Environmental Engineering Reference
In-Depth Information
Figure 5.4
Cumulative DVI for the northern hemisphere: dust from an individual eruption is apportioned over
4 years-40% to year 1; 30% to year 2; 20% to year 3 and 10% to year 4
Source:
From Bradley and Jones (1992)
The DVI provides an indication of the
potential disruption of weather and climate by
volcanic activity. Dust in the atmosphere reduces
the amount of solar radiation reaching the earth's
surface, and at high index levels that reduction
can be considerable. This is particularly so in
higher latitudes where the sun's rays follow a
longer path through the atmosphere, and are
therefore more likely to be scattered. Major
eruptions, producing a DVI in excess of 1,000,
have caused reductions in direct beam solar
radiation of between 20 and 30 per cent for
several months (Lamb 1972). The effect is
diminished to some extent by an increase in
diffuse radiation, but the impact on net radiation
is negative. Observations in Australia, following
the eruption of Mount Agung in 1963, showed a
maximum reduction of 24 per cent in direct beam
solar radiation, yet, because of the increase in
diffuse radiation, net radiation fell by only 6 per
cent (Dyer and Hicks 1965). Similar values were
recorded at the Mauna Loa Observatory in
Hawaii at that time (Ellis and Pueschel 1971). El
Chichón reduced net radiation by 2-3 per cent
at ground level (Pollack and Ackerman 1983),
while the reduction caused by Pinatubo averaged
2.7 per cent over a 10-month period following
the eruption, with individual monthly values
reaching as high as 5 per cent (Dutton and Christy
1992).
A number of problems with the DVI have been
identified since it was first developed, and these
have been summarized by Chester (1988). The
DVI was based solely on dust and did not include
the measurement of sulphates or sulphuric acid
aerosols, which are now recognized as being very
effective at scattering solar radiation. As a result,
the impact of sulphur-rich eruptions such as El
Chichón or Mount Pinatubo would be
underestimated. At the same time, there is no way
