posted on Mar, 8 2005 @ 11:28 AM
Hybrid EB says:
"I’m not stupid dude; I meant that when a trail stops abruptly, it isn't caused by anything natural. I didn't say that the trail itself is
Hybrid, have you ever seen an abrupt stop to a cloud -- in other words, have you ever seen a cloud with sharply-defined edges?
I have, too; and so has everyone else here.
There is a difference between the atmosphere not being homogeneous and there being a drastic temperature difference in such a short space. The
distinct boundaries you talk about are not abrupt but gradual.
See comment above. From 30,000 feet, though, they look pretty abrupt.
And remember, you don't have to have a drastic shift in temperature or humidity in order for there to be persistent contrails. If the temperature is
below minus 40 deg and the relative humidity is 100% (saturated), the contrails will persist. If either of those two conditions are not met, then the
contrail will dissipate.
Now you're probably aware that relative humidity is not determined solely by the percentage of water vapor in a given volume of air; it is the
percentage of water vapor in a given volume of air at a certain temperature. The colder it is, the less water vapor you need before the air
Think of it this way:
Pour some cold water into a cup and add a teaspoon of sugar. Stir it up until it dissolves. then add another teaspoon of water and another, stirring
well. Sooner or later, you will reach a point where, no matter how much you stir, you will not be able to dissolve the suger. In other words, the
sugar solution is saturated.
Now pop that puppy in the microwave and nuke it for about a minute and a half. take it out and stir again.
Voila! the sugar has dissolved!
You can get more of a solute (whether water vapor or sugar) in a solvent (whether a cup of water or a volume of air) if the
solvent is hotter!
Therefore, if the temperature rises by just a couple of degrees (from, say, minus 41 deg to minus 38 deg) then, even if the relative humidity
was 100%, that little bit of temperature rise will ensure that it is no longer 100%.
And that tiny temperature/humidity change is all you need to exceed the envelope for persistent contrails
"Plus, responding to an earlier post, the size of clouds doesn't depend on the air around them but the amount of water vapor in them."
It also depends on the temperature, as I discussed above.
"No I am not familiar with the concept of super saturation but I will gladly look into it if you provide me with the necessary links."
Supersaturation is when the relative humidity is greater than 100%.
Now this may seem impossible, since you'd think that a theoretical maximum can't be exceeded, but it's not.
Remember that cup of hot water we took out of the microwave that had too much sugar to dissolve at low temperature but dissolved at high temperature?
Well, you might want to try this yourself, for grins.
As the hot water cools, there's now too much sugar in it to stay dissolved, so the sugar crystals appear (or they "precipitate out") on the sides
and the bottom of the cup. And more and more sugar crystals will precipitate out until you have the 'saturated' amount of sugar water at whatever
the new (room) temperature is.
But if you very carefully let the water cool slowly without shaking up the water or adding any impurities, the sugar crystals won't
precipitate out. the cooler water is now supersaturated.
Now, drop about ten grains of sugar into the supersaturated cooled solution. Within about a minute, the dissolved sugar molecules will grab on to the
new additions and you will have a big crystal appear as if by magic.
And that is why sometimes contrails spread out and cover the sky with cirrus clouds. If portions of the atmosphere are supersaturated and the cold
air is holding more moisture than it "should", the addition of the ice crystals from the aircraft engines, along with the turbulence, acts just like
the grains of sugar you dropped into the supersaturated cup of sugar water, and ends up "seeding" all the water vapor.
And there's your new cirrus cloud (which, of course, like all cirrus clouds, is just ice crystals).
[edit on 8-3-2005 by Off_The_Street]