posted 10-27-2000 04:57 PM            

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Canex

Back to the sounding for Nola's sighting:
Jay mentioned the dry bias in the soundings and the fact that relative humidity with respect to water requires lower dewpoints than relative humidity with respect to ice expecially below -40C. In other words, ice will form at higher dewpoint temperatures than liquid water. It just takes something to get the ice started. That is why you get persistent contrails in otherwise clear air. The aircraft exhaust with its water vapor and soot and other particles provides that something to get the ice crystals to start forming. Once they form, the excess water vapor is attracted to the "seed" crystals like hogs to a trough full of corn. I copied part of the sounding over Birmingham that was posted earler:
30 329 08956 -32.7 -38.7 55 6.0 -33.4 267 29 330.5 330.5 292.3 332.0 0.41
31 313 09305 -35.1 -42.1 49 7.0 -35.8 272 30 331.9 331.9 292.5 333.0 0.30
32 300 09600 -37.3 -41.6 64 4.3 -37.7 275 31 332.8 332.9 292.8 334.1 0.33
33 264 10468 -44.9 -50.9 51 6.0 -45.2 267 32 334.1 334.1 293.0 334.6 0.13
34 250 10830 -47.9 -52.9 56 5.0 -48.1 265 33 334.9 334.9 293.2 335.3 0.11
35 220 11656 -55.1 -58.6 65 3.5 -55.2 268 44 336.2 336.2 293.4 336.5 0.06
36 200 12260 -57.1 -63.1 46 6.0 -57.2 270 54 342.3 342.3 294.9 342.5 0.04

You will notice that between 8,000 and 12,000 meters, the temperature and dewpoint temperatures differ by 3 to 7 deg C. The layers where the difference is less than 5 deg is definitely supersaturated with respect to ice and will form persistent contrails while the layers with the 5 to 7 deg differences may form persistent contrails but more likely will produce short-lived contrails. No wonder a lot of contrails were observed. Those are flight altitudes.

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quote:

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Canex

To Thermit:
Unfortunately, the Hanson and Hanson reference is not the greatest for determining contrail formation. A paper by Ulrich Schumann ("Contrail formation from aircraft exhausts," Meteorologischen Zeitschrift, Vol. 5, 1996, pp. 4 - 23 ) is probably the best one out there for giving a technical overview of contrail formation. There is no particular simple formula for determining the threshold temperature for contrail formation at a given pressure altitude. A series of non-linear differential equations describing the thermodynamics of formation is necessary to arrive at the threshold temperatures. These can be solved iteratively, but can also be approximated with a series of linear equations that require several variable inputs like engine efficiency factors, fuel hydrogen content, ambient water vapor mixing ratios, pressure, and temperature. Some simple curves can also be constructed for various values of relative humidity. These equations can be found in the appendix 2 of the Scumann paper. Also included in that paper is a plot showing the thresold temperatures for contrail formation for a particular fuel efficiency and several ambient relative humidities. In that plot, a temperature less than -42C or 231K is required for contrail formation when RH < 100%.
Persistent contrails will only occur when the ambient RH with respect to ice (RHI) > 100%. Thus, one should expect persistent contrails the Schmidt-Appelton criteria (threshold temperature for contrail formation) are met and the RHI > 100%. I will provide that fromula whenever i can dig it up.

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posted 10-27-2000 04:58 PM            

quote:

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Cydoniaquest

canex,
It seems to me that you are saying that, although RH conditions do not have to be at 100% for contrail formation to occur, it has to be close enough to where the addition of moisture by the jet engine can raise RH to 100% thereby enabling condensation and sublimation.

I'm not sure if this information can be found, but it would be interesting to know how close the temp/dewpoint spread has to be in clear air to allow the passage of a jet engine to introduce enough water vapor and condensation nuclei to produce contrails.

I guess what I'm asking is, what is the temp/dewpoint spread in degrees celsius that would allow formation of contrails, and does the RH need to be 100% for their formation to occur after the introduction of water vapor from a jet engine (irregardless of temperature)?

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posted 10-27-2000 04:58 PM            

quote:

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Thermit

Thanks Canex, will be looking forward to seeing the Schumann equations.
Still wondering about the 0-5 and 5-7 degree difference between temp and dewpoint guides...are they applicable to only a given range of altitudes, or somewhat universal?

Really appreciate you digging up the information...

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posted 10-27-2000 11:29 PM            

The temperature difference (dewpoint depression) less than 5C is a rule of thumb for esytimating the conditions ripe for persistent contrail or cirrus cloud formation at temperatures well below the freezing point. It may even be as much as 7 or 8C. Dewpoint depression is rarely 0C because of two factors
1. the sensors have a dry bias of a few degrees or so at cold temperatures because dewpoint temperature is difficult to measure when the water wants to freeze, espcially on the sensor surface.
2. The dewpoint depression should never be zero for ice clouds because the ice forms in drier air than liquid water below freezing.

posted 10-27-2000 11:47 PM            

Canex: I agree that these are not natural clouds. They would not be there except for the formation of the contrails. Contrails will spread out and form murky looking cirrus clouds if the moisture conditions are right (see previous post). I see this in my area all the time.

cydoniaquest
Senior Member

Posts: 53
From:Sacramento,
California
Registered: Aug 2000
posted 10-27-2000 04:30 PM

canex,

It seems to me that you are saying that, although RH conditions do not have to be at 100% for contrail formation to occur, it has to be close enough to where the addition of moisture by the jet engine can raise RH to 100% thereby enabling condensation and sublimation.

Canex: Yes, condensation, freezing, and deposition.

I'm not sure if this information can be found, but it would be interesting to know how close the temp/dewpoint spread has to be in clear air to allow the passage of a jet engine to introduce enough water vapor and condensation nuclei to produce contrails. I guess what I'm asking is, what is the temp/dewpoint spread in degrees celsius that would allow formation of contrails, and does the RH need to be 100% for their formation to occur after the introduction of water vapor from a jet engine (irregardless of temperature)?

Canex: The exact dewpoint spread for contrail formation is fairly complicated to compute as I tried to get across in one of the earlier posts. But generally, the colder the air, the greater the dewpoint spread can be for contrail formation. The contrail forms because the mixture of the exhaust air and ambient air together initially raises the relative humidity above 100% and so permits the condensation and immediate freezing of the droplets. If the surrounding air is dry (RHI < 100%), then the contrail will dissipate (the ice crystals sublimate) as the exhaust is further diluted by the dry air. The drier the air the shorter the contrail.

posted 10-29-2000 10:08 PM            

1. Your first appears to be a proposition for a simple means for judging whether a contrail will form and persist. While that may be a good approach, it is just not that simple for the following reasons.
a. Contrails have been observed at -35C (and very persistent at that because so much moisture was available)
b. Soundings are taken only twice per day and in very few locations. The profile of temperature and moisture that is taken with a balloon sounding represents a pencil thin column of air angled from the zenith above the launch site by wind speed and direction aloft. Thus, the sounding is not necessarily representative of the air between the station launching the balloooooon and any other station. And the sounding does not necessarily represent the air over that site even a few minutes later (frontal passage or thunderstorm for example). In some instances, one can use the soundings upwind of a particular location to estimate the profile or linear interpolation between sites or soundings. However, that does not always work because the atmosphere does not work linearly for very long and the atmospher can be very patchy (as I noted on another post). In a given situation , you really need to look at all of the variables available. The profiles can be used to confirm that a contrail formation is possible, but it is more difficult to use it to rule out a contrail.
For example, a moist sounding coincident with a contrail is clear evidence that the contrail seen is not unusual. I.e., you can expect to see contrails if a plane flies over at the moist, cold altitude. However, if you don't see the contrail, it may be because the air is sinking due to topographic forcing (mountains, valleys) or vertical waves. Conversely, you may have a dry sounding and see contrails. That may occur because you have local lifting (mountains, waves) occurring or a thin moist layer that was not detected in the sounding because of limitations of the sensor on the balloon.

When distant from a sounding in space or time, you can try assessing the situation by trying to advect (move with the wind) a sounding from upwind to the location of interest. If that does not provide the right moisture fro contrail formation, you can then check the water vapor imagery from the satellites. The light and dark patterns represent the temperature variation of the highest level of moisture (or cloud top) over a given spot. The dark areas represent emission form low (hot, warm ) levels, while the whitest areas represent the coldest (highest) levels. The various shades of gray represent something in between. From these images, you can see why a sounding from a given location may not necessarily represent a much larger area surrounding the location all of the time.

Given all of that, allowing for reasonable error, knowing the altitude of the planes, making some reasonable assumptions, it will then it may be possible to hypothesize that something other than contrails is observed. But rememember the atmosphere is patchy at many scales, so a plane could fly through a spurious patch of moisture that is not large enuff to show up on the 8-km resolution water vapor imagery and produce a PC that may not be very large or spread much.

Whether it is chemical spraying or not is another matter. I would contend that spraying from commercial flight altitudes is a nonsensical endeavor because whatever is sprayed would be diffused and carried along with the winds as far as several thousand miles from the injection point before it even comes close to the surface. That's why people spraying crops or for mosquitos try to fly slightly above the canopy level during the early morning or late evening. They want to make sure that the spray reaches the target, is not diluted, and is not wasted. Put it at 30,000 ft and it will not reach a desired target and it will be diluted to parts per zillion by the time the air parcel containing it reaches the ground.

2. Yes, I am confident that there is some lower limit of RH below which a PC cannot occur. But remember the complexity of assessing the temp-humidity field.

posted 10-29-2000 10:44 PM            

Or is your bottom line in fact, that ANY persistent contrails that are observed are in themselves proof of a priori atmospheric conditions supporting their formation...

posted 10-30-2000 10:06 AM            

However, I would be suspicious of the existence of multiple spreading contrails in a situation in which all nearby soundings and water vapor imagery indicate that the upper troposphere is dry and should not support contrails. Widespread contrail activity should be a sign of a significant moisture layer at flight altitude. If there is no corroborating evidence (like a nearby cloud at a similar altiude or the soundings/wv imagery), then I would find such an occurrence unusual and warranting further investigation.

posted 12-13-2000 10:11 AM            

In a previous post you provided a general guide to persistence based on the degrees of separation between temperature and dewpoint temperature: less than 5 degrees difference "will form persistent contrails" and 5 or more "may form persistent contrails but more likely will produce short-lived contrails".

How would you define persistent and short-lived ranges in specific units of time?

I was hoping you might be able to go into a little more detail on this scale. For example, where on this scale would a contrail that persisted for 5-10 seconds or a contrail that persisted for 2 hours fit?

Thanks.

posted 06-04-2002 09:37 AM            

I'm trying to marry this info with the ADDs graph that Lulu provided.

posted 02-13-2003 07:46 PM            

Bumped for canex...