Contrails - how do I predict them?

[Layla Curtis]

I am an artist currently researching contrails to make a new video work. Does anyone have any information regarding how i might predict when contrails are likely to occur so that I can organise a filming schedule

[Capt H Peacock]

The Met Office used to include a flight level called MINTRA or minimum trail altitude. This was primarily for the use of bomber pilots. Flying just below MINTRA would ensure that your position was not given away by contrails. I'm sure such a product should still be available.

If your use is intended for professional purposes, the Met Office are usually happy to oblige, but there will be a charge. 08700 767 868

[KC-10 Driver]

When I flew in the U.S. Strategic Air Command (SAC), we used to file a contrail report after every flight. This report contained data such as altitude, temp, flight vis, lat/long, presence or absence of contrails, etc.

I'm sure that the USAF has this data somewhere, and they may have done something useful with it. But, I do not know if it is accessible. A little research may give you an answer.

[Earthmover]

There's no simple answer I'm afraid Prof. Chen (University of Wisconsin) outlines the problem:

This problem involves several components. First, (s)he must estimate the temperatures of jet exhaust. A good model developed from another PCOL project, 'How Hot is The Flame' [Zielinski, 2001] would be a starting point. Second, the temperatures and humidity of the atmosphere must be known. These data can be either estimated (by barometric formula [Atkins and de Paula, 2002a], and adiabatic air expansion formula [Atkins and de Paula, 2002b]) or found from satellite data bank [Thompson,2000]. Third, (s)he must construct vapor-pressure diagrams of ice and water[Atkins and de Paula, 2002c]. These diagrams can be found in many references. Lastly, (s)he must articulate the cooling mechanisms of jet exhaust and predict the frost/fog formation behind the jet in the sky.
Yeah, right!

Here's a link- but it's not a great deal of help in waking up, looking at the sky, saying "today's perfect" & phoning the cameraman, focus-puller, clapper-loader and soundman, I'm afraid!

www.tpub.com/weather3/6a-26.htm


Good luck

[Captain Airclues]

The trail level can be calculated manually using a tephigram. The ascent curve for the current conditions is plotted until it reaches the line marked 'Mintra'. The level where the two lines cross is the 'exhaust trail' level. This used to be a required excercise in the APTL exams, but thankfully it isn't now.

Airclues

[Dick Whittingham]

UK Met tephigrams used to have MINTRA lines on them, but I don't think they are plotted now. I have just called up a current Irish tephigram - no MINTRA. I think there used to be a DRYTRA line as well to mark the top of the contrail band.

I think, but memory is falible, that the old MINTRA related to piston engines, which had fairly consistent exhaust content and mixing, so contrails could be forecast with some accuracy. I do not recall ever using the MINTRA forecast when in a jet.

Dick W

[Captain Airclues]

Dick

Many thanks for the correction. I'm afraid that I was using my ATPL notes from 37 years ago. Things move on!

Airclues

[Earthmover]

I believe that it was common to subtract 11 to 14deg C from MINTRA for jet aircraft - this predicts short non-persistent trails (but persistent in high humidity) More than14deg C below MINTRA one can expect long persistent trails, irrespective of the humidity.

Which is bl@@dy useful for an artist whose equipment probably consists of a piece of seaweed and a wet finger held up to the wind.

She appears to have (somewhat understandably) given up in despair! ;)

[psy clops]

I do not recall ever using the MINTRA forecast when in a jet.Hmmm, I'm pretty sure that they were available pretty recently - especially when we were flying where the locals were keen to 'greet us' with the odd sharpened mango (or similar)!

Psy

[Layla Curtis]

hello and thanks to everyone who has replied to my question regarding contrails. Your advice has been very useful and I've been very busy following all your leads.

I'm now looking for information on what weather conditions are more likely to allow contrails to form/be visible so that I can look at the weather forecast at any point and plan whether filiming is going to be likely.

As I understand it contrails last longer in the sky when the weather is moist and rising, with the arrival of a storm in a day or two. When the weather is dry and sinking and fair weather is set to continue contrails are short and fade quicker. Does anyone have any thoughts / further info on this.

thanks

[Earthmover]

Ye-e-s, but the trouble is, that it can be wet 'down here' and dry 'up there'!

I think that the advice given by Capt H Peacock on the second posting is the best - the UK Met Office is excellent, and forecasters are very obliging - but as he or she says, they may charge.

[mcdhu]

Found this on a Google search for 'Mintra Level'. The last sentence seems to give the sort of advice you are looking for. I well remember struggling with Tephigrams in met lectures in the late 60s and Mintra levels used to come into the preflight met briefs in those days and was indeed used. A Master Pilot QFI went off one day in a JP and drew a set of male 'tackle' in the sky overhead Cranwell. There couldn't have been much wind that day because his artwork 'hung' around for quite a while much to everybodies' mirth. Anyway, I digress........

'' To aid the forecasting of condensation trails emitted (or not) from high-flying aircraft, a line marking the critical temperatures (altitude dependent), above which trails are not possible, is marked on a tephigram (q.v.). The values are approximately -24degC at 1000 hPa (i.e. roughly sea-level), -39degC at 250 hPa (34000ft / 10.4 km) and about -45degC at 130 hPa (50000feet/15km). Using the MINTRA line (as it has come to be called - based on experiments by JK Bannon during World War II with the piston-engined Spitfire), a forecaster will mark two further lines on a tephigram: MINTRA minus 11degC (A) and MINTRA minus 14degC (B). If the ambient temperature (from the tephigram air temperature plot) lies between (A) and (B), then short, non-persistent trails are possible. If colder than (B), then long, persistent trails should be expected. However, some note should be paid to the relative humidity - high values will tip the balance to trailing (or longer/persistent trails.), even with air temperatures warmer than (A); ultra-low rh% will reduce the risk of condensation trails - the design of engines will have an effect as well. In broad terms, warm Tropical Maritime airmasses with a high but cold tropopause will result in a good deal of trailing, whilst cold, polar air-masses with a low, relatively warm tropopause will seldom give rise to significant aircraft trails.''

Cheers,
mcdhu

[Dick Whittingham]

Now that's interesting! We did get contrails at ground level once at Portage-la-Prairie in Manitoba with a surface OAT of near minus 20ºF - below minus 24ºC - when the first aircraft out blanketed the field with fog.

Dick W

[SIGMET nil]

As I understand it contrails last longer in the sky when the weather is moist and rising, with the arrival of a storm in a day or two. When the weather is dry and sinking and fair weather is set to continue contrails are short and fade quicker. Does anyone have any thoughts / further info on this.

I think, what you wrote, sums it up fairly accurately.

I stand to be corrected by the more knowledgable but I think, what you need is a weakening surface high pressure area at your location and an approaching disturbance of some kind. Which is, what you said already.

This weather situation makes sure that the lower and middle atmosphere is still dry and transparent, so you can observe the contrails at all. But you need the large scale atmospheric lifting and moist air moving in at high levels, so there is enough humidity at high levels and contrails can form at all.

I think this should be the case far ahead of an approaching typical warmfront. They are marked on a surface chart (see below) as lines with breast shaped icons on them.With a warmfront normally moist air moves in at high levels first. Lots of cirrus veils will be visible along with contrails, but soon the high level clouds will increase and blot out the contrails. Then medium level clouds arrive and soon it will start to rain.

I'd also expect contrails near trough lines. This kind of disturbance means, that atmospheric lifting (caused by the high level wind current) triggers thunderstorms in an unstable airmass without an airmass change.
The UK surface charts (see link below) mark them as lines without symbols on them.

An approaching coldfront (lines adorned with triangles on the surface chart) may also yield opportunities, as there is often fair weather ahead of the front.

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The following requires that you have a fast internet connection ...

To get an overview of the next 7 days try the surface charts from the UK Met Office (http://www.westwind.ch/?page=ukmb) (assuming you want to film in the UK)

To see what is going on, try the high resolution satellite pictures (http://www.btinternet.com/~wokingham.weather/wwp2.html) They are wonderful. You need to select images from the UK by looking at the previews or look for UK in the file name below the previews.
Sometimes you can detect areas with contrails on those images.

Lots has been said about atmospheric soundings by other posters. You can easily view current atmospheric soundings at this link: European vertical soundings (http://www.westwind.ch/w_0sok.php)
Select a profile near your location by clicking on the green symbol.
Ignore all the stuff on the right of the plot and just have a look at the plotted diagram. Enlarge it by clicking on it.
You see a red line depicting the temperature against altitude. Altitude is given in meters to the right and in mb to the left of the diagram. You see a light blue line which is depicting the dew point, i.e. humidity. The closer together the lines are, the moister is the atmosphere. You need these two lines to be far apart below 400 mb (see left vertical axis) and close together at levels above 400 mb.
Again this is an oversimplication but maybe useful.

Maybe you might even try to make sense from the upper level charts given here: (http://www.westwind.ch/?page=gfsm)
In the first line marked SLP, H500, T500 you get a certain type of forecast chart in 6 h intervals. You see the surface pressure lines known to you from surface charts. You also see areas painted in different colours which outline the airflow at roughly 5.5 km altitude. Precisely the colours indicate the altitude of the 500 mB (=hPa) layer, but well ..

Like at the surface the airflow aloft is clockwise around highs and counterclockwise around lows for the Northern Hemisphere. The airflow is parallel to the lines that divide the various coloured areas and the closer these lines are, the stronger is the wind.

On this chart you can recognize patterns which look like the letter U (upper troughs) and patterns that look like a hump (upper ridges).

Speaking very general, if you cut both patterns in half along their (mostly north south) symmetry axis, then the upstream (often western, left) half of the 'hump' and the downstream (often eastern or right) half of the trough will enhance contrails, because there is atmospheric lifting present.
This is just an oversimplification but maybe it helps.

Apart from this you can use aforesaid link to check for precipitation and medium level clouds in order to rule out periods with downright bad weather ..

Finally here is a link to the UK Met office in case you decide to ask for a
specialised briefing from an aviation met officer (http://www.metoffice.gov.uk/services/talkfc/) It's not cheap though.

good luck

[SIGMET nil]

This (Monday) morning I saw some persistent contrails on the early passes of the NOAA polar orbiting satellite and the earliest visible pictures of Meteosat 8. They were located over East Anglia and initially also in the London Area.

It fits in the picture outlined in my earlier post. An airmass boundary with unstable air and thunderstorms over the channel is adjacent to weak high pressure influence over Southern England.
I would expect the contrails to grow less persistent and to disappear entirely in the course of the day, as subsidence (sinking motion) is setting in over the Southeast of England.

Tomorrow morning I expect persistent contrails to be visible over the area from London to East Anglia before the clouds of the reactivated airmass boundary, by then a warmfront, will blot them out.

I must add, that I'm not very experienced in this matter and these thoughts are rather experimental.

Another link for great satellite pictures from polar orbiting satellites courtesy of the UNI Bern:
http://saturn.unibe.ch/rsbern/noaa/dw/realtime/

Click in the line marked level1b on the resolution you would like. With a fast connection choose fullres for the best quality.