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Treebeard's Stumper Answer
27 September 2002

National Defence Light Show

Do'h, I missed that spectacular Vandenberg AFB missile launch last week. I've seen other twilight Minuteman launches so I know what I missed, and I have photos here to prove it. The low part of the missile contrail is reddish and twisted like a corkscrew. The high part is bright white. There are expanding rainbow colors at the very top. We only see this light show at twilight. How does this work? Sometimes the sky over school is full of commercial jet trails, but this week the sky has been clear. What do these contrails depend on?

LA Times photo by Gary Friedman showing the Sep. 19, 2002
Minuteman III launch from Vandenberg AFB.
My friend Mike Davidson sent this photo of the
same launch taken some time later in Paradise.
.
I took this photo of a different Minuteman missile launch
on Oct. 2, 1999, from my porch on San Marcos Pass.
Beautiful photo by Hayne Palmour of a June 23, 1997
launch, taken in Vista near San Diego.
.

There are many more missile launch photos at Fred's Astronomy Stuff, SoCal Sky Lights, Art Bell, and The Goleta Air and Space Museum. John Parres has a nice sequence of photos of the Sep. 19 launch.

These photos of different missile launches over the years have a lot in common:

I wanted to shoot a photo of commercial jet contrails for this stumper. The sky is usually full of them here under the flight path between LA and SF, but this week the sky was clear. It's also been an odd weather week. It was 105° at school on Tuesday, but now we have cool temperatures and real rain on Saturday. Sometimes I see commercial jet contrails go on and off in the sky, as though the pilot has a "smoke switch" like a sky-writer. Sometimes I see dark "shadow trails" behind the jets instead of the usual white cloud-like trails. How do all these contrails work?

By sheer coincidence, NASA this week will show us what a rocket launch looks like from the rocket! On October 2nd, NASA plans to broadcast the first live video from a space shuttle's fuel tank as it soars into Earth orbit on the belly of Atlantis and falls back again. Read about it at Science@NASA and watch it Wednesday on NASA TV.


Imagine a mountain at sunset. The bottom is dark, but the summit still glows in the setting sun. The lower part of a twilight missile launch is also reddish because it is still sunset up there. Even higher, the bright white contrail still has direct sunlight. The rainbow at the very top is the twilight phenomenon caused by sunlight passing through the cloud of fuel and water ice crystals formed when the rocket stages separate. The strong upper level winds distort this cloud, but I still don't understand why the contrail holds together so well and twists like a corkscrew.

Notes:

The cloud-like trails we often see behind commercial jets and missiles are not smoke. Contrails are the condensation trails of (mostly) water vapor in the exhaust. On cold winter days, you can "see your breath" (and try to make "smoke rings"!), but it depends on the weather, and some days are better than others. Contrails depend on the temperature and humidity and sunlight up there in the stratosphere and beyond, just like the high-level clouds we sometimes see.

We can see our breath on cold mornings when the water vapor in our breath turns to water droplets. In the upper atmosphere, water vapor from exhaust can quickly turn to ice crystals that can last a lot longer as a cloud in the sky, but only when conditions are right. Low temperature and high humidity are the right conditions for contrails. It's been very hot (105°!) and bone dry here in Central California this week. If these conditions extend to the upper atmospher then maybe that's why I haven't seen commercial jet contrails lately. In the humid East Coast it's different.

This started as a simple stumper, but it got complicated. Maybe you do have to be a rocket scientist to understand these beautiful missile launches! I think I understand the colors and the twilight phenomenon, but I'm still stumped by why the contrails hold together so well and twist like a corkscrew. There's lots of contrail questions for future stumpers!

Many web sites say the missile contrails twist because of upper-level winds, eg Tom Chester's fine Vandenberg AFB Missile Contrails page:

Contrary to what most people think, the contrails in general do not look like they were formed from a rocket! The upper-level winds, with typical speeds around 100 mph, quickly blow the contrails into corkscrews or spirals. It takes only 5 minutes for 100 mph winds to move a given spot on the contrail 5 miles away from the rocket's path. Since the contrails can persist for an hour, there is plenty of time for the wind to play with the contrail.

Further, wind direction, as well as speed, varies dramatically at different altitudes and locations. Watch weather satellite picture loops and you will often be able to see high clouds moving in different directions than the lower clouds. This change of wind direction with height causes the corkscrew shapes that always make observers think that the rocket has blown up, which it never has if you are seeing the contrail... (Rockets are blown up at much lower altitudes than those which produce the contrails visible to us.)

I just can't visualize how wind can expand the contrails into a corkscrew pattern without blowing them away, and there's still the question why contrails twist at all.

My first thought was that the missile spins on it's longitudinal axis like a rifled bullet, leaving a twisted path. I don't think that's true. You can also see these twists in commercial jet contrails (right). You can even see it in the water from your kitchen faucet if you set the flow just right. I'm sure there's some interesting physics at work here. There are names for this twisting motion in fluids (including thin air): helicity and vorticity. These definitions are from NOAA:

Helicity - A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity).
Vorticity - A measure of the local rotation in a fluid flow. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e., rotation about a vertical axis) and is used most often in reference to synoptic scale or mesoscale weather systems. By convention, positive values indicate cyclonic rotation.


These national defence light shows are beautiful, and I wish that was reason enough for these launches. I'm glad there's some mystery. Here are a few Web links I found for more research:

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Copyright © 2002 by Marc Kummel / mkummel@rain.org