Ship tracks

Ship tracks (otherwise known as ship trails or ship plumes) are analogous to jet contrails, cloud trails that form at low level, usually a few hundred or a thousand metres altitude, in the wake of ships. They are caused by particulates from ship exhaust stacks rising into clean air which is nearly saturated (or even supersaturated) with water vapour. The particulates, such as sulphur dioxide, act as condensation nuclei and encourage the formation of droplets. The droplets so caused are smaller and more numerous than those condensing spontaneously in natural cloud, so the total droplet surface area per unit volume is higher than for natural cloud, meaning that the albedo is higher and the trails reflect sunlight more efficiently than natural cloud.

Ship tracks often occur embedded in a layer of natural altocumulus (see Fig.32 ) but sometimes occur in isolation in a relatively clear sky. They generally plume many tens of km downwind, spreading to between 0.5-5 km in breadth, and persist for days because the droplets are too small to rain out quickly, clearly visible at optical wavelengths as bright "contrails" on satellite photographs. (They are especially bright in infrared, but they are usually too deep in the atmosphere for effective detection by satellite IR.)

Some of the points raised in relation to aerodynamic fog contrails are also applicable to ship tracks. But ship tracks do not dissipate rapidly like fog contrails, and we do at least have radar and other evidence (Section 4) that at least two ships probably of moderate size, one a Channel ferry, were in the approximate area of the UAPs at the sighting time.

Fig.32 Ship tracks off Brittany, France. Photographed at 500m resolution, January 2003 s1Aqua MODIS, NASA
Fig.32 Ship tracks off Brittany, France. Photographed at 500m resolution, January 2003 s1Aqua MODIS, NASA

We can imagine that long ship tracks from two ships heading broadly N-S and S-N were oriented at an oblique angle to the LOS from both observer locations and thus foreshortened. Also a fortuitous cloud hole in dissolving altocumulus at 10-12,000ft (Section 5) could conceivably allow just portions of these side-by-side trails to be illuminated. Moreover it would not be impossible for (say) a single jet contrail or cloud streak at higher altitude, oriented E-W, to cast a shadow across both illuminated trails in a similar way, giving rise to two bright cigar-shaped UAPs each bisected by a dark band.

During the course of the sighting the trails could retain their relative positions and orientations under winds of only a few knots, spreading slightly and drifting slightly NNE towards the approaching Trislander. Thus the growth in apparent size and reduction of the subtended bearing angle would be a complicated function of these various movements plus the fact that the seedinghead of the nearer northbound trail would be shifting to the right whilst that of the farther southbound trail would be shifting to the left. The LOS from the Jetstream to the eastern trail at 1414 would be around 340° (nearer 7 o'clock than the reported 8, but not too far off) with the sun angle meaning that the Jetstream pilot did not have the benefit of forward scattered sunlight or possible transmissive effects that might have enhanced the brilliance for the Trislander witnesses in the N but instead saw a relatively dull reflection that was "yellow/beige".

The yellow and/or yellow/beige colouration mentioned by all observers (even "orange" in two instances) has no obvious interpretation. Ship trails would normally appear white under high angle sunlight (45° elevation rules out horizon reddening). Some reddening due to scattering extinction by the haze layer reported at ~2000ft is possible, and there is some witness evidence that the colouration deepened during the Trislander's descent towards the layer, which would be consistent with a longer optical path length through a scattering medium.

But there are several problems with this interesting theory.

The least serious is that there is no sign of ship trails miles long on satellite images (either in visual or IR). Granted the resolution is poor at 1415 but both suspect ships (among others) should already have been underway in the area during our high resolution MODIS look at 1328. Given that the trails are to be brilliantly illuminated by direct sunlight (ex hypothesi), and so should be at least partially unobstructed by higher cloud, some glimpse might have been hoped for.

The stability of the illusion over many minutes of binocular observation is also a difficulty, despite what was said above. And one wonders why such prominent and unusual trails were not visible from the ground and the air by observers not in a position to be deceived by the illusion. It was reported to us second-hand that an unidentified pilot said that the weather that day was "unusual" in some unspecified way, but if the pilot in question had seen monster ship trails we think that we might have heard about it. In any case this is mere hearsay, whereas we know from the ATC audio record that the BAe 146 asked to look down from a few thousand feet above the area reported nothing unusual that might account for the sightings.

A big problem with the radar-plotted positions of the suspected ships (Section 4) is that the initial LOS to the western ship not only makes a somewhat large angle with the LOS to the eastern ship (we could cope with a factor 1.5 visual error) but it is also well to the W of Casquets Lighthouse, by something like 5°, and ex hypothesi this south-moving ship is at the head of a trail cloud pluming ~N from this position (surface winds being roughly SSW) so any sunlit section of trail would lie somewhere still further to the W of this LOS. Yet Capt Bowyer stated that his LOSs to both objects lay to the left (E) of the Casquets Light which was visible at the time (Section 3). At no point on the Trislander's track would the western ship position have been seen to the left of Casquets and any ship-trail cloud UAP would have to be significantly to the right of Casquets at all times. And especially telling is that these LOSs to the ship trails could not possibly have rotated (due to parallax) so as to cross one another, as the bearings to the UAPs explicitly did.

In addition to these objections, the Jetstream pilot would have to have flown right between or possibly over both of these hypothetical ship tracks en route past Guernsey and would potentially have had many minutes of good views at changing angles of sight and angles of illumination. It makes little sense that only a couple of minutes later he would look back at one of them and fail to recognise what he was seeing, given that the illumination conditions and visual appearance of the cloud (seeming unchanging during binocular observation at the time) should have remained rather constant. Moreover, it is hard to see how trail clouds associated with either ship could be the cause of an object which the Jetstream pilot located "NW of Alderney".

Finally, prominent ship track clouds at ~2000ft the Guernsey-Alderney area might be reported by observers at sea or on land. We made inquiries via the Guernsey Harbour Authority, who were aware of no reports of unusual weather phenomena on that day n1 Letter from AJB Pattimore, Deputy Harbourmaster, Guernsey Harbour Authority, to Paul Fuller, 14 Aug 2007.. Such clouds would constitute striking low level altocumulus bands that might be recorded in routine local weather observations. Alderney and Guernsey airport half-hourly meteorological records (Appendix C) show no low-level cloud at all observed between 1150Z and 1550Z, with only remnants of dissipating 10-12,000ft altocumulus (1/8 cover or less) at the sighting time.

Plausibility (0-5): 1