Microbursts

Bill Daniels wrote:
"5Z" wrote in message
oups.com...

wrote:
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.
I guess lightning/thunder does that ?
Perhaps a laser could too.

Don't have time to get into details, but the best example of
microbursts here in Colorado, is the "virga bomb" as often mentioned in
a forecast discussion.

The air is dry, there's a thunderstorm with cloudbase at 18K or so. It
starts raining, so there is a localized parcel of air containing
raindrops. As the rain falls, it evaporates due to the dry air below.
The evaporation pulls heat from the nearby air and it rapidly chills.
This cool air is now much heavier and begins to fall faster, etc, etc.

I've been in situations where the air is falling so fast, that in a 45
or more degree nose down attitude, my airspeed is still decreasing (in
an ASW-20B). Luckily, the few times I've encountered this, I was in or
near the landing pattern, and I flew out the side before reaching the
ground. Others have not been so lucky, and end up "landing" in
whatever is nearly directly below them.

-Tom


To 5Z, yep! BT,DT got the t - shirt.

The real power behind downburst is the amazing amount of heat it takes to
evaporate the raindrops before they hit the groumd. This cooling effect
chills millions of tons of air that litterally free falls to earth. The
impact has leveled humdreds of square miles of forrest in "blowdown areas"
across the western USA.

They can be seen as they happen. First virga appears below a high based Cu
Nim then a dust ring appears on the ground below. The dust ring can grow
until it's miles across.

The good news is that the mass of falling air displaces warm air near the
surface creating a ring of strong, smooth lift around the downburst - a good
thing since you don't want to land anywhere near one.

Bill Daniels
Microbursts are very common near Greeley, Colorado, where the build ups
from the Front Range often collapse and the resulting winds blow for
20-30 minutes and may peak at 50mph. Also the wikipedia article
differs a bit from my understanding that microbursts cover up to 10
square miles and macrobursts up to 100 square miles. The downburst
link mentions heat bursts, something I'd not heard of until earlier
this summer when they were reported in Nebraska with 4-5am temperatures
in several small towns reported at 96-102F.

Frank Whiteley

wrote:

...I've got problems with this
http://en.wikipedia.org/wiki/Microburst

This text seems to suggest that you can take an unenclosed 'parcel'
of air, and move it through the surounding air....

Chris,

It is true. Microbursts are real, and they can be deadly.

Do you feel the air from a fan "impact" your hand? Can you see the
effect of its impact on a curtain? I believe you will agree that the
answer is, Yes. Obviously then, there can be parcels of air that
move in a different way then the larger air mass. The surface effect
of the microburst may be seen on the surface of a lake, or a field
of grain, or even on a forest.

Several airline accidents, including Delta 191 in Dallas
http://www.airdisaster.com/special/special-dl191.shtml
have been caused by microbursts.

The NWS (US National Weather Service) says:
"Microburst - A small, concentrated downburst affecting an area
less than 4 kilometers (about 2.5 miles) across. Most microbursts
are rather short-lived (5 minutes or so), but on rare occasions
they have been known to last up to 6 times that long."

Google "microburst" and you'll find much useful information.


Jack

588 wrote:
wrote:

...I've got problems with this
http://en.wikipedia.org/wiki/Microburst

This text seems to suggest that you can take an unenclosed 'parcel'
of air, and move it through the surounding air....

Chris,

It is true. Microbursts are real, and they can be deadly.

Do you feel the air from a fan "impact" your hand? Can you see the
effect of its impact on a curtain? I believe you will agree that the
answer is, Yes. Obviously then, there can be parcels of air that
move in a different way then the larger air mass. The surface effect
of the microburst may be seen on the surface of a lake, or a field
of grain, or even on a forest.

Several airline accidents, including Delta 191 in Dallas
http://www.airdisaster.com/special/special-dl191.shtml
have been caused by microbursts.

The NWS (US National Weather Service) says:
"Microburst - A small, concentrated downburst affecting an area
less than 4 kilometers (about 2.5 miles) across. Most microbursts
are rather short-lived (5 minutes or so), but on rare occasions
they have been known to last up to 6 times that long."

Google "microburst" and you'll find much useful information.


Jack
The routinely run that flight profile at the United Training Center in
Denver. A few make it, but only by luck according my 777 instructor
friend.

Frank

wrote:
Remarkably valuable material is available these days on wikipedia.

But I've got problems with this
http://en.wikipedia.org/wiki/Microburst
text.

Sailplaners will have a good understanding of natural air flow.
This text seems to suggest that you can take an unenclosed 'parcel'
of air, and move it through the surounding air, like you can throw
a solid object through the air.

I can't find good explanations of why the text is 'wrong'.

Microburst

From Wikipedia, the free encyclopedia

[5]A photograph of the surface curl soon after an intense microburst
impacted the surface

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.

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.
I guess lightning/thunder does that ?
Perhaps a laser could too.

The term was defined by severe weather expert [9]Tetsuya Theodore
Fujita as affecting an area 4 km (2.5 mi) in diameter or less,
distinguishing them as a type of [10]downbursts and apart from common
[11]wind shear which can encompass greater areas. Dr. Fujita also
coined the term macroburst for downbursts larger than 4 km (2.5 mi).

A distinction can be made between a wet microburst which consists of
precipitaiton and a dry microburst which consists of [12]virga. They
generally are formed by precipitation-cooled air rushing to the
surface, but they perhaps also could be powered from the high speed
windsofthe [13]jet stream deflected to the surface in a
[14]thunderstorm (see [15]downburst).

Microbursts are recognized as capable of generating wind speeds higher
than 75 m/s (168 mph; 270 km/h).

Danger to aircraft

See also: [17]downbursts

The scale and suddenness of a microburst makes it a great danger to
aircraft, particularly those at low altitude which are taking off and
landing.The following are some fatal crashes that have been
attributed to microbursts in the vicinity of airports:
* [18]Delta Air Lines Flight 191
* [19]Eastern Air Lines Flight 66
* [20]Pan Am Flight 759
* [21]USAir Flight 1016

A microburst often causes aircraft to crash when they are attempting
to land. The microburst is an extremely powerful gust of air that,
once hitting the ground, spreads in all directions. As the aircraft is
coming in to land, the pilots try to slow the plane to an appropriate
speed. When the microburst hits, the pilots will see a large spike in
their airspeed, caused by the force of the headwind created by the
microburst. A pilot inexperienced in microbusts would try to decrease
the speed. The plane would then travel through the microburst, and fly
into the tailwind, causing a sudden decrease in the amount of air
flowing across the wings. The sudden loss of air moving across the
wings causes the aircraft to literally drop out of the air. The best
way to deal with a microburst in an aircraft would be to increase
speed as soon as the spike in airspeed is noticed. This will allow the
aircraft to remain in the air when traveling through the tailwind
portion of the microburst.

OTOH I've heard the big-jet's 'exhaust' and downwash also
'stays together like a solid' and doesn't disperse.

How much of this is true ?

If you've got a conical bucket of white-water, with a mechanism
to close off the lower 25% of the cone, can you project a black-ball
of water down through the white-water, and capture it by closing
of the lower clone section ?

Or will the black-ball of water just be dispersed ?

If an aircraft/bomber had it's front blown-off so that the
pilots had no shielding in front of them, would they necessarily have
near flying speed winds 'impacting' them, if the airflow had no
'reason' to flow in, 'cos it's got no low resistance path to flow out ?

== Chris Glur.

You are thinking of air moving at slow speeds. It takes some time for
the kinetic energy of a moving body of fluid to disperse to the
surrounding environment, so the material at the center will remain
virtually constant while the edges are slowly un-defined. Because of
the difference of speed, this system takes on properties like one fluid
moving through another.

Yes, if done properly you could fire some black water into a container
of white water and somehow close off the bottom at the right time and
trap the black water. It would be impossible, however, to trap all the
black water, as there will be some mixing. There will definately be a
moment of impact, though.

Have you ever dropped food coloring into a glass of water? Initialy it
falls downward in a column, and when it hits (impacts) the bottom of
the glass it spreads out and will eventually diffuse throughout the
glass.

wrote:

Sailplaners will have a good understanding of natural air flow.
This text seems to suggest that you can take an unenclosed 'parcel'
of air, and move it through the surounding air, like you can throw
a solid object through the air.
== Chris Glur.

Chris,
What is happening is that the parcel has momentum, which is conserved.
The momentum vector remains intact in the air until it is dissipated by
friction. Think of it as billiard balls passing along the momentum.
Jim Ketcham

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.

wrote:
Remarkably valuable material is available these days on wikipedia.

But I've got problems with this
http://en.wikipedia.org/wiki/Microburst
text.

Sailplaners will have a good understanding of natural air flow.
This text seems to suggest that you can take an unenclosed 'parcel'
of air, and move it through the surounding air, like you can throw
a solid object through the air.

I can't find good explanations of why the text is 'wrong'.

Under certain conditions, a toroidal vortex can & does move an
'enclosed` volume of fluid through the surrrounding fluid.
A smoke ring is a good example in air.
MadDog

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

In article .com, "5Z" wrote:


wrote:
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.
I guess lightning/thunder does that ?
Perhaps a laser could too.

Don't have time to get into details, but the best example of
microbursts here in Colorado, is the "virga bomb" as often mentioned in
a forecast discussion.

The air is dry, there's a thunderstorm with cloudbase at 18K or so. It
starts raining, so there is a localized parcel of air containing
raindrops.

I think that's the secret: "localized parcel of air containing
raindrops" effectively constrain/contain the air parcel, preventing
dispersion.

As the rain falls, it evaporates due to the dry air below.
The evaporation pulls heat from the nearby air and it rapidly chills.
This cool air is now much heavier and begins to fall faster, etc, etc.

I've been in situations where the air is falling so fast, that in a 45
or more degree nose down attitude, my airspeed is still decreasing (in
an ASW-20B). Luckily, the few times I've encountered this, I was in or
near the landing pattern, and I flew out the side before reaching the
ground. Others have not been so lucky, and end up "landing" in
whatever is nearly directly below them.

-Tom

Bill Daniels wrote:
The good news is that the mass of falling air displaces warm air near the
surface creating a ring of strong, smooth lift around the downburst

That's my point: the surrounding air which needs to be displaced
makes the analogy of a solid object 'impacting the ground' wrong.

I don't doubt that the described dramatic effect and results exist,
just that the explanation is simplistic.

OTOH if the water falls through the air, the air above a particular
air 'parcel' has the air-column above it already cooled, by the same
water which visited there earlier. So the air-column above is
already primed to move down. So it's not a sphere of air that
falls, but rather a self generating cylinder.

An analagy is: an individual can't 'run through a crowd' because
it will be constrained by the individuals; but a core of the crowd
can run. But you can't get away from the fact that there will be a
speed difference between adjacent 'atoms' of the crowd.
Boundry layer effect.

Frank Whiteley wrote:
Also the wikipedia article
differs a bit from my understanding that microbursts cover up to 10
square miles and macrobursts up to 100 square miles.

In that case there's no mystery.
But when they talk about the microburst 'impacting' and flattening
a 10 meter area, there have to be pressure gradients which seem
impossible to sustain without a solid container.

top-posted:
Have you ever seen someone blow smoke rings? Eventually they disperse,
but they can stay together for a surprising amount of time.

True, except it doesn't move much with respect to the surounding air.
Although the inside of the toroid could be moving.
Which would be like the core of the microburst ?

== Chris Glur

In article om, wrote:


wrote:

Sailplaners will have a good understanding of natural air flow.
This text seems to suggest that you can take an unenclosed 'parcel'
of air, and move it through the surounding air, like you can throw
a solid object through the air.
== Chris Glur.

Chris,
What is happening is that the parcel has momentum, which is conserved.

All momentum is conserved.
But the molecules escape from the 'parcel' since it's not sealed.
So the combined system conservs momentum.

The momentum vector remains intact in the air until it is dissipated by
friction. Think of it as billiard balls passing along the momentum.

This is valid for each molecule.
The macro model is more complex.

Jim Ketcham

For the physicists: what's the AVERAGE velocity of the nitrogen
molucules at 25 degree C & atmospheric pressure ?

Now I know what they mean by "herding cats".

== Chris Glur.