# Relative humidity difference between persistent and non-persistent contrails

#### mygovtlovesme

##### New Member
Hi there

Does anyone have the info/source on the relative humidity needed for persistent contrails. Non-persistent
ones need at least 70% but presumably the persistent ones need higher humidity?

#### Mick West

Staff member
Contrails can form at any level of RH, if it's cold enough. But if it's lower than a certain value, then they will not persist.

The 70% is a rule of thumb number for contrail persistence, and the real number varies with pressure (i.e. altitude). The numbers behind formation and persistence of contrails are actually a little complicated, as there are several variables involved. There's the temperature, pressure and relative humidity (RH) of the air, and the temperature, pressure, speed, and humidity of the exhaust gas.

For a contrail to persist, the relative humidity with respect to ice (RHI) needs to be above 100%. Note the RH is not the same as RHI. It differs in the same way that dew point differs from frost point. It's also not a simple matter to convert from one to another, as they derive for different complex equations (polynomials), even the approximations of which (such as Goff-Gratch) are overly complex, so conversion is done with tables, or computers.

This link will give a taste of the complexity. Note that the equations given at the bottom of the page are approximations.
http://www.yesinc.com/education/moist-convert.html

Anyway, that's just to explain why there's not a precise figure. 70% is ABOUT the RH where RHI is above 100% in average contrail condition.

To answer your question: no, persistent contrail do not need higher humidity. If the RHI is above 100% (approximately 70% RH), then a contrail will persist. Contrails will fade away if the humidity is below this level. The dryer the air, the quicker they will fade away.

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#### Jay Reynolds

##### Senior Member.
If I were to assume you were seeking to use the RHI and forecast or actual measurements at altitude to determine if conditions exist for contrail persistence, and thus to infer if what is seen is possibly a peristent contrail, you have several problems.
1. The state of the art in prediction of RH at altitude is really no better than forecasts of rain, temperature, or cloudiness, sometimes the model used gets it right, sometimes it doesn't.
2. Actual measurements of humidity and temperature are taken by balloons sent up maybe twice a day, and travel with the wind from the point of release. So what is being measured is only a reflection of the air mass the balloon passed through, which might be hundreds of miles away from it's point of release and in various directions north south east or west from that point depending on wind speed and direction.
3. There is sometimes a 'dry bias' with the instrumentation on these balloons, which tends to give RH lower than actual humidity.
4. Saturated air masses can exist which are spotty, only a few miles across, so even if you took a measurement in one spot a few miles or less away, conditions can be different.
So, you can only expect to get a very rough idea of what actual conditions for persistence are at any one spot in the sky.

Here is a fairly technical discussion by one of the world's foremost contrail experts, who cites the research you can read to learn more:

http://www-pm.larc.nasa.gov/sass/sass-ref.html

However, I suggest two means by which one can personally use remote sensing to actually idenitfy the aircraft you see in real time. By doing so, you can determine and record the actual aircrat seen. The first method is by using internet based live flight tracking such as Flightaware.com. It uses FAA generated data. The second method is using a telescope to zoom in and even photograph the aircraft's unique tail number.

Very few people believe that ordinary commercial flights are spraying anything. The logistics and secrecy involved make it rather unlikely, so generally most chemtrail believers are under the impression that military or other non-commercial flights are spraying. Simply identifying the flights seen takes most of the mystery out of chemtrails. Even if you insist that commercial flights are spraying, having their identity and destination would allow you to follow up on that belief.

I have written more on this in detail:

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#### lee h oswald

##### Banned
Banned
Man, as soon as someone asks a question, you guys are all over it! The question was: Does anyone have the info/source on the relative humidity needed for persistent contrails. Non-persistent ones need at least 70% but presumably the persistent ones need higher humidity?
So where does:
However, I suggest two means by which one can personally use remote sensing to actually idenitfy the aircraft you see in real time. By doing so, you can determine and record the actual aircrat seen. The first method is by using internet based live flight tracking such as Flightaware.com. It uses FAA generated data. The second method is using a telescope to zoom in and even photograph the aircraft's unique tail number.

Very few people believe that ordinary commercial flights are spraying anything. The logistics and secrecy involved make it rather unlikely, so generally most chemtrail believers are under the impression that military or other non-commercial flights are spraying. Simply identifying the flights seen takes most of the mystery out of chemtrails. Even if you insist that commercial flights are spraying, having their identity and destination would allow you to follow up on that belief.

I have written more on this in detail:

...come into it?

The truth is this: 70% RH is pretty rare at 30000ft and above where I come from and where aircraft fly; if you're seeing a lot of 'persistent contrails' you might ask why and investigate for yourself.

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#### Mick West

Staff member
70% RH is pretty rare at 30000ft and above where I come from and where aircraft fly; if you're seeing a lot of 'persistent contrails' you might ask why and investigate for yourself.

How would you define "pretty rare"? Once a month? Once a week? 10% of days? 20% of days?

#### lee h oswald

##### Banned
Banned
The 70% is a rule of thumb. The numbers behind formation and persistence of contrails are actually a little complicated, as there are several variables involved. There's the temperature, pressure and relative humidity (RH) of the air, and the temperature, pressure, speed, and humidity of the exhaust gas.

This is interesting - when your mate Ross Marsden said there were only five variables in this...you didn't debunk him. You list seven. How come you didn't correct Ross, the meteorologist? i can find the relevant bits if you need?

#### Mick West

Staff member
Ross did not count the pressure and speed of the exhaust gas, which is fine because they really only affect the mixing a bit. It does have an effect, but not a huge one, so can safely be ignored. I probably should not even have mentioned them - I was just thinking of all possible variables. I should debunk myself

If we are going down the road of all possible variable, you'd probably want to add the amount of incident sunlight (only really relevant if it's non-persistent, and then not much), and availability of condensation nuclei (not much, as there's plenty in the exhaust)

Contrails, as I said, are surprisingly complex.

#### lee h oswald

##### Banned
Banned
How would you define "pretty rare"? Once a month? Once a week? 10% of days? 20% of days?

I'd suggest checking out radiosonde data in a google search and going to the Uni of Wyoming for those in the US. There you can get two radiosonde readings a day from numerous stations and they will give you an idea. Have a look every day for a month if you really want to know. Log it - percentage? My guess (for altitudes between 30 and 40 thousand feet) would be 5 percent of days. Why don't you do the experiment? Publish the figures every day. Show me to be wrong

#### lee h oswald

##### Banned
Banned
Ross did not count the pressure and speed of the exhaust gas, which is fine because they really only affect the mixing a bit. It does have an effect, but not a huge one, so can safely be ignored. I probably should not even have mentioned them - I was just thinking of all possible variables. I should debunk myself

If we are going down the road of all possible variable, you'd probably want to add the amount of incident sunlight (only really relevant if it's non-persistent, and then not much), and availability of condensation nuclei (not much, as there's plenty in the exhaust)

Contrails, as I said, are surprisingly complex.

Fair enough. But, and I think I responded to Ross before on this 'five variables' thing: There are an indefinable number of variables can be derived from these 'five' - don't you agree?
I have said before that they (contrails) are hard to understand; prediction methods are unreliable; we don't know that much etc. Is this common ground? we appear to agree on something!

#### Mick West

Staff member
I'd suggest checking out radiosonde data in a google search and going to the Uni of Wyoming for those in the US. There you can get two radiosonde readings a day from numerous stations and they will give you an idea. Have a look every day for a month if you really want to know. Log it - percentage? My guess (for altitudes between 30 and 40 thousand feet) would be 5 percent of days. Why don't you do the experiment? Publish the figures every day. Show me to be wrong

Planes don't just fly between 30,000 and 40,000 feet. A better measure would be the number of days where there exists at some level RH at or above 70%, and temperature below -40 degrees (at the same altitude, obviously). We can check the historical record on UWYO (they do Europe too).

Would you think that would be 5% (or less) of days for, say, Camborne (03808)? Which I believe is near you (as you used it before).

And based on your observations, how many days in the year have you seen persistent spreading contrails?

(Note the individual radisonde soundings are not good at predicting, contrails in a particular place, as they are two low resolution. But they should give a good sampling of the conditions over the entire year).

#### Mick West

Staff member
Fair enough. But, and I think I responded to Ross before on this 'five variables' thing: There are an indefinable number of variables can be derived from these 'five' - don't you agree?
I have said before that they (contrails) are hard to understand; prediction methods are unreliable; we don't know that much etc. Is this common ground? we appear to agree on something!

Hard to understand for the layman, yes. It takes a bit of time to get familiar with the various concepts.

The prediction methods are unreliable like all weather forecasting. There's issues of instrument bias, lack of data resolution and frequency, the usual things. There's enough to give a fairly reasonable forecast, just like with most aspects of the weather.

I would not say "we don't know that much". We know a lot about the basic physics, but there are some interesting unanswered questions that are still the subject of on going research.

We DO know what conditions are required for contrails to persist. That's pretty basic science (even if it involves some complicated looking math).

#### Jay Reynolds

##### Senior Member.
I was not addressing you. I have asked you about that information several times with no response. After you read and understand it, we can discuss it, but if you cannot grasp the concept, it is probably not relevant to you.

lee said:
The truth is this: 70% RH is pretty rare at 30000ft and above where I come from and where aircraft fly; if you're seeing a lot of 'persistent contrails' you might ask why and investigate for yourself.

I also have previously directed you to this link, lee:

If you had taken time out from your intentionally abstruse performance art, you would have learned this:

Dr. Ulrich Schumann said:
As shown by Figure 5, more than 40 % of all data collected during a measurement campaign over
the North Atlantic [43, 44] were taken in ice-supersaturated regions. Measurements on modern airliners
[51] indicated that such aircraft fly in ice-supersaturated air masses about 15 % of the flight time[42].

Figure 5. Relative humidity with respect to liquid water versus ambient temperature. The thin full
curve denotes the relative humidity for ice saturation. The dashed line denotes liquid saturation. The
symbols denote measured data for cases exceeding ice saturation as derived from a frostpoint hygrometer
(Ovarlez et al. [43]) and temperature sensors (extended from Schumann et al. [44]). The pair
of curves with various line notations represent the relative humidity for constant water vapour content
at various temperatures.

Ice-supersaturated air masses form in regions with rising air motion and are partially filled with cirrus
clouds [52]. Regions with ice supersaturation have been found with horizontal extensions of the
order 150 km [53] and vertical extensions of about 500m in the mean [47]. Supersaturation is also
observed at least occasionally in the lower stratosphere up to the hygropause in the polar winter and
also, rarely [47, 54], in the lower stratosphere at mid-latitudes and in the tropics up to a about one
kilometre above the local tropopause [55, 56].

The area size of the regions with ice-supersaturated air masses defines the potential contrail cover
which would appear if aircraft were to fly everywhere and at all times. Global distribution maps of ice
supersaturated regions have been produced from satellite data [48] and from analysis of meteorological
data [57]. Such analysis suggest that the global average contrail cover (partially overlapping with
cirrus clouds) could approach 16 % [57]. Over Europe the potential contrail cover reaches 12 %,
which is consistent with estimates derived from satellite observations of the size of regions with clusters
of persistent contrails [58] and by in situ humidity measurements [42, 53].

If these numbers were realised it would mean a very large change in high cloudiness. For comparison, the
mean high cloud(cirrus) cover at northern mid-latitudes is about 20 - 30 % according to different
observations [59, 60]. Subvisible clouds with optical depths below about 0.03 are even more abundant [61].

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#### Jay Reynolds

##### Senior Member.
Concerning the extent of ice supersaturated regions, researchers took measurements on ordinary commercial airliners flying their normal routes. They found some huge areas of ice supersaturation, and some perplexing data they had a lot of trouble analyzing, mainly because the flights were not dedicated to the research. For that reason, their conclusions were tempered with some qualifications.
Gierens etal. said:
Abstract. In order to determine typical sizes of ice-supersaturated regions (ISSRs) in the upper troposphere and lowermost stratosphere we set up the frequency distribution of path lengths flown by MOZAIC aircraft within ISSRs. The mean path length is about 150 km with a standard deviation of 250 km. We analyse the influence of a selection bias (viz. that large ISSRs are more often crossed by aircraft than small ones) on the obtained path length statistics and derive a mathemat-ical equation that relates the path length distribution to the underlying size distribution of ISSRs, assuming that they have circular shape. We solve the equation (by trial and error) and test the result using numerical simulations. Surprisingly, we find that there may be many more very small ISSRs than apparent from the data such that the true mean diameter of the ISSRs may be of the order a few kilometres only. The relevance of the result is discussed and dedicated research flights to measure the true extension of ISSRs are recommended
http://www.ann-geophys.net/18/499/2000/angeo-18-499-2000.pdf

#### lee h oswald

##### Banned
Banned
Concerning the extent of ice supersaturated regions, researchers took measurements on ordinary commercial airliners flying their normal routes. They found some huge areas of ice supersaturation, and some perplexing data they had a lot of trouble analyzing, mainly because the flights were not dedicated to the research. For that reason, their conclusions were tempered with some qualifications.

...in order to determine typical sizes of ice-supersaturated regions (ISSRs) in the upper troposphere and lowermost stratosphere

Commercial aircraft don't fly in the stratosphere which begins at approx 60,000ft - so irrelevant - the upper troposphere is what? alt 12000m - 14000m or 39600ft - 46000ft - the lowest being right at the cusp of commercial traffic limits. What good is that?

#### Mick West

Staff member
Commercial aircraft don't fly in the stratosphere which begins at approx 60,000ft - so irrelevant - the upper troposphere is what? alt 12000m - 14000m or 39600ft - 46000ft - the lowest being right at the cusp of commercial traffic limits. What good is that?

It says the area in the "upper troposphere and lowermost stratosphere"

Wikipedia says:
http://en.wikipedia.org/wiki/Stratosphere

The stratosphere is situated between about 10 km (6 mi) [32,800 feet] and 50 km (30 mi) altitude above the surface at moderate latitudes, while at the poles it starts at about 8 km (5 mi) [26,246 feet] altitude.
[...]
Commercial airliners typically cruise at altitudes of 9–12 km (30,000–39,000 ft) in temperate latitudes (in the lower reaches of the stratosphere).[2] They do this to optimize fuel burn, mostly thanks to the low temperatures encountered near the tropopause and the low air density that reduces parasitic drag on the airframe. It also allows them to stay above any hard weather (extreme turbulence).
The key word there being "about", as the height of the stratosphere varies with location (being lower closer to the poles) and season. See:

http://www.weatherquestions.com/What_is_the_troposphere.htm

The boundary between the troposphere and the stratosphere is called the "tropopause", located at an altitude of around 5 miles [26,400 feet] in the winter, to around 8 miles high in the summer [42,240], and as high as 11 or 12 miles in the deep tropics.

And here's a random verification from New Scientist, 1962, pinning it at around 33,000 feet

So it would seem Jay's source is very relevant.

#### Jay Reynolds

##### Senior Member.
Mick, lee doesn't read any of my links, it seems. Pity the fool.
The research was done on commercial flights by ordinary AIRBUS planes.
MOZAIC=
M- Measurements
OZ- of OZone, water vapor, CO, NOx
AI-AIrbus
C- airCraft

Lee, if you don't follow along, what use is having a conversation with you?

Here's their website, which you can ignore:
http://mozaic.aero.obs-mip.fr/web/

##### Senior Member.
Pity the fool.
Oh, 'cos he doesn't like planes either- I see what you did there

M

#### Mick West

Staff member
That's rather an unfortunate amount of misconceptions about contrails. Note thought that it was written in 2010. I wonder if he's changed his opinion at all since then.

#### CbIncus

##### Member
Because a lot of chemtrail believers say that "Those lines in the sky are chemtrails: relative humidity is too low for contrail persistence. Radiosonde data proves it", I'd like to offer a way for debunking these claims. Weather sondes are affected by dry bias or even stop working at high altitudes: https://www.metabunk.org/threads/ac...midity-soundings-for-contrail-prediction.758/, so satellite info should be used. There are some model reanalyses: NCEP (GFS), ERA-Interim. The second one should be used for obtaining precious data. It is rather difficult to visualize it. I ask all the debunkers: should I describe the way to do it? It will take me a long time to write the post.

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#### Belfrey

##### Senior Member.
Because a lot of chemtrail believers say that "Those lines in the sky are chemtrails: relative humidity is too low for contrail persistence. Radiosonde data proves it", I'd like to offer a way for debunking these claims. Weather sondes are affected by dry bias or even stop working at high altitudes: https://www.metabunk.org/threads/ac...midity-soundings-for-contrail-prediction.758/, so satellite info should be used. There are some model reanalyses: NCEP (GFS), ERA-Interim. The second one should be used for obtaining precious data. It is rather difficult to visualize it. I ask all the debunkers: should I describe the way to do it? It will take me a long time to write the post.
I would appreciate that very much. By the way, I notice that the ERA-Interim page has a message saying, "This server is been replaced by a more powerful system that can be found at http://apps.ecmwf.int/datasets/. Please start using this new system, as this server will be discontinued in the near future."

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#### Mick West

Staff member
I would be interested in that. It's a difficult thing to communicate though. Very annoying that once you can actually convince people that contrails would persist in certain conditions, they then become convinced (because of the bad radiosonde data) that those conditions don't exist. It's difficult for them to take the next step to understand the problems of measuring humidity.

#### CbIncus

##### Member
I'd like to ask about the quantity for plotting. Maximum RHi in 350-150 mb layer would be an appropriate variable.

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#### Ross Marsden

##### Senior Member.
That looks good. Persistent contrails in the yellow, orange, red shades; above the 100% contour.
It would be good to see the 300 -250 hPa layer as well (FL300 to FL390).

#### CbIncus

##### Member
Ross, I'm still thinking about the plotting form. It will be better to create several maps of RHi for many isobaric surfaces (350, 300, 250, 225, 200 ,175, 150 hPa) along with their geopotential heights and/or temperatures.

#### Jay Reynolds

##### Senior Member.
Many folks who aren't visual/spatial thinkers will have trouble interpreting this. In addition to showing the humidity fields, perhaps alongside the image you could also show a representative set of white line grids depicting flight tracks in which persistent contrails would start/stop as they pass through ice supersaturated areas would help in the visualization.

Animation of the action would make that even better. Ross' idea about finer resolution of altitude would help those who think that ice supersaturated regions are consistent with altitude. Imagine a short 3D video of a plane flying through patchy or consistent ISSR's:

This page seems to be discussing software which might be useful in accomplishing that:
http://vterrain.org/Atmosphere/Clouds/

Considering that I've noted a significant portion of chemtrail believers are artistic types who seem to respond to imagery, such a project should pay off in better understanding.

#### CbIncus

##### Member
Jay, this would be a good idea. But it's important to understand physics of humidity in ERA-Interim. Yesterday I searched ECMWF site for saturation vapor pressure equations and found this: http://www.ecmwf.int/research/ifsdocs/CY25r1/Assimilation/Assimilation-06-6.html. The page says RH is calculated w.r.t. ice when T < -23 deg. C. No conversion is needed. ISSRs in this case become much smaller and even non-existent in dense cirrus/cirrostratus, which is unlikely. I'll probably switch to model's high level cloud cover.

#### TEEJ

##### Senior Member.
I noticed that Len Duggan referenced this thread in one of his recent posts. It appears that Len posted within the thread? It stems from a full blown argument after David R Haynes called out Max Bliss in regards to MINTRA. Max admits that he had never heard of MINTRA.

#### Vostok

##### New Member
(Perhaps this is not the correct place to post such a message and, if I am in the wrong please just point me in the right direction)

My question is related to the conditions in which a Contrail should form. I have heard from place to place, including, many times within this site, that Contrails need a relatively high humidity to form. While that seems consistent with many sources, I'd come across this the other day:
"Appleman Chart"
http://science-edu.larc.nasa.gov/contrail-edu/images/page-graphics/chart1.jpg
Found on the NASA website here:
http://science-edu.larc.nasa.gov/contrail-edu/resources-activities-appleman_student.php

Anywho, If i am not mistaken, this graph appears to make the case that Contrails will always appear when the relative humidity is 0%, while at 100% they don't form
I am almost certain that I am in some way misinterpreting the graph, and if one of you could explain it, it would be appreciated.

Thanks, Vostok.

#### jonnyH

##### Senior Member.
The chart shows that at greater humidity contrails will form over a wider range of temperatures and pressures.

The 100% humidity line marks the edge of an area representing the conditions in which contrails will form at 100% RH, this area is to the left of the line (lower temperatures) and includes the area demarcated by the 0% RH line which is labelled "always contrails" and the central stripe labelled "maybe contrails."

As an example, at 100% RH, 300hPa and -44 degrees you will get contrails because the coordinate (-44 degrees, 300hPa) is to the left of the 100% RH line. That same point on the chart is to the right of the 0% RH line so contrails would not form at 300hPa and -44 degrees at 0% RH the conditions being too warm.

#### Vostok

##### New Member
Thank you jonnyH, now I understand.
It had me confused as i thought it was saying 0% relative humidity was ideal (hence the "Always Contrails") but now you've cleared it up a lot.

#### Paso Winemaker

##### New Member
There might be some confusion between types of contrails. Aerodynamic contrails require plenty of atmospheric humidity, where engine exhaust contrails can form in the absence of atmospheric RH because the required moisture is produced in abundance by the combustion of jet fuel.

Here's a pic of an airplane producing both types simultaneously...

#### Mick West

Staff member
There might be some confusion between types of contrails. Aerodynamic contrails require plenty of atmospheric humidity, where engine exhaust contrails can form in the absence of atmospheric RH because the required moisture is produced in abundance by the combustion of jet fuel.

Neither type of contrail can persist at low humidity though.

#### Lisa P

##### Active Member
The chart shows that at greater humidity contrails will form over a wider range of temperatures and pressures.

The 100% humidity line marks the edge of an area representing the conditions in which contrails will form at 100% RH, this area is to the left of the line (lower temperatures) and includes the area demarcated by the 0% RH line which is labelled "always contrails" and the central stripe labelled "maybe contrails."

As an example, at 100% RH, 300hPa and -44 degrees you will get contrails because the coordinate (-44 degrees, 300hPa) is to the left of the 100% RH line. That same point on the chart is to the right of the 0% RH line so contrails would not form at 300hPa and -44 degrees at 0% RH the conditions being too warm.

This explanation on the NASA website is easier to understand;
http://science-edu.larc.nasa.gov/contrail-edu/resources-activities-appleman_student.php

My understanding is imagine the Always Contrail side coloured in blue, the No Contrails side pink and the middle section purple is where there Maybe Contrails. You plot your temperature and pressure and where the spot lands is what you expect of contrail formation? Always, No or Maybe. On the left side of 0% there will always be contrails because the engine produces enough moisture.

Not sure if that is what jonnyH is saying but I am understanding his explanation differently. I am so confused about now. Just reread NASA's explanation and it makes sense.

#### Mick West

Staff member
You can also think of it as a bunch of separate graphs, on graph for each line for a particular RH. Each graph has just two regions. A "No contrails" on the right of the line where the temperature is to high for that RH, and "always contrails".

If you know what the RH is, then there's no "Maybe contrails". Generally though, you don't.

#### Lisa P

##### Active Member
Contrails can form at any level of RH, if it's cold enough. But if it's lower than a certain value, then they will not persist.

The 70% is a rule of thumb number for contrail persistence, and the real number varies with pressure (i.e. altitude). The numbers behind formation and persistence of contrails are actually a little complicated, as there are several variables involved. There's the temperature, pressure and relative humidity (RH) of the air, and the temperature, pressure, speed, and humidity of the exhaust gas.

For a contrail to persist, the relative humidity with respect to ice (RHI) needs to be above 100%. Note the RH is not the same as RHI. It differs in the same way that dew point differs from frost point. It's also not a simple matter to convert from one to another, as they derive for different complex equations (polynomials), even the approximations of which (such as Goff-Gratch) are overly complex, so conversion is done with tables, or computers.

This link will give a taste of the complexity. Note that the equations given at the bottom of the page are approximations.
http://www.yesinc.com/education/moist-convert.html

Anyway, that's just to explain why there's not a precise figure. 70% is ABOUT the RH where RHI is above 100% in average contrail condition.

To answer your question: no, persistent contrail do not need higher humidity. If the RHI is above 100% (approximately 70% RH), then a contrail will persist. Contrails will fade away if the humidity is below this level. The dryer the air, the quicker they will fade away.

I am trying really hard to understand this and I won't give up. I am going to study this thread and annoy you and ask questions til I do.

Different subject but I told my Mum the other day about how they do the cloud seeding at the Snowy Hydro in New South Wales Australia. She nearly fell off her chair and said there is no way they could be doing that. I told her the State Govt had to pass legislation to approve it, they had to vote on it. I had looked into this when I was a chemtrail believer and downloaded the legislation. She just didn't believe me and said the Govt lie you know! I just shook my head what else could I do. I know she will eventually absorb it.
That said I think it shows how important it is to know the ins & outs so I can explain it to friends and family that are starting to think 'chemtrails' are a thing.
I just tracked a Hong Kong Airline on 9/1/16 close to 11:30am near Noosa making a rare (I have only seen 2 -3 of these in the last 2years, there could be way more I don't see) non-persistent contrail. I looked up the Appleman chart and the weather conditions it was -52.7C @ 200hPa & 6% humidity (if I haven't made any errors which is so easy to do). And it sits in just the right place for contrail formation I think. Being non-persistent the RHi must be less than 100%? I am even too scared to ask about dew point. (I understand the readings were 100km away and almost an hour & half earlier so not entirely accurate).
Now I understand why some of the chemtrail believers say you lie about contrails Mick because this is dang complicated! Such a shame they don't have the trust and patience to stay with it and work it out. At least my learning curve is helping me to understand why they could think that. It should help me when having discussions with them. I have had people private msg me lately asking questions so that is interesting. I just want to be as on top of it as I can.
Would it be best to ask questions in chit chat?

#### Mick West

Staff member
I just tracked a Hong Kong Airline on 9/1/16 close to 11:30am near Noosa making a rare (I have only seen 2 -3 of these in the last 2years, there could be way more I don't see) non-persistent contrail. I looked up the Appleman chart and the weather conditions it was -52.7C @ 200hPa & 6% humidity (if I haven't made any errors which is so easy to do). And it sits in just the right place for contrail formation I think.

There's a few things to understand here.

Firstly, the 6% humidity @200hPa - where did that number come from? Unfortunately atmospheric soundings of humidity are very inaccurate (due to cheap instruments, which need to bee disposable), and widely spaced, so there's not much use for them directly. In the absence of a precise reading at that time and place, you need to use a computer model incorporating corrections and satellite readings - which is really only going to give you a fuzzy estimate.

Secondly, there's a mathematical relationship between humidity (the common term for the relative humidity with respect to water, or RHw) and the relative humidity with respect for ice (RHi). They are not independent values. It would be nice if it were a simple one like RHi = 1.3*RHw, but unfortunately it's a more complicated one, varying with pressure and temperature, and involving mathematical notion that's beyond the comprehension of the average person.

At it's "simplest", to convert from RHw to RHi you multiply by the saturation vapor pressure for water (at that temp and pressure) and divide by the saturation vapor pressure for ice.

It also seems to be something that's not settled yet, with several different equations in use.

Here's the code I use to convert RHW to RHI based on pressure and temp:
Code:
``````# convert RHw to RHi, as per
# ta = temperature of atmosphere, in Celsius
# p = pressure in hPa (hectopascals), which are the same as mb (millibars)
# note this is defined as accurate from -0C to -50C, but it's unclear how inaccurate it is
# at temperatures below -50C
def rhw2rhi(rh, ta, p)
e_sat_w_Ta = (1.0007 + 0.00000346 * p)*6.1121*Math.exp((17.966*ta)/(247.15+ta));
e_sat_i_Ta = (1.0003 + 0.00000418 * p)*6.1115*Math.exp((22.452*ta)/(272.55+ta));
return rh * (e_sat_w_Ta/e_sat_i_Ta);
end``````

From:

Something I think might be useful it to write a web calculator that calculates this, something that can be extended to draw graphs etc.

Finally, it's interesting that you see short contrails as "rare" - this is maybe a function of where you live, or possibly related to your personal observation history (short trails are hard to spot, and somewhat seasonal)

#### Trailspotter

##### Senior Member.
I just tracked a Hong Kong Airline on 9/1/16 close to 11:30am near Noosa making a rare (I have only seen 2 -3 of these in the last 2years, there could be way more I don't see) non-persistent contrail. I looked up the Appleman chart and the weather conditions it was -52.7C @ 200hPa & 6% humidity (if I haven't made any errors which is so easy to do). And it sits in just the right place for contrail formation I think. Being non-persistent the RHi must be less than 100%?
If I read your info correctly, the flight in question was HX16 from Gold Coast to Cairns (Airbus A330-343) that passed over Noosa Heads at 40,000 ft. This is higher than 200 hPa, so temperature could be even lower than -53°C. Perhaps, it was just cold enough for the formation of a short contrail even in dry ambient air. In a more humid air, the contrail would last longer and be longer as well. Other conditions being equal, for RHi below 100%, there would be a correlations between relative humidity and the contrail length/duration. Non-persistent contrails can be quite long. A one minute contrail will extend for nine miles behind the plane that flies with the speed of one mile in about seven seconds.

Could you recall how long was the non-persistent contrail that you saw?

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