posted on Jan, 19 2005 @ 02:27 PM
"ive seen those huge trails that go from horizon to horizon, and ive also seen planes flying right by that trail and not making any trail of their
Because although they're "right by" the contrail, they're not at the same altitude.
You hear a lot of stuff by "chem-trail" believers who say, "I can tell the difference between chemtrails and contrails!" Yet, when pushed
to explain how they can tell them apart, they either change the subject or say that the "chem-trails" persist and contrails do not.
But that's not the case. As our colleague Kano says, the chief exhaust product of a jet engine is water vapor. As a matter of fact, the components
of a typical jet engine (especially the more modern, cleaner- and hotter-burning ones) is water vapor: around 99.7 percent by weight, unburned
hydrocarbons, about 0.3 percent. What you see, of course, is not water vapor, because water vapor is invisible. What you do see is an
ice-crystal cloud, because the water vapor flashes into ice crystals within a half-second or so of hitting the air.
At 30 thousand feet, a typical altitude for contrail formation, the ambient temperature is somewhere in the vicinity of minus forty degrees. If the
temperature is either above -40 degrees or the relative humidity is less than 100 percent, the ice crystals will sublime (that
is, go directly from solid to gaseous states, like dry ice does). So if you see a contrail that dissipates within a minute or so, you can be pretty
sure that the regime is either a bit too warm, a bit too dry, or both.
But if the temperature is below minus forty degrees and the humidity is saturated (100% or greater), then the ice crystals will
stay solid, which means the contrail will persist. Often, the ice crystals act as a 'seed' or catalyst to freeze the surrounding cold and
wet atmosphere; the result is that the contrail will not only persist, but spread.
Bear with me, spangbr; I'm getting to your question LOL!
Anyway, the air around us is not homogeneous; you will have areas of slightly colder/slightly wetter atmosphere surrounded by slightly warmer/slightly
dryer air. This can be shown simply by noticing the you will have a cloud in the sky, and right next to it, there will be clear sky. Very often
these changes are a factor of altitude; it is just a teensy bit too warm at 31,500 feet, but cold enough for the contrails to persist and spread at
32,000 or 32,500 feet.
Now you or I can't tell a 500-foot or even a thousand-foot difference in altitude when you're looking six miles straight up with nothing like a
mountain to give you a frame of reference. We assume that two aircraft are at the same altitude -- but, of course, the FAA makes sure that adjacent
aircraft are seperated by both time and vertical difference.
And it is that separation which leads us to think that two aircraft are at the same altitude when they're not, which is why you will often see an
aircraft near a persistent contrail that is making its own contrail which rapidly dissipates.