Gary wrote:
>One note. Nearly all of the photos you see of supersonic airplanes with
>various clouds of condensing vapor surrounding them have captions claiming
>to record the instant of "breaking the sound barrier!" They aren't. They
>record the condensation of atmospheric water vapor as the local pressure is
>reduced by the accelerating flow around the airplane. This flow can become
>locally supersonic, though the airplane isn't. As the pressure drops, the
>air temperature drops below the dew point and vapor condenses. The cloud
>often terminates at a disk-shaped boundary near the middle of the airplane.
>This is the transonic configuration shock - the boundary where the locally
>supersonic flow shocks back down to subsonic speed. The instantaneous
>increase in pressure across that normal shock raises the temperature above
>the dew point and the visible water once again becomes invisible vapor. All
>of this happens at aircraft speeds typically 5 to 10 or even 150wer than
>the speed of sound. In really humid conditions, you can often see that cloud
>around airliners even at approach speeds
snip
Yep. I observe that all the time near SeaTac on aircraft both taking off and
landing, so they are rarely exceeding 200 kts. Especially on humid days
(which we used to have a LOT of, until global warming). It's a neat way to
observe airflow characteristics of the aircraft.
Same phenomenon happens over Mt. Rainier. Cool, moist ocean air flows
onshore, hits that big sucker, rises and cools, and voila, up above the
summit, you see this neat disk-shaped cloud (a "lens" cloud), just "hanging"
there. The air is moving, of course, so the moisture is just condensing out
at the dew point, then being re-vaporized as it flows down the east side of
the mountain and warms up again, giving the impression of a stationary cloud.
Happens on lower hills and mountains too, when conditions are right.
And speaking of low passes, the Navy EA-6B Prowlers from NAS Whidbey Is.
practice that hill-and-valley sneaky stuff over the N. Cascades. First time
it happened to me was a shock.
And just this weekend they made the news because two of them made a low pass
over the new Seahawks football stadium and then went off to the east over the
suburbs and scared a few folks. From the film I saw on the news (which could
have been file footage -- you can't believe anything you see on the news any
more) they looked to be well above 600 ft.
But after 9/11, folks are REAL jumpy. Two Al Queda suspects have been
arrested here.... so far. One had a trunk full of explosives, and was
heading to LAX. A customs agent got suspicious when he disembarked from the
ferry in Pt. Angeles after crossing from Victoria, B.C., Canada. If it
hadn't been for her...
Rich
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