of reports to be higher following famous eruptions.
For example, 1951 is the only year from 1950 to 1978 to
have more than 10 eruptions reported with lightning. Ten of
11 reports were from Lamington volcano, which had a large
explosive eruption that year. In other years with high
numbers of cases generally one volcano contributed most of
the cases. The large number of reports for 1979 (19)
includes many reports for Aso and Sakurajima; apparently
some sort of systematic study of lightning/electrical activity
was under way in Japan.
Lightning occurrence and volcanic explosivity index
We found that lightning has occurred in association with
212 eruptions at 80 volcanoes (Appendix). These are
significantly higher numbers than the 55 volcanoes cited
in McNutt and Davis (2000) or 58 cited in Mather and
Harrison (2006) and reflect our additional work since then.
VEI values were known or could be estimated for 177
eruptions, and a histogram of these events is shown in
Fig. 1 (bottom). Note that heights are estimated above the
vent for eruptions with VEI=2 and smaller, but above sea
level for eruptions with VEI=3 and larger following the
standard conventions (Simkin and Siebert 1994). For
comparison, a histogram of all known VEI is shown in
Fig. 1 (top). There are many more small eruptions than
large ones, so the numbers increase from right to left. The
drop off for VEI=1 is likely due to under reporting (Simkin
and Siebert 1994).
The number of occurrences of volcanic lightning at
various VEI values is also shown in Table 2 along with all
known VEI and percent of cases showing lightning versus
all VEI. The percent of eruptions with lightning is nearly
the same for VEI=3, 4, and 5. This suggests a standard
reporting efficiency. The percentage of VEI=6 eruptions is
somewhat higher. Such large eruptions attract attention and
are generally well reported. We also note the common
observation that large eruptions create local darkness
because of the dense ash plumes, a condition under which
lightning can be more easily seen. Further, the heights of
the ash plumes are >15 km for this range of VEI, so these
are taller than most thunderclouds (also referred to as deep
convection by atmospheric scientists) and we infer that
similar water based charge generation or separation
mechanisms may be acting (see Discussion section below).
The percentage of eruptions with recorded lightning with
VEI=2 and smaller drops off sharply. This behavior
suggests either a systematic reporting problem, or that
lightning is simply less common in these smaller eruptions,
or both.
Bimodality of eruption plume heights
Eruptions with VEI=3 have ash plume heights of 3–15 km
(Simkin and Siebert 1994); these straddle typical thunderstorm heights, and we find many cases with lightning and
also many without. For VEI=1 and 2 eruptions there are
relatively fewer cases, but these are important because the
ash plume heights are <5 km, less than summertime
thunderclouds (tops at 7 to 20 km a.s.l.; Byers and Braham
1949; Williams 1985), and they suggest that some other
lightning producing mechanisms may be acting. A histogram of ash plume heights is shown in Fig. 2a for the 142
cases for which we have data. The histogram shows a
bimodal distribution with one peak in the range of 7 to
12 km and another peak in the range of 1 to 4 km. The
higher altitude peak represents the typical heights of
ordinary thunderstorms. The low altitude peak, however,
is significantly lower than thunderstorm values.
No comprehensive or extensive compilation of plume
heights is known to us, based on numerous searches and
discussions with colleagues. We expect plume heights,
which are a component of VEI, to have a similar
distribution to the VEI data in Fig. 1 (top). Restated, a
histogram of all plume heights would be expected to have
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Table 1 Number of eruptions
with lightning per year
Prior to 1960 only years with
eruptions are listed
Prior to 1900, the year 1707 is
the only year with more than
one report of lightning
Year
No. Eruptions
Year
2009
2008
2007
2006
2005
2004
2003
2002
2001
2000
1999
1998
1997
1996
1995
6
6
2
15
13
5
0
0
0
3
1
1
1
3
2
1972
1971
1970
1969
1968
1967
1966
1965
1964
1963
1962
1961
1960
0
1
0
0
2
0
1956
1
1994
1993
1992
1991
1990
1989
1988
1987
1986
1985
1984
1983
1982
1981
1980
1979
1978
1977
2
1
4
9
12
1
2
1
4
1
1
1
13
2
3
19
7
1
1953
1952
1951
1950
1949
1948
1947
1946
1945
1944
1943
1937
1933
1931
1929
1924
1914
1912
1
1
11
1
1
1
1
1
1
2
1
1
1
1
1
1
2
1
1976
1975
1974
1973
5
1
1
2
1911
1906
1902
1
1
3
1707
2
Augustine (14)
Colima (11)
Redoubt (12)
Galunggung (6)
Aso (7), Sakurajima (8)
smoothly increasing numbers moving to smaller heights.
Because no compilation exists, we created a synthetic
distribution of heights as follows. We used VEI data to
generate a plot of number of plumes at each km of altitude
by assuming all heights for a given VEI were at the midpoint of the altitude range. For example, the height range
for VEI=2 is 1–5 km so we assumed 3 km as the mean
height. For VEI=5 we used 25 km and for VEI=6 we used
31 km. We then computed a linear regression for the VEI
values from 2 to 6 (heights of 3 to 31 km) and used the
No. Eruptions
1
2
1
1
0
0
Lamington (10)
linear relation to estimate the number of plume heights for
each km. Data were then renormalized so the total number
of eruptions was the same as the starting values. This
procedure gives us estimates of the total number of
eruptions for each km of plume height. We then can plot
the number of eruption plumes with lightning for each
height as a percentage of all eruptions, as shown in Fig. 2b.
Here we observe that all the values for heights of 1–6 km
are 0.3% or less (mean 0.018%), whereas those between 7
and 13 km are 0.3% or greater (mean 0.07%). This
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Fig. 1 Histogram of Volcanic
Explosivity Index (VEI) for all
eruptions in Volcanoes of the
World, by Simkin and Siebert,
1994 (a) and for eruptions
accompanied by lightning (b)
using data from the Appendix.
Note that the eruptions accompanied by lightning are skewed
towards higher VEI values. The
plume heights for VEI 3 and
larger are similar to the heights
of thunderclouds
difference of more than a factor of 3 suggests that eruptions
with ash plume heights of 7 to 13 km, the same range as
thunderstorms, are more efficient at producing lightning
than smaller eruptions. Additional reasons for this systematic difference are given in the discussion section.
Recent instrumental data from Augustine volcano in
Alaska may also shed some light on this issue. The eruption
on January 28, 2006 at 05:31 UT had an ash plume 10.5 km
high (a.s.l.), similar to thunderstorms. It had abundant
lightning in the plume, with over 300 lightning flashes
starting about 5 min after the onset of the main phase of the
eruption (Thomas et al. 2007, 2010). Most of these events
were intracloud flashes with durations of 30–600 ms,
typical of values for thunderstorms. Measurements of
duration were made using Lightning Mapping Array
(LMA) data (Thomas et al. 2004). In contrast, a smaller
eruption at 08:37 UT January 28 had an ash plume only
Table 2 Number and percent of occurrences of volcanic lightning at
various VEI values
VEI
1
2
3
4
5
6
7
8
No. cases
with Lightning
No. cases
all eruptions
7
61
81
22
7
4
0
0
845
3477
869
278
84
39
4
0
percent
0.83
1.75
9.32
7.91
8.33
10.26
0.00
–
3.8 km high and this was accompanied by only a single
lightning flash. The latter flash showed a duration of about
10 ms, substantially shorter than typical thunderstorm
lightning (Thomas et al. 2010). Thus the available instrumental data, although sparse, do suggest a systematic
difference in lightning between large plumes and small ones.
The study of photographs of volcanic lightning also
supports a systematic difference. Photos of small plumes
(1–3 km), typically time exposures, show lightning with
few branches and typical lengths of a few tens to hundreds
of meters (e.g. National Geographic Sept. 2007, p. 14–15).
Some photos from larger eruptions, on the other hand, show
lightning with many branches and lengths of several km
(e.g. Galunggung; Katili and Sudrajat 1984). We note that
the dozen or so photos we examined are suggestive but not
definitive. A more comprehensive study of this topic is
warranted.
Magma composition
The chemical composition of erupting magma is of interest
in studies of volcanic lightning because this characteristic is
known to influence the dissolved water content in the
magma (with important effects on electrification in its own
right), with important consequences for the style of the
eruption (i.e., Strombolian, Plinean, etc.). The broad
categorization for chemical composition in eruptions ranges
from basalt to andesite to dacite to rhyolite (48–77% SiO2),
with a systematic decline in equilibrium magma temperature from about 1,200°C to 800°C over this range. Water
contents range from about 0.1 to 6.5 weight percent, and
are systematically higher with increasing silica content
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Fig. 2 a Histogram of ash
plume heights for eruptions
accompanied by volcanic lightning. The broad peak >7 km
includes heights of similar
dimension and larger than typical thunderclouds. This suggests
that similar mechanisms may be
acting. Also note the second
peak from 1 to 4 km. These
heights are significantly smaller
than thunderclouds and suggest
a possible second mechanism or
mechanisms. b Histogram of ash
plume heights for eruptions
accompanied by volcanic lightning plotted as percentages of
the numbers of all eruptions for
given ash column heights
(Wallace and Anderson 2000). In an ideal case we would
have eruptions of identical size (plume height and volume,
etc.) with only the composition (SiO2 percent) varying.
Unfortunately the tests enabled here with real-world
observations fall short of this mark. To address this issue
we compiled information concerning the magma composition for as many of the cases in the Appendix as possible.
Some of the cases were compiled previously (Mather and
Harrison 2006) and we added 30 or so new cases to that
list. One problem is that information on a volcano is often
given based on study of geological samples over a long
time span, rather than for a specific eruption that produced
lightning. Hence we were constrained to choose representative values for magma composition based on published
data.
All identified eruptions with lightning as functions of
plume height and magma composition are listed in Table 3.
Basaltic eruptions span the range from 1 to 21 km, with no
clear groupings as a function of plume height. The same is
true for the basaltic-andesite cases, which range from 0 to
19 km. Andesite cases, which were the most numerous,
give the first indication of two groups of plume heights as
also seen in Fig. 2. They also span the largest range from 0
to 21 km. Andesitic-dacite cases form a single group
between 7 and 13 km plume height. Dacite cases range
from 4 to 33 km (Pinatubo at the maximum) with no
obvious grouping. Four of the six rhyolite cases were
associated with small eruptions from 0 to 2 km plume
height; the high value is for Chaiten, May 2008. This is too
small a sample to be able to generalize. Thus the overall
data do not appear to display any simple compositiondependent trends. The number of cases in Table 3 is fairly
high and supports the general conclusions given here,
however, we suspect that the sample is not uniform. This
topic will benefit from better data from future eruptions.
Ideally, future cases of instrumental recording of lightning
will also include the supporting information on magma
composition for each individual eruption.
Latitude effects
The current data set on volcanic lightning (Appendix) is
now sufficiently extensive to explore the variation of events
with latitude, with results that serve to constrain source
mechanisms for the lightning. Figure 3 (top) shows the
number of volcanoes with at least one eruption with
lightning, binned in 5° increments of latitude and normalized for surface area within these increments, and summed
for northern and southern latitudes. Figure 3 (bottom),
constructed in similar fashion for sake of contrast, shows
the latitudinal distribution of all volcanoes in the Holocene
record (Smithsonian Institution 2009; Simkin and Siebert
1994). Figure 3 (middle) shows the latitudinal variation of
all eruptions known to produce lightning.
Despite a notable gap in all the distributions in Fig. 3 in
the 20°–30° range, attributable to a deficit in island arc
length in this region, the overall distributions are decidedly
flat with latitude. Indeed, the percentage of all historical
volcanoes with documented lightning (Fig. 4, top) is largely
independent of latitude. If the entrainment of meteorological water was significant, then the tropical values should
be significantly higher. As a check, the percentage of
eruptions with lightning is also plotted versus latitude
(Fig. 4, bottom). Here several volcanoes contribute heavily
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Table 3 Volcanic Lightning
occurrence, magma composition, and ash plume height
Plume Height km
0–1
1–2
2–3
3–4
4–5
5–6
6–7
7–8
8–9
9–10
10–11
11–12
12–13
13–14
14–15
Repeated letters indicate multiple observations
B basalt; BA basaltic-andesite; A
andesite; AD andesitic dacite; D
dacite; R rhyolite
15–16
16–17
17–18
18–19
19–20
20–21
21–22
22–23
33
B
B,B
B
B,B
BA
A
BA
BA
A,A
A,A,A,A
A,A,A,A,A
A,A
AD
B
D
R
A
BA,BA
BA
BA
A,A,A
A,A,A
A,A,A,A
A,A
B
B
B
R
R,R
R,R
BA
B
D
BA,BA
BA
AD
AD
AD,AD,AD
AD,AD,AD,AD
D
AD,AD
D,D
D
A
A
B
BA,BA
R
B
to specific latitude bins, but overall the distribution is still
flat with no significant increase for the tropics. These
observations suggest that magmatic water already brings
volcanic plumes above the threshold needed to produce
lightning, hence the role of meteoric water is secondary.
In marked contrast with the latitudinal behavior of
volcanic lightning, Fig. 5 shows the latitudinal distribution
of natural (meteorological) lightning after Williams (1992).
A pronounced decline with increasing latitude is evident.
Similar findings are evident in the classical study of thunder
days for the globe (Brooks 1925). These trends are
generally attributable to the temperature dependence of
saturation water vapor embodied in the Clausius-Clapeyron
relation (Williams 1995, 2005). For a rough doubling of
saturation water vapor mixing ratio for every 10°C of
temperature change in the latter relation, this can amount to
an order-of-magnitude effect between equatorial and polar
regions. For thunderstorms the water substance is responsible for generating and moving charges between water
droplets, ice, and graupel. Hence more water in tropical air
means more lightning there.
Water substance is fundamentally important for volcanic
lightning, according to at least one idea (Williams and
McNutt 2005). If this water is derived from the magma
rising in the eruption, one does not expect a latitudinal
A
D,D
D
D
dependence in the available water, because geochemistry—
and water content—is independent of latitude. If, on the
other hand, the water is derived by entrainment of water
vapor from the local atmosphere (e.g., Rose et al. 1995a, b;
Sparks et al. 1997; Carey and Bursik 2000; Textor et al.
2003), then a pronounced latitudinal dependence in
volcanic lightning is expected. The latter expectation is
not supported by the available observations in Figs. 3
and 4. Some water vapor is added to ash columns via
entrainment, but our data suggest this is not in sufficient
amounts to alter the electrical activity. In other words, there
may be a threshold of water substance concentration
required in a plume for lightning to occur, but this appears
to be a function of the large amount of water in the magma
and not the smaller amount in the entrained air.
Seasonal effects
A second test of these ideas pertaining to water substance
involves the comparison of volcanic lightning in the winter
and in the summer, on account of the pronounced differences in surface temperature between these two seasons.
For purposes of this comparison, only extra-tropical
volcanoes (with latitudes greater than 23° in both hemispheres) were considered. ‘Winter’ eruptions were consid-
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Fig. 3 a Number of volcanoes
with lightning versus latitude;
b number of eruptions with
lightning versus latitude; and
c number of volcanoes versus
latitude. The three plots show
basically the same trend: the
distribution of volcanoes, eruptions, and volcanoes at which
lightning is observed occur
rather uniformly from 5 to 20
and from 35 to 65° latitude, with
a conspicuous gap from 25 to
30°. This plot is very different
from the plot of thundercloud
lightning versus latitude shown
in Fig. 5. The numbers for
volcanoes and eruptions are
normalized for the total area
within each 5° interval of
latitude
ered bounded by the autumnal and vernal equinoxes, and
‘summer’ eruptions, vice versa. With these definitions, 44
eruptions from the Appendix are wintertime, in contrast
with only 36 in summer. This result runs counter to the
expectations based on the entrainment of local water vapor,
whereas geochemistry is seasonally independent. The
greater number of eruptions in winter may find an
explanation in the temperature structure of the local
atmosphere, a circumstance that assures that more of the
magma-derived water substance is transformed to ice
during the eruption. Note that the results found here do
not mean that entrainment of water vapor does not occur; it
Fig. 4 a Percent of volcanoes
with lightning versus latitude.
b Percent of eruptions with
lightning versus latitude. High
values in (b) between 30–35 and
60–65° represent observations
from Japan and Alaska. These
distributions are essentially flat,
suggesting that entrainment of
meteorological water does not
play a major role in volcanic
lightning occurrence
does indeed occur. The available evidence, however,
suggests that it is not an important effect in terms of
increasing the water content of volcanic plumes. Previous
calculations (Williams and McNutt 2005) show that the
magma-derived water content in volcanic plumes is already
considerably higher than that of thunderstorms.
An additional effect must be noted here. We had
assumed in the previous paragraph that eruptions were
evenly distributed by season. However, Mason et al. (2004)
demonstrate seasonality of eruptions with higher numbers
occurring in boreal winter. Thus it is possible that the
greater number of eruptions with lightning in winter may be
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observations from the constellation of US DoD satellites
(Moore and Rice 1984; Pack et al. 2000).
Ash plume heights
Fig. 5 Latitudinal distribution of lightning from space (i.e. global
ISS-b satellite observations) showing a dominant contribution from
the tropics (+23°). This strong decline with latitude is attributable to
the distribution of water in the atmosphere. Dominant signals are from
thunderstorms, not volcanoes. Note the differences between this figure
and volcanic lightning data in Figs. 3 and 4, particularly between 25–
30° and >50°. Figure from Williams (1992) based on data from
Orville and Henderson (1986)
a function of more eruptions in that season, and the relative
ease of forming ice at colder winter conditions.
Discussion
Lightning occurrence
Our compilation shows that volcanic lightning is quite
common, having occurred at 212 eruptions from 80
volcanoes (Appendix). However, Simkin (1993) estimates
that 55–70 volcanoes erupt worldwide each year. From
Table 1 we find the year with the greatest number of
lightning reports is 1979 with 19 cases. This suggests that
only 27–35% of eruptions are accompanied by lightning,
assuming one eruption per volcano per year. These must be
over-estimates because most volcanoes erupt more than
once per episode. There are several reasons that such low
percentages of eruptions are accompanied by lightning.
First, our table may be incomplete; we did our best, but we
cannot guarantee completeness. Second, specialized conditions may be necessary, and these may occur during only
a small percentage of eruptions. Third are reporting
problems mentioned above, such as daylight or tropical
environments and the need for alert observers in areas
where the general population may be very sparse. To
improve reporting and to remove possible bias, we
recommend that a representative suite of volcanoes be
instrumented and a systematic reporting scheme be set up.
A more comprehensive documentation of eruptions worldwide (with and without lightning) could also be achieved
through greater exploitation of continuously available
The bimodal ash plume height distributions of Fig. 2 (top;
and normalized version in Fig. 2 (bottom)) suggest that two
broad classes of charging mechanisms may be acting. It is
important to distinguish what we actually measured—
plume heights—from the lengths of lightning flashes which
can be much smaller. First, note that the ash plume heights
are for the tops of the plumes. Lightning occurs within the
ash plume at various azimuths (not necessarily vertical) and
flashes typically have smaller dimensions. For the higher
ash plume peak from 7 to 12 km, and including all reports
greater than 7 km, the mechanism is plausibly the same as
for ordinary thunderstorms. The concentration of water in
eruption plumes is likely greater than for typical thunderstorms (Williams and McNutt 2005), so the stage is set for
efficient charge separation via the mixed phase microphysics of water substance (Williams 1995). Indeed, the
expected abundance of water may lead to an ice coating
of the ash particles that interferes with a charge separation
mechanism based on the collisions of dry silicate particles,
but it is suggested here that tall (>7 km) volcanic ash
plumes are essentially dirty thunderstorms. Further evidence for this point of view may be found in Williams and
McNutt (2005). A recent study by Durant et al. (2008)
concluded that volcanic plumes are “overseeded” with ice
particles compared with thunderstorms, and have higher
concentrations of ice particles of smaller average size. This
lab based finding agrees with our inferences based on
lightning distribution. Modeling studies by Textor et al.
(2005a, b) also discuss formation of ice and water
droplets. Durant et al. (2008) noted that ice formation
occurs on ash particles at temperatures of 250 to 260 K
(−10 to −20 C); these temperatures are found at heights of
several km depending on the atmospheric model, latitude,
and time of year. Plumes may be somewhat warmer than
the surrounding ambient air, so heights of ice formation
would be a bit higher. We suggest that the formation of ice
particles within plumes larger than 7 km is the reason for
the systematic increase in lightning percent shown in
Fig. 2 (bottom). Once ice particles exist, the processes
separating charges operate efficiently as in ordinary
thunderstorms. The charge distribution in thunderstorms
as a function of height and temperature is shown by
Krehbiel (1986) and lightning initiation is common at
heights of 5–6 km where temperatures are −10 to −20 C
and ice particles form. This is the same range of temperatures found by Durant et al. (2008) for ice formation on
ash particles. The data of Fig. 2 (bottom) suggest that this
process—involving water and ice—is about 3 times more
Bull Volcanol
efficient than the process or processes dominant at
altitudes of 6 km and below.
The secondary peak of Fig. 2 (top), from 1 to 4 km,
suggests that a second process or suite of processes is
acting. The ash plume heights for these eruptions are
smaller than those for thunderstorms and the near-vent
temperatures are likely quite elevated, both effects casting
doubt on any significant ice phase. Accordingly, a
mechanism similar to thunderstorms seems unlikely. We
suggest that several mechanisms are operating simultaneously. The first is ash particle collisions, which have been
shown to generate reasonable charges in laboratory experiments (James et al. 2000). A second related mechanism
consists of fracturing particles, so called fracto-emission
(James et al. 2000; Gilbert and Lane 1994b). A possible
third mechanism involves moving insulating bodies relative
to ionizing fluids (also called streaming potential; e.g.
Morgan et al. 1989). A familiar example of this is gasoline
moving through hoses. (This is why gas stations have signs
requiring containers to be placed on the ground for filling.)
This effect requires an electrical double layer at the contact
between the fluid and wall or pipe. This effect, if it occurs,
may be most likely for basalt because basalt is the most
fluid lava. Alternatively, gases separating from the magma
or groundwater may be involved. With the cases we have
for study, however, we cannot verify or quantify this
mechanism. It is mentioned for the sake of completeness.
Boiling of seawater, as discussed by Blanchard (1964),
Blanchard and Björnsson (1967) and James et al. (2008) is
a very efficient charge generating mechanism. However, the
number of cases in the Appendix that involve seawater is
very small, so we infer this to be a special case rather than
the more ubiquitous general case involving ash.
Recent instrumental observations
The 2006 eruptions of Augustine (Power et al. 2006)
provide some clues about charging mechanisms (Thomas et
al. 2007; Thomas et al. 2010). The explosive eruption on
January 28, 2006 at Augustine 08:37 UT produced
significant nearly continuous radio frequency electrical
discharge activity (vent discharges; 1–2 ms, 10 s of m
scale length) for the 20 s during which the tephra and gases
were exiting the vent (Thomas et al. 2010; these were
similar to the continuous charges as shown in Fig. 1a of
Thomas et al. 2007). The 08:37 UT eruption had the highest
pressure amplitude (105 Pa) of the 13 explosive eruptions
as measured on an infrasound sensor 3 km from the vent
(Petersen et al. 2006). From this we infer relatively higher
gas content and high ejection rates. However, the ash plume
was only about 3.8 km high, and this eruption produced
only a single lightning flash (near vent lightning; 10–30 ms,
100 m to km scale length). This behavior contrasts with the
subsequent eruptions at 11:04 UT and 16:42 UT, which had
ash plumes 7.2 and 7.0 km high. Each of these produced a
weaker (but longer lasting) vent discharge signal at the time
of the eruption, and correspondingly weaker pressure
amplitudes on the infrasound sensor (Petersen et al.
2006), but each had more near vent lightning flashes, 28
for the 11:04 UT eruption and 6 for the 16:42 UT eruption.
To summarize, the continuous electrical signals at the time
of the eruptions (vent discharges) were positively correlated
with pressure amplitudes, but negatively correlated with the
number of lightning flashes. We can reconcile these
observations by noting that the eruptions with longer
durations produced more lightning flashes (McNutt et al.
2010). This suggests that efficient charge producing
mechanisms (high pressure amplitudes at the vent) alone
are not enough to produce lightning; there must also be a
sufficiently large amount of tephra to carry the electric
charge and redistribute it in space to produce conditions
favorable for lightning generation.
A later phase of sustained eruptions at Augustine (in the
interval January 29-February 2, 2006) showed lightning
only in association with four stronger pulses (Thomas et al.
2010), even though the ash columns were persistently 5 km
or more in height. The ash plume height was 3.8–7.2 km
for each of the four pulses (Thomas et al. 2010). However,
the pressure amplitudes were low (all less than 13 Pa at
3 km distance), so even though abundant tephra was
available, the charging mechanisms were only sufficiently
vigorous for lightning production when the stronger pulses
occurred. We infer that virtually all the visual observations
of lightning in the Appendix are either near vent lightning
or plume lightning; the vent discharges (continuous signals
during eruption) have only been observed instrumentally
and there are not yet any corroborating visual observations.
From a theoretical point of view, the maximum electric
field at the surface of a uniformly charged spherical volume
of radius R is
E ¼ rR=3"o
where ρ is the space charge density (C/m3) and εo is the
permittivity of free space. The maximum field between two
uniformly charged spherical volumes of opposite polarity is
twice this value, by superposition. These simple calculations show that dielectric breakdown can be achieved in
clouds of small size only if the space charge density is very
large. Typical values of ρ for thunderstorms are 1 nC/m3
which requires R of several km to produce lightning. For
some observed volcanic lightning, the scale length is a few
hundred meters, which suggests space charge densities
about one order of magnitude larger. Dielectric breakdown
initiated in the high field region is sufficient to produce
VHF radiation, but subsequent thermalization of the
streamer channel is required to form lightning flashes.
Bull Volcanol
(Thermalization is movement of the channel towards
thermal equilibrium, a transition in the state from high
electron energy alone to high energy for all constituents
(Gallimberti et al. 2002)). Small volumes of space charge
may not provide sufficient charge (and current) for
thermalization, if laboratory studies are a guide (Boschung
et al. 1977).
These considerations suggest that the nearly continuous
electrical activity—vent discharges—in the high pressure
emission at Augustine on January 28, 2006 at 08:37 UT
was caused by a mechanism generating large space charge
density in a relatively compact cloud (3.8 km height) that
produced discharges with durations much shorter than
typical lightning flashes in thunderclouds. The larger,
deeper clouds (7.0–7.2 km) in the 11:04 UT and 16:42
UT eruptions plausibly involved an in situ mechanism,
possibly involving ice particle collisions, which allowed for
both dielectric breakdown and sufficient charge to thermalize channels and provide for discharges with durations
more typical of those in thunderclouds. In the later January
29-February 2, 2006 Augustine eruption phase, involving
smaller clouds (<~5km) and also low ejection rates, again,
only four episodes of near vent lightning activity were
present. The behavior during most of the continuous phase
is plausibly explained by the absence of high space charge
density and by a cloud radius R insufficient for either
dielectric breakdown or channel thermalization.
Other meteorological phenomena
Electrical charge separation is known to occur in two other
meteorological phenomena that are distinct from thunderclouds: dusty gust fronts (Williams et al. 2009) and dust
devils (Freier 1960; Crozier 1964; Ette 1971). These
involve lofting of silicate minerals into the boundary layer
by strong winds by an entirely dry process. Though the
detailed physical process for charge separation is still
poorly understood at the particle scale, the observations
support a selective charging of the smaller dust particles
with negative electricity. This polarity is opposite to the
behavior of ordinary thunderclouds and the few volcano
clouds that have been documented electrically (Cobb 1980;
Hobbs and Lyons 1993; Lane and Gilbert 1992; Gilbert and
Lane 1994b; Hoblitt 1994; McNutt and Davis 2000). Miura
et al. (2002) showed cases with both positive and negative
charge in fine particles from eruptions of Sakurajima.
Furthermore, dusty gust fronts and dust devils occur in
dry environments, whereas even ‘dry’ volcanic eruptions
contain a significant quantity of water. These considerations
and comparisons cast doubt on the gust front/dust devil
process alone in accounting for volcano electricity.
To better constrain these ideas, instrumental data are
needed. For example we need to know particle velocities
and electric field intensities as functions of time. Are
particles already charged and simply hoisted into the air by
eruptions? Or does the eruption process itself produce the
charges? Previous works by Gilbert and Lane (1994b) and
James et al. (2000) investigated charging mechanisms and
rates but not with respect to lightning per se. If particle
collisions are needed, then we would expect greater
concentration of fines in ash columns with greater charge,
and textural evidence of collisions in the particles that fall
to the ground. We are not aware of studies that looked for
these features in particular with respect to volcanic
lightning production, and we suggest that such studies are
needed. Information is also needed on the distribution of
electric charge carried by tephra particles over the full range
of particle sizes. Some work on charge versus size has been
documented by Miura et al. (2002) but the eruptions
studied did not produce lightning. In ordinary thunderclouds, the ability to access such information is difficult. In
volcanic eruptions, the access is extraordinarily difficult.
Practical considerations
A certain concentration or density of charge is needed to
cause lightning, so there ought to be a relation between ash
particle concentration and lightning occurrence if indeed
the ash particles are the charge carriers. This suggests that
lightning may be used as a real-time predictor of charge
density, hence ash particle density. The occurrence of
lightning would then add information of value to determine
ash hazards to aviation. Lightning could thus improve the
ability to verify when significant ash clouds are being
produced, and by identifying the different types or regimes
of lightning, such as near-vent or plume lightning, also
improve estimates of the sizes of eruptions. This would be
especially true for volcanoes where lightning detection
systems are in place, which could provide information in
poor weather or darkness.
Better instrumental data are needed, including specific
experiments for individual eruptions as well as better
utilization of existing resources. For example, there is a need
for a systematic search for volcanic lightning in the Lightning
Imaging Sensor (LIS) and Optical Transient Detector (OTD)
satellite data (Christian et al. 2003). Volcanic lightning
presents many interesting research opportunities, which are
complementary to both atmospheric sciences and volcanology. It is hoped that future research with better data will
further illuminate the trends identified in this work.
Conclusions
Data from a comprehensive global database show that
lightning has been documented at 80 volcanoes in
Bull Volcanol
association with 212 eruptions. Eight percent of VEI=3–5
eruptions have reported lightning, and 10% of VEI=6, but
less than 2% of those with VEI=1–2. These findings
suggest consistent reporting for larger eruptions but either
less lightning, a different mechanism, or possible underreporting for small eruptions. Ash plume heights show a
bimodal distribution with main peaks at 7–12 km and 1–
4 km. The former are similar to heights of typical thunderstorms and suggest involvement of water substance,
whereas the latter suggest other factors contributing to
electrical behavior closer to the vent. Reporting of lightning
is more common at night (56%) and less common in
daylight (44%). Reporting also varied substantially from
year to year. Several weak trends in lightning occurrence
based on magma composition were found. The bimodal ash
plume heights are obvious only for andesite to dacite; basalt
and basaltic-andesite evenly span the range of heights; and
rhyolites are poorly represented. The distributions of the
latitudes of volcanoes with lightning and eruptions with
lightning roughly mimic the distribution of all volcanoes,
which is generally flat with latitude. Meteorological
lightning, on the other hand, is common in the tropics and
decreases markedly with increasing latitude. This finding
supports the idea that lightning in large (deep) eruptions
depends on water substance, and the origin of the water is
primarily magma and not entrainment from the surrounding
atmosphere. Seasonal effects show that more eruptions with
lightning were reported in winter (bounded by the respective autumnal and vernal equinoxes) than in summer, again
suggesting that the water is primarily from magma. The
small number of modern instrumental observations from
Augustine Volcano shows features that are complementary
to the data from the global compilation.
Acknowledgements Discussions on this subject with RV Anderson,
P Arason, M Baker, D Blanchard, CGJ Ernst, J Gilbert, T Grove,
P Herzegh, R Hoblitt, P Hobbs, C Kessinger, P Krehbiel, T Mather,
J Murray, C Newhall, A Oswalt, D Pack, C Rice, W Rison, W Rose,
D Schneider, T Simkin, C Textor, R Thomas, A Tupper, J Ewert, and
R Wunderman are gratefully acknowledged. We thank two anonymous
reviewers for their comments which helped to improve the manuscript.
ERW’s work on this problem has been assisted by the NASA ASAP
(Advanced Satellite Aviation Assets) program under J Murray. This
work was partially supported by the National Science Foundation
under contract ATM-0538319.
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