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veil index (DVI) was high in the northern middle and high latitudes (Lamb, 1973).
This latter point deserves further explanation. In a series of ground-breaking stud-
ies, Lamb compared the statistical relations between historic volcanic eruptions of
well-constrained magnitude with any unusual changes in atmospheric circulation,
near-surface temperature and net radiation flux (Lamb,
1970
Lamb,
1972
, pp. 410-
435; Lamb,
1977
). In the process of doing this, he devised a series of volcanic dust veil
indices (DVI) since 1500 AD, with the 1883 Krakatau eruption used as the standard
DVI of 1000. The dust veil index of any explosive eruption can be estimated using a
simple set of equations. For example, Lamb's DVI equation 3 is expressed as:
DVI
=
4
.
4
·
q
·
E
max
·
t
(23.1)
In this equation,
q
is the estimated volume in km
3
of solid matter dispersed as dust
in the atmosphere,
E
max
is the greatest proportion of the earth at some time covered
by the dust veil (taken as 1 for eruptions between 20
S) and
t
is the total
time in months elapsed after the eruption to the last readily observed effects in middle
latitudes.
In regard to potential impact, only the finest ash particles (size range 0.5 to 2
°
N and 20
°
m)
are likely to persist in the stratosphere for periods of several or more years (Lamb,
1972
, p. 411). The coarser particles (such as the 30-50
μ
m YTT deposited across
India) will be rapidly removed from the atmosphere, in accordance with Stokes' Law
(see
Chapter 9
, Equation 9.1). Niemeier et al. (
2009
) considered that the residence
time of even fine ash in the stratosphere was more likely to be limited to months
than years, so for the effects to persist for decades, some strong positive feedback
processes and/or an interlude of more frequent global eruptions are needed (Lamb,
1970
;Lamb,
1972
).
A further impact is the direct change in albedo as a result of the deposition of
a reflective ash mantle, leading to local near-surface cooling (Jones et al.,
2007
).
Albedo, or reflectivity, is a measure of the amount of incoming solar radiation reflec-
ted back into the atmosphere Williams et al. (
2010a
) noted that the colour of fresh
YTT ash in India is white to very pale grey, which would imply an initial albedo value
of 0.25-0.45 for the original ash mantle. Because the albedo under deciduous forest is
0.15-0.20, there may have been a 5-30 per cent increase in the reflection of incoming
solar radiation immediately after deposition of the YTT, leading to cooler surface
temperatures, reduced mesoscale convectional instability and less convectional rain.
However, this effect is unlikely to have persisted for more than a few years (Williams
et al.,
2010a
). Future work will need to examine the processes involved in the asso-
ciation between major inputs of volcanic dust into the upper atmosphere and the
weakening of the Asian summer monsoon.
Less well documented is the impact of eruptions on sea surface temperatures and
on sea level fluctuations (Cazenave, 2005), with the ocean cooling persisting in some
cases for decades (Gleckler et al.,
2006
). In this context, it is interesting to note that
μ
