Microbursts

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