News agencies are reporting cloud heights of 20 miles, which is a great exaggeration of the actual cloud height of about 3-7 miles based on scientific analysis.
The May 2, 2008 eruption of Chaiten Volcano in Chile has created a media frenzy. It’s not very often that volcanoes erupt, much less erupt with the kind of intensity that hasn’t been seen since the 1991 eruption of Mt. Pinatubo. As with most natural disasters, popular media has a tendency to embellish, and reports of Chaiten Volcano cloud heights of 20 miles are being greatly exaggerated.
How Scientists Estimate Volcano Eruption Cloud Heights
There are several methods that volcanologists and meteorologists use to estimate volcano eruption cloud heights. These heights need to be as accurate as possible because they are used in volcanic ash transport and dispersion (VATDM) models and in volcano ash advisories released to pilots and air traffic controllers.
Some methods that the scientific community uses to estimate volcano cloud heights are:
Cloud shadow clinometry
Temperature of the cloud related to ambient temperature of the atmosphere
Trajectory of the cloud
Cloud Shadow Clinometry
These calculations are done by looking at satellite images of the volcanic cloud and trying to find any shadows that are being cast on the ground below by the cloud. Knowing the angle of the sun – which can be inferred from the time of day the satellite image was taken – scientists can use simple trigonometry to calculate the height of the cloud. It is important to point out, though, that by using this method, you are really only calculating the height of the edge of the cloud; the portion of the cloud that you can see is creating the shadow.
Calculations of cloud shadow clinometry have been applied to the Chaiten eruption cloud. These calculations have produced values of about 6-11 km (3.7-6.8 miles) above ground level – significantly lower than heights reported by news media.
Temperature of the Volcanic Ash Cloud
When a volcano erupts and sends an ash column into the atmosphere, the initial temperature of this cloud is hundreds of degrees Celsius. Adding to the energy of the eruption, the heat of the ash cloud makes it extremely buoyant and it easily ascends through the atmosphere like a rocket ship taking off.
However, once the cloud has been in the atmosphere for a few hours, it loses its heat and buoyancy, and the temperature of the cloud is controlled by the surrounding atmosphere. Thermal infrared data of volcanic ash clouds in the atmosphere show that these clouds are the same temperature as the surrounding atmosphere. So if one knows the temperature of the atmosphere at any given height, it can be correlated to the temperature of the ash cloud and therefore the ash cloud’s height.
Trajectory of the Volcanic Eruption Cloud
One of the most utilized methods of estimating volcano cloud heights is correlating the trajectory of the cloud to meteorological movement. Atmospheric data on meteorological cloud movement, both direction and speed, at differing heights throughout the atmosphere can be used to calculate the heights of volcanic ash clouds.
According to the trajectory of the Chaiten Volcano ash cloud, scientists have determined that the cloud must be residing at heights of about 8-10 km (5-6.2 miles) and is definitely being carried along by the same winds that are moving meteorological clouds in the area. Again, this is a science-based cloud height estimation that is significantly less than those being reported by the media.
Personal Communication, Dr. Fred Prata, Norwegian Institute for Air Research
The copyright of the article Chaiten Volcano Cloud Height in Volcanoes is owned by Alexandra Matiella Novak. Permission to republish Chaiten Volcano Cloud Height in print or online must be granted by the author in writing.
The information being reported is coming from the National Service of
Geology and Mining in Chile which is the authority on Chilean Volcanoes.
The web site & story are here http://www.sernageomin.cl/index.php?plantilla=detalle&option=com_cont
ent&task=view&id=298&Itemid=4§ionid=4
Here
is the traslated version.... (note Lateral expansion to 30km high... i.e.
18.64 miles, or roughly 100,000 feet)
The National Service of
Geology and Mining reports that according to background visual and seismic
collected from the date of the event Eruptive Volcano Chaiten, we conclude
that can not be ruled out new explosion (s) mayor (s) and an eventual
collapse of the eruptive column and / or dome Rhyolite.
Accordingly, SERNAGEOMIN maintains Volcanic Red Alert and launched a
remote monitoring since the craft of the Navy Achilles, with daily
overflights to monitor the eruptive behavior, especially the evolution of
the stability of the dome and the eruptive column, as well as developments
the seismicity associated.
It is noteworthy that during the
morning on May 6, at 8:20 pm., The eruptive cycle flared with explosions
and vigorous rhythmic higher energy holding a column Eruptiva wider and
lateral expansion of about 30 km high in its initial phase, which was
subsequently declining.
During a helicopter overflight
conducted with police, at 10:00 pm. it was observed that the two craters
explosion located in northern flank of the dome, joined into one with a
diameter of approximately 800 m. The column declined slightly, and
apparently there were no major pyroclastic flows associated, at least to
the north, west and south. It was verified that the rivers increased their
load of material.
The possibility exists that there is a
collapse of the eruptive column and / or dome Rhyolite, which would
generate pyroclastic flows. On the other hand, the fall of ashes will
continue with accumulations whose thickness and accumulation rather than
depend on the intensity of explosions and prevailing winds. It was ruled,
by far, the emission of lava.
May 7, 2008 10:50 AM
Alexandra Matiella Novak :
Thank you for the update. It is definitely worth noting that initial
column heights are usually higher than settled cloud heights. For the
purpose of dispersion and transport modeling, an accurate cloud height is
absolutely critical. For aircraft hazard mitigation, the cloud's
trajectory height is used, in this case, only about 3-7 miles high. It's
important for the public to understand that, based on current satellite
data, the dispersing ash cloud is not at a height of 20 miles.
The Pinatubo 1991 eruption height was 25 km (15.5 miles). I'm not sure
that this eruption is larger than the Pinatubo eruption, so I am skeptical
about a higher ash column. But only time will tell. This volcano looks
like it has much more in store.
Alexandra Matiella Novak
May 7, 2008 2:17 PM
Guest :
Is this ash cloud going to have an effect on southern hemisphere/global
atmospheric temperatures once it disperses more widely?
May 8, 2008 7:46 AM
Alexandra Matiella Novak :
Great question. The 1991 Mt. Pinatubo eruption ejected 8.4-10.4 cubic
kilometers of material into the atmosphere and the effects were summer
surface cooling and and winter surface warming in the northern hemisphere
for about 1-2 years after. It's not known yet the amount of material
ejected by the Chaiten eruption into the atmosphere and we may not know
what the global climate changing effects are for another few months. But
it has happened with other historical large eruptions (Pinatubo,
Krakatau).
The 1980 Mt. St. Helens eruption was smaller than
Pinatubo and had minimal climate effects. Some localized warming occurred,
but this diminished as the cloud dissipated and dispersed over the north
western U.S.
For more information, please see this article
published in Nature:
McCormick, A.P.L., W. Thomason, and C.R.
Trepte (1995), Atmospheric effects of Mt. Pinatubo eruption, Nature, 373,
399-404.
May 15, 2008 2:46 PM
Guest :
Hello alexandra, I´m from chile, and I have been taking all the information
about the Chaiten Volcano eruption, but my question is the next: What
is the real ash quantity that the volcano has emitted until now?
I now that from the first day to the 7 day of eruption (may 9)it was 2.0
cubic kilometers.....
May 15, 2008 9:58 PM
Guest :
No doubt the media have a tendency to exaggerate... however, you give no
sources for your estimated height or even by which of the above methods the
estimate of 8-10 km were arrived at (by either yourself or others).
On the other-hand here is an note from NASA that estimates the
height of the ash plume at 19 miles:
Guest, My sources are listed at the end of the article. The methods
used for obtaining these heights are described in the article. These
values were not calculated by myself, rather by the sources provided.
The NASA post you offer is referring to the May 6 event - the
second explosive eruption. This article is about the May 2 event, the
initial explosive eruption, as stated in the lead paragraph.
Thank you for reading my article closely and I look forward to future
comments.
