MadSci Network: Earth Sciences
Query:

Re: What is the velocity/strengh of a pyroclastic cloud compared to a tornado

Date: Fri Apr 9 15:48:20 2004
Posted By: John Christie, Faculty, Dept. of Chemistry,
Area of science: Earth Sciences
ID: 1081438611.Es
Message:

Pyroclastic flows from volcanoes and tornadoes are very different types of thing. Both are very 
destructive. Otherwise they are quite different, and very difficult to compare directly.

In a tornado, the destructive energy comes from the speed of the whirling air and debris about the 
core, and the pressure difference between core and surrounding atmosphere. The high speeds, 
over 150 km per hour, are the speeds in the whirlwind. The speed of a tornado over the ground is 
usually much slower -- perhaps around 30 km per hour.

With pyroclastic flows, the destructive energy is in the heat of the rocks and ash (mainly) and gas 
(minor) in the flow. There is also a problem with the momentum of large, fast-moving rocks. 
Pyroclastic flows move very rapidly down the sides of the volcano, down valleys, and across flat 
country, at typical speeds across the ground of 100 to 250 km per hour.

Looked at overall, it would probably be fair to say that a pyroclastic flow is much more destructive 
than a tornado in terms of the extent of damage caused in its immediate path. When a tornado 
strikes a populated area, there might be a death toll in the tens; pyroclastic flows cause death tolls 
in the thousands if they strike a town or city. People survive tornadoes; as far as I am aware, no 
one caught in a pyroclastic flow has ever survived.

Information about pyroclastic flows can be obtained from the websites of US Geological Survey,
San Diego State University, and the British Geological Survey.


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