We’ve all become familiar with contrails — the cloud tracks that planes leave as they fly through certain altitudes:
But, did you know that ships apparently do the same thing? (I sure didn’t!)
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UW professor of Atmospheric Sciences Cliff Mass (author, and weather blogger) has a nice post up discussing the phenomena. Both contrails and “ship trails” are produced when microparticulates released by combustion serve as seeds for condensation. Over the ocean, the typical droplet size is much smaller in ship exhaust than in the natural cloud cover, producing different reflectivity, leading to high contrast white ship trails. As Cliff discusses, you can imagine the interest that national security agencies might have in this effect….
(EDIT: Actually, it turns out that contrails are primarily due to vapor released during combustion, not nucleation, so the processes are somewhat different.)
April 24th, 2009 at 10:18 am
For God’s sake, do not let the “chemtrails” (http://en.wikipedia.org/wiki/Chemtrails) get hold of this!
April 24th, 2009 at 10:47 am
[...] Blogs / Cosmic Variance « Ships that Pass in the Day [...]
April 24th, 2009 at 7:50 pm
No — contrails are caused by the rapid changes in pressure as moist air travels by the wing, causing a change in vapor pressure (as vapor pressure is a function of pressure), which causes droplets to rapidly nucleate and form as the air passes by the wing.
April 25th, 2009 at 2:38 pm
Actually, sorry, it seems we’re both right — low altitude contrails are primarily caused by vapor pressure changes, whereas high-altitude contrails are indeed caused by nucleation from combustion particulates.
April 26th, 2009 at 1:40 pm
Am I the only one who thought that top picture was a chalk board at a university?