AviationBanter - View Single Post

**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