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#2
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In article om, "tadchem" wrote:
wrote: snip A falling potatoe may 'impact' the floor, but air can't impact the floor any more than a 'swirl' [being a separate volume of the liquid] inside your coffee cup can impact the surface. I think you may be reading too much into the word "impact." A microburst is simply a wind that blows *downward* - usually in association with a cloudburst-type thunderstorm. What word would *you* use to describe what happens to a wind that is moving downward at considerable speed and then runs into the ground? It is the same effect as a regular wind running into a wall, only rotated 90 degrees. Impact implies a significant *rate of change* of force. The critical difference is that the potatoe doesn't have to displace other potatoes in front of it, whereas the air does. A microburst is a very localized column of sinking air, producing damaging divergent and [7]straight-line winds at the surface that are similar to but distinguishable from [8]tornadoes which generally have convergent damage. The 'localisation' is the problem. To move a small volume with respect to its surroundings, you have to apply energy to this 'localisated package' and not to its surroundings. Gravity combined with the viscous drag of falling raindrops and the cooling effect of trhe evaporation of the falling rain (to compress the air, making it more dense) does the trick. On the Great Plains of the US I have seen cloudburst thunderstorms less than a km across. You'll see the same in deserts. I guess lightning/thunder does that ? Not enough energy, not directed. - thunder is omnidirectional, lightning is too fast and too localized (a few cm wide) to overcome the inertia of a large mass of air. Perhaps a laser could too. No, for the same reasons that lightning can't do the job. Also, we have no lasers anywhere near energetic enough. In Amarillo, TX one afternoon I witnessed a damaging downburst that peeled the sheet metal roof of a 110' square building and crumpled it like aluminum foil, but left adjacent structures untouched. The weather service estimated the speed at 100 mph. [The building had previously withstood 60 mph winds.] I'm not implying that lightning or lasers make microbursts which are dangerous to aircraft. But that lighning is the only natural force which I know that produces such a massive velocity gradient. Ie. the air-packet is forced to greatly accelerate despite the 'surrounding constraints' - viscosity wrt surounding air. OTOH I've heard the big-jet's 'exhaust' and downwash also 'stays together like a solid' and doesn't disperse. Google "vortex gun" and find some interesting pages, including this: http://amasci.com/amateur/vortgen.html which has a crude but accurate animation of a travelling vortex of air. I'm more interested in the theoretical physics. Consider a 100m long rope suspended & dropped from 200m height. So the head has 100m free fall to ground. And the tail has to 'displace' rope in front of it..... ? == Chris Glur. |
#3
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![]() wrote: In article om, "tadchem" wrote: wrote: snip A falling potatoe may 'impact' the floor, but air can't impact the floor any more than a 'swirl' [being a separate volume of the liquid] inside your coffee cup can impact the surface. I think you may be reading too much into the word "impact." A microburst is simply a wind that blows *downward* - usually in association with a cloudburst-type thunderstorm. What word would *you* use to describe what happens to a wind that is moving downward at considerable speed and then runs into the ground? It is the same effect as a regular wind running into a wall, only rotated 90 degrees. Impact implies a significant *rate of change* of force. The critical difference is that the potatoe doesn't have to displace other potatoes in front of it, whereas the air does. You *are* demanding too much of the word "impact." If you would like to join a physics discussion, you should try to become familiar with the definitions of words as *others* use them, not just with the meanings *you* assign to them. This will avoid a lot of confusion arising from semantic differences later. In physics, and "impact" does not even require contact, only an approach close enough that the *momentum* (not 'force') is measurably altered: http://hyperphysics.phy-astr.gsu.edu...ear/impar.html If a moving mass of air encounters an obstacle and has its speed or direction measurable altered, it may be considered an impact. No, for the same reasons that lightning can't do the job. Also, we have no lasers anywhere near energetic enough. In Amarillo, TX one afternoon I witnessed a damaging downburst that peeled the sheet metal roof of a 110' square building and crumpled it like aluminum foil, but left adjacent structures untouched. The weather service estimated the speed at 100 mph. [The building had previously withstood 60 mph winds.] I'm not implying that lightning or lasers make microbursts which are dangerous to aircraft. But that lighning is the only natural force which I know that produces such a massive velocity gradient. Gravity is a very formidable natural force, too. Gravity acts on masses of air with different densities through Archimedes' principle to lift the masses with lower densities and pull the ones with the higher densities down, resulting in storms like Katrina. Now *there* was a velocity gradient!!! I'm more interested in the theoretical physics. That is a shame. The theoretical physics must be supported by empirical observations to be known to be reliable. Consider a 100m long rope suspended & dropped from 200m height. So the head has 100m free fall to ground. And the tail has to 'displace' rope in front of it..... ? The rope is free-falling as a unit. The tail has no need to displace anything. It just falls. Until the lower end "impacts" the ground, both ends will fall freely and there will be no tension on the rope. Once the rope does touch the ground, then the material properties (stiffness, compressibility, etc) of the rope become important as the distance between the ends gets smaller. Air is a fluid. It does not have the same properties as the rope. It has a tensile strength of zero, and does not resist torque or shear. "Analogies are like ropes; they tie things together well, but you won't get very far if you try to push them." - Thaddeus Stout Tom Davidson Richmond, VA |
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"tadchem" wrote
wrote: "tadchem" wrote: Well, this is interesting. Why are 9 and 10 day-old posts just now showing up on google/deja? They seem to be duplicates? BTW, this has been a really nice discussion - the first active and useful one on this group for a year or more. FloydR |
#5
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![]() Floyd Rogers wrote: "tadchem" wrote wrote: "tadchem" wrote: Well, this is interesting. Why are 9 and 10 day-old posts just now showing up on google/deja? They seem to be duplicates? I see them on Google in moments. Are you using Deja? Tom Davidson Richmond, VA |
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