cieldumort
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Posts: 2497
Loc: Austin, Tx
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Re: Very large whirlwind/dust devil....
Sat Jun 09 2007 12:58 PM
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A dust devil is never a "true tornado," in and of itself, and without any special extenuating circumstances, as existed at that exact moment I was so very fortunate to witness.
A tornado by very definition is a violently rotating column of air in contact with the ground and generally pendant from deep, moist convection. In other words, you need a "deep" cumulus-type of cloud. This is often cumulonimbus - (a thunderstorm-) or frequently in Florida simply a towering cumulus over water. AND you need a violently rotating column of air spinning between it and the ground, or water. Waterspouts ARE very simply simply tornadoes ...over water, and are usually NSTs (Non-Supercell Tornadoes) ie: classic "waterspouts." In much the same way a waterspout develops, so do "landspouts." - I'm digressing to add more background to the question you did ask:
Dust Devil: Rotating - but rarely "violently rotating" - updraft usually found on cloudless days - or at least most usually NOT directly under, or influenced by, deep moist convection. These are most common on clear, hot summer days with otherwise gentle ambient winds, but with uneven surface heating. There is deep convection going on, but nothing related to deep, "moist" convection.
NST (Non-Supercell Tornado): Rotating column of air - often "violently rotating" - that forms as preexisting, but rather diffuse or otherwise rather benign spins or shears in the lowest 100 mb or so of the atmosphere, are drawn into the updraft of a cumuliform cloud, tilted much more into the vertical, and then stretched. This stretching results in the broad area of shear or rotation picking up speed and become far more focused (Think of the proverbial ice skater pulling in his/her arms).
NSTs are most of what you see in Florida, especially over water. They are also very common in northeastern Colorado.
Conversely, a tornado born from a supercell thunderstorm is associated with many very similar, but fundamentally different processes, having much more to do with the cyclogenesis of the parent thunderstorm (mesocyclone, etc.)
Roger Hill (Silver Lining Tours), has some impressive footage of landspouts on their homepage.
For visual and descriptive comparison, here's a great page on dust devils from a JC in Maricopa County, Arizona.
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