Microbursts

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.

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

"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

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

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

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.

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

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

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.

"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