Naked Science Forum
Life Sciences => The Environment => Topic started by: DoctorBeaver on 12/08/2009 09:19:39
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clbuckle posted a question about dust devils & it brought to mind something I've been meaning to ask for a while but keep forgetting. What distinguishes a dust devil from a tornado?
Tornados seem to start at the top & work down. Is that, in fact, what happens? Having heard the cry of "Tornado on the ground" while watching storm chasing on TV, I imagine that is so.
With dust devils, on the other hand, the first indication that 1 is forming is a swirling of dust/dirt at ground level which then gets raised into the air. However, the dust devils I've personally seen have occured on hot days with very little, or no, cloud. So is the swirling of dust on the ground merely an indication that 1 has formed higher in the air & has touched down?
So, is the difference that 1 starts at ground level and goes up whereas the other starts in the air and comes down? Or do they both form in the same way? If they do both form in the same way, is the difference just 1 of magnitude?
(waits for our FRIENDly resident windy chap to respond [;D] )
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The main difference between dust devils and tornadoes is that tornadoes are always attached to a cloud, dust devils are not. No matter how or where the tornado forms it always decends from the cloud. Dust devils on the other hand do mainly form out in deserty regions but can form in parking areas, or areas with a similar surface and surroundings.
One thing they both have in common is that, as yet, there is now odds on way of predicting where one will happen. Ala, Vortex 2 earlier this year.
"Tornado on the ground" Is mainly a film cry, but it is also used to distinguish between a tornado and a funnel cloud. A funnel cloud is a funnel cloud until it touches the ground when it then becomes a tornado. You may have heard that the "UK has the highest reported frequency per unit area, in the world. " or words to that effect, well, it's not true. This comes from a statement by Dr Ted Fujita, but it's no longer true. Holland now has that title.
Here is a picture of a funnel cloud i took last year in York.
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Here is a short item about dust devils, WITH PICTURES!!!
http://www.gi.alaska.edu/ScienceForum/ASF2/227.html
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clbuckle posted a question about dust devils & it brought to mind something ...
Did you see the link I included in that reply? Guess how many people clicked on it, go, guess! The answer is one! One! ONE!!!
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Thanks, Paul. So dust devils do form at, or close to, ground level? That's what I thought.
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clbuckle posted a question about dust devils & it brought to mind something ...
Did you see the link I included in that reply? Guess how many people clicked on it, go, guess! The answer is one! One! ONE!!!
If you mean the link to the PDF, that would have been me.
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A great resource and interesting read is anything by Jack Williams, He used to answer the weather questions in the USA Today.
His website is http://www.weatherjackwilliams.com/
He is also on twitter http://twitter.com/weatherjackwill
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Found this by Jack.
Different kinds of small vortexes
The strongest tornadoes come from the kind of long-lasting, especially fierce thunderstorms known as supercells. As the name implies, these are super thunderstorms with more than their share of potential danger. In addition to tornadoes, supercells can produce large hail and downbursts. Some bring heavy rain while others are relatively dry. Supercells are most common on the Plains, in the Southeast and across the Midwest, but do occur elsewhere. Not all tornadoes come from supercells, but the strongest twisters usually have a supercell as a parent.
Weaker vortices, such as waterspouts like those common in the Florida Keys, can come from cumulus congestus clouds, also known as "towering cumulus." These are tall, thick cumulus clouds that might be producing rain, but not lightning and thunder. Some researchers are using the term "landspouts" for similar twisters that form over land, not water. At times, waterspouts move ashore and can do the same damage as weak tornadoes.
"Gustnadoes" are weak vortices that are not connected to the cloud base, and by definition are not tornadoes. They are relatively shallow vortices associated with intense, small-scale shear in a thunderstorm gust front. They are not directly linked with rotation in the thunderstorm itself. Because they can produce whirling dust clouds (sometimes with small debris), they are very often erroneously reported as tornadoes. It takes a very alert and experienced spotter to tell the difference sometimes. Roger Edwards, a meteorologist and mesoscale forecast specialist at the Storm Prediction Center in Norman, Okla., says that, "On several occasions, I've personally witnessed tornadoes and gustnadoes produced by the same storm; but they have no mandatory link."
Cold-air funnel clouds - or simply cold-air funnels - form mainly during the late fall and winter months. Like waterspouts, cold-air funnels also form beneath cumulus congestus clouds, rather than thunderstorms. The cold air above the warmer ground creates instability that grows the fair weather cumulus. Small-scale wind shear forms the funnels along narrow boundaries on the edges of the clouds, much like gustnadoes. So, technically, they could be called "gust funnels" because they are like gustnadoes that never reach the ground.
Dust devils are swirls that go upward to fizzle out in clear air; they aren't attached to clouds. While they are most commonly found on deserts and form when air at the ground becomes much hotter than the air above. The lighter, hot air begins rising and takes on a whirling motion that carries dust and sand upward. Top wind speeds seem to be around 60 mph.
By Jack Williams and Chris Cappella, USA TODAY.com
http://www.usatoday.com/WEATHER/WTKINDS.HTM?loc=interstitialskip
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Very interesting. Thank you, Paul.
I've never heard of gustnadoes. What a horrible word [xx(]