Chaiten Volcano Cloud Height
Overestimates of the Chaiten Eruption Cloud
© Alexandra Matiella Novak
May 6, 2008
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.
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 are used 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 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 of.
However, once the cloud has been in the atmosphere for a few hours, it looses 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 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.
Sources:
Smithsonian Institute’s Global Volcanism Program
US Geological Survey
Personal Communication, Dr. Fred Prata, Norwegian Institute for Air Research
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Comments
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 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 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 6, 2008 8:57 PM
Guest
:
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_content&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.
