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