Microbursts

[[email protected]]

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.

[5Z]

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

[[email protected]]

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


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.

[Floyd Rogers]

> 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
> ...
>> 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.

Read the explanation of "downdraft" and "gust front", beginning on
page 247 in Pagen's "Understanding the Sky". Note particularly:
"...brings a shift in wind up to 180 degrees ..., an increase in velocity
commonly around 30 mph BUT OCCASIONALLY SEVERAL TIMES
this amount..." (Emphasis mine.)

National Audubon Society's "Field Guide to North American Weather"
also has an excellent explanation on pp 104-5, and an explanation of
plates 202-5 on p.509.

Might be hard to find the field guide in South Africa. ;->

FloydR

[Nyal Williams]

Point this out to those who de-ride (pun intended)
the High Parasitic Drag Approach.


At 15:00 03 August 2006, 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. 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
>
>

[Sorcerer]

> wrote in message
...
| 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.
|
Wackypedia can be edited by anyone, so you'll inevitably get
nut case rants along with solid information. Jimbo Wales doesn't care
as long as he profits by it.
Androcles.

[tadchem[_1_]]

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.

> > 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.]

> 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.

Make your own long-range vortex generator for a few pennies:
http://www.geocities.com/davidvwilliamson/vortex.html

Or buy one pre-made for a few bucks:
http://dansdata.com/airzooka.htm

Tom Davidson
Richmond, VA

[CWatters]

> wrote in message
...
> To move a small volume with respect to its surroundings, you have to
> apply energy to this 'localisated package' and not to its surroundings.

Yes but that's not a problem. Read up on how thermals are produced. A micro
burst isn't that different. It's a bit like a strong "anti thermal". ... The
sun warms up a large area then a small rain cloud cools part of it causing
that part to sink rapidly.... is one way to look at it. The ultimate source
of the energy is the sun just the same as for a thermal.

[Bill Daniels]

"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

[Floyd Rogers]

"Sorcerer" > wrote
> > wrote in message
> | Remarkably valuable material is available these days on wikipedia.
> |
> | But I've got problems with this
> | http://en.wikipedia.org/wiki/Microburst
> | text.
>
> Wackypedia can be edited by anyone, so you'll inevitably get
> nut case rants along with solid information. Jimbo Wales doesn't care
> as long as he profits by it.

Actually, the microburst article (as well as most wikipedia entries, IME)
is quite good, as far as it goes. The text is very close to that in the two
references (Pagen, NAS Field Guide) that I mentioned, as are the
explanations.

FloydR

[Frank Whiteley]

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

[588]

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

[Frank Whiteley]

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

[Peter]

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.

[[email protected]]

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

[[email protected]]

In article m>, "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.

[[email protected]]

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

[tadchem]

wrote:
> In article m>, "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/hbase/nuclear/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

[[email protected]]

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

[[email protected]]

In article m>, 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.

[tadchem]

wrote:

<snip>

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

>From kinetic molecular theory of gases:

The *most probable* velocity (peak of the distribution)
c(p) = SQRT(2*k*T/m)

The average scalar velocity
c-bar = SQRT(8*k*T/pi*m)

The velocity of the molecule with average kinetic energy
SQRT(c^2-bar) = SQRT(3*k*T/m)

where k = Boltzmann's constant, T is absolute temperature, and m is
molecular mass.

Note that pressure is not a significant system variable in these
results - only temperature. Pressure *does* contribute to other
quantities such as collision frequencies and mean free path.

FWIW, c-bar for a nitrogen molecule at 25° C is about 1.31*10^5
cm/sec.

Tom Davidson
Richmond, VA

[tadchem]

tadchem wrote:
> wrote:
>
> <snip>
>
> > For the physicists: what's the AVERAGE velocity of the nitrogen
> > molucules at 25 degree C & atmospheric pressure ?
>
> >From kinetic molecular theory of gases:
>
> The *most probable* velocity (peak of the distribution)
> c(p) = SQRT(2*k*T/m)
>
> The average scalar velocity
> c-bar = SQRT(8*k*T/pi*m)
>
> The velocity of the molecule with average kinetic energy
> SQRT(c^2-bar) = SQRT(3*k*T/m)
>
> where k = Boltzmann's constant, T is absolute temperature, and m is
> molecular mass.
>
> Note that pressure is not a significant system variable in these
> results - only temperature. Pressure *does* contribute to other
> quantities such as collision frequencies and mean free path.
>
> FWIW, c-bar for a nitrogen molecule at 25° C is about 1.31*10^5
> cm/sec.

Oops! Make that 1.50*10^5 cm/sec

>
> Tom Davidson
> Richmond, VA

[Capt. Geoffrey Thorpe]

"tadchem" > wrote in message
oups.com...

tadchem wrote:
> wrote:
>
> <snip>
>
> > For the physicists: what's the AVERAGE velocity of the nitrogen
> > molucules at 25 degree C & atmospheric pressure ?
>
> >From kinetic molecular theory of gases:
>
> The *most probable* velocity (peak of the distribution)
> c(p) = SQRT(2*k*T/m)
>
> The average scalar velocity
> c-bar = SQRT(8*k*T/pi*m)
>
> The velocity of the molecule with average kinetic energy
> SQRT(c^2-bar) = SQRT(3*k*T/m)
>
> where k = Boltzmann's constant, T is absolute temperature, and m is
> molecular mass.
<...>

Is this the point to jump in and argue that velocity is a vector quantity
and, therefore, the AVERAGE velocity is nearly zero? I would think that with
a little effort, it should be possible to spin this thread off into a
urinating contest about units or something like that.

:-)

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

[Floyd Rogers]

"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

[tadchem]

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