Aircraft contrails

A section of brightly sunlit contrail might appear strange in certain conditions - for example if broken cloud produces unusual contrasts of light and shade - and could resemble a distant bright cigar shape lying parallel to the horizon. The high resolution 1328Z MODIS image (Fig.19) shows several contrails above the clouds. Note their shadows on the cloud deck: If a higher contrail happened to be oriented so as to cast a shadow across a lower this could possibly also explain a "dark band".

Jet contrails are of two types. High altitude exhaust contrails as in Fig 19, which are formed of ice crystals nucleating around exhaust particulates at atmospheric temperatures far below zero (around -40°C), and aerodynamic fog contrails at low level. The latter may occur around aircraft surfaces in humid air when moisture condenses due to a local pressure drop caused by the aerodynamic lift.

The line of sight to the UAPs from both aircraft (Aurigny Trislander to the north and Blue Islands Jetstream to the south) was close to the horizon or even at a small depression angle. UAP#1 was observed from the Trislander "against the sea and the island [Guernsey]". These factors would all indicate local contrails - i.e, located approximately where the triangulated lines of sight intersect in Fig.7 - and at an altitude significantly lower than ~4000ft. This is >6000ft below the freezing level and 20,000ft below typical exhaust contrail heights, requiring pressurecaused water droplet contrails. But since the pressure change is local to the wing it is transient and such trails would not normally persist for many minutes after the passage of the aircraft, which gives added emphasis to the question of what jet(s) might have been responsible? Very dense and persistent highly-reflective low-level contrails would be unusual and ought also to have been visible to other aircraft asked to look out for anything in the area. Nothing was seen by the BAe146 flying above the immediate area.

Civil traffic during the sighting time was not in the right area. The FlyeBe 146 did not even taken off until some minutes after the start of the sighting and the Jetstream was still 25-30nmi W of the line of sight to the UAP. No unusual aircraft activity was reported by these or other aircrews. The Channel Islands Zone and Jersey Approach radio recordings from 1406:47 contain no reference to unusual traffic prior to Capt Bowyer's first radio contact at 1409:32. During this time SVW 23 AR is handed off to Jersey Approach at FL60, an Aurigny island-hopper approaches Jersey, Thomson 742B leaving the Control Zone to the south is instructed to contact Brest, and N9MS leaving the Control Zone to the north and crossing 50°N at FL70 is instructed to "continue VFR and contact Plymouth military". This is because the area NW of the Control Zone is a designated military exercise area, but ATC advises "no known traffic" in the area. These are clearly all routine commercial flights. Unidentified military jets crossing civil air lanes inside the Control Zone at ~2000ft without ATC clearance and/or the prior issuing of NOTAMS n1 NOTAMs (Notices to Airmen) are hazard warnings required under an international ICAO convention. They are distributed electronically to aviation and navigation service providers. Military exercises and the like require NOTAMs. None were issued. seem most unlikely. Inquiries were made to the UK MoD in respect of any military exercises carried out in the Channel area on 23 April. The MoD responded as follows:

The MoD did not conduct any military exercises (naval, RAF or army) in the English Channel, nor is it aware of any known military activity involving other nations (e.g. France) on or about Monday 23 April 2007.

Also relevant to the possibility of low-altitude contrails is the humidity at the hypothetical altitude, which would be about 2000ft. The closer the relative humidity is to saturation (RH = 100%) or supersaturation (>100%) then generally speaking the more likely is condensation to occur with a given aerodynamic reduction in pressure and temperature. Since the pressure change will tend to be proportional to aircraft performance and acceleration, the RH clearly relates to the likelihood of aerodynamic fog being produced by light commercial traffic.

A reported haze layer close to 2000ft altitude is believed (see Section 5) to be a particulate haze intruded from the continent n2 Moisture is still a factor in the optical thickness of such a haze even if RH is far below saturation. Hygroscopic salt particles in a salt sea haze only begin to swell at more than 70% RH, but dusts and biological aerosols such as pollen will react similarly to a lower RH.. The noon Brest radiosonde ascent shows RH at 52% at the surface and <40% through the first 3000ft, which is quite dry, falling to an unusually dry 10% at about 2000ft. At the sighting time, Guernsey surface RH was recorded at 59% (17°T, 9°D, well below the local 22-year historical April average n3 of 73%), and Alderney surface RH at 77% (14°T, 10°D). The mean of these values is 68%. This tentative evidence on the whole does not suggest saturation in the lower atmosphere, although an elevated humid layer isn't ruled out.

If low-altitude contrails are unlikely, short sunlit sections of exhaust contrail due to aircraft at high altitude above and beyond the Control Zone could conceivably appear as bright "cigar shaped" objects, and these ice crystal contrails can persist for a considerable time. However this also seems most unlikely for several reasons.

We would first need to assume an angular elevation error on the part of the Trislander witnesses, so that their UAPs were not observed against the sea and islands at any time but remained just above the visual horizon. Then a LOS tangential to the horizon from an aircraft at 4000ft intersects an altitude of 25,000ft (the minimum realistic level for exhaust contrails) at a slant range of ~235nmi n4 The 1328 high-res MODIS image (Fig.19) shows a pair of high contrails lying just S of Guernsey and casting shadows on the cirrus tops. Similar nearby contrails would, if any were visible at 1406, be a number of degrees above the horizon..

At this range the LOS is running into an area of frontal cloud over the Bay of Biscay SW of Finisterre shown on the 1418 (overhead time) NOAA 18 satellite images (Fig 18), likely to lead to obscuration of the LOS by intervening cloud below the trail height, and/or to obscuration of the sun by thick cloud above the trail height. Visibility was estimated by Capt Bowyer at 100nmi above the haze. But even assuming brightly sunlit contrails to be visible at >235nmi in these conditions, we run into problems with the scale and rates of displacement.

The angular thickness of the UAPs was established to be in the range 0.05° - 0.1° (Section 3). UAP#1 with an angular thickness of 0.1° would be equivalent to a horizontal length of contrail almost ½ mile thick, in vertical depth, or much more than this in horizontal breadth if we consider the perspective projection of a thin layer, which is the normal form of wind-dispersed contrails. These are improbable dimensions for even a very dissipated contrail, especially given the required extreme brilliance (implying high crystal density and little dispersal) and the fact that the LOS intercepts the underside of the high-altitude trail in this case, requiring light to be transmitted through the contrail rather than reflected off the directly sunlit top.

Some unusual coronal diffraction effect might be indicated. However, let us remember that there were two of these UAPs, and that both were observed to have a similarly-located dark band, a detail which was preserved during an overall westward azimuth rotation of the UAP#1 LOS by about 10° and a simultaneous superimposed eastward counter-rotation of the UAP#2 LOS by about 5° relative to LOS #1 (or in other words #2 rotates to the west at half the rate of #1). At the likely minimum slant range of an exhaust contrail #1 has probably moved westward by ~20nmi, corresponding to a real ground speed of ~100 knots. Winds were less than about 50 kts at all levels at all of the radiosonde stations examined, and upper winds, dominated by the approaching frontal system, were generally from the southwest/west, as indicated by noon radiosonde ascents and satellite photo sequences. Winds at ~25,000 ft recorded at Brest were SSW, less than 20 kts, falling lighter at higher altitudes. Even if the #1 LOS is assumed static, and the relative #2 rotation is then interpreted as a real 25°/min eastward drift, then the minimum real (easterly) speed exceeds twice that of the maximum (northeasterly) wind vector, and the implied ~50 knot wind velocity shear between the two trails is at odds with the persistence of their identical form and internal detail during some 12 minutes of binocular observation.

Finally of course there is the corroborating report of the Blue Islands Jetstream pilot. The description of a "yellow/beige" cigar might be less problematical in terms of reflectivity than the"brilliant sparkling yellow" of the Trislander report. But on this hypothesis there is no natural physical relation between the sightings. The coincidence of two geographically very remote sunlit contrails producing a similar visual effect at reciprocal azimuths at the same time is not very attractive.

In summary, aircraft contrails are a rather poor explanation.

Plausibility (0-5): 1