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.