Many years before this event  and a long time back in aircraft evolution the first aircraft to have reached height's in excess of 20,000ft or so may have first noticed a strange trail behind them (contrails are rare below 20,000ft  or 460hPa). As things turned out, pilots were very much aware of this very exposing give away to an aircraft's location during the war years, it seems then that meteorologists and scientists became very interested in the higher atmospheric conditions that occurred to result in the development of contrails, a lot of research has been carried out over the past sixty years...... So what is a contrail?, incidentally there is another atmospheric phenomena the opposite to a contrail... a distrail!.. more about them a little later.

The best description I have seen about contrails  is discussion presented by Richard Scorer in his book Clouds of the World 1972.....Now just over thirty years on I still frequently study his descriptions on just about anything associated with clouds and this book just does not seem to age. Condensation trail are initially composed of water droplets formed by the motion produced by aircraft or by the condensation of water vapour in the aircraft exhaust. This effect, caused by aircraft, frequently freezes into a line often visible from the ground. There are a lot of surprisingly complex interactions required to form and maintain a contrail. When a contrail first forms it is composed of water and exhaust gasses, below about -40°C the contrail freezes almost immediately (about one second after entering the free atmosphere), then the conditions encountered will dictate the persistence or the disappearance of the contrail.

For the observer on the ground this man-made modification to the upper atmosphere is a very useful weather tool as it can tell the observer about conditions in the upper atmosphere above 30,000ft (300hPa). The basic information is the condition of the moisture loading and the direction and speed of the wind at those levels. If the contrail persists and even enlarges when conditions are positive for the development of ice crystals, occasionally I have seen contrails develop into large cirrus sheets especially when the ice particles are spread out by wind shear. On other occasions, a contrail may only be visible in an already existing patch of cirrus, this indicates a moist patch at that level - as the aircraft passes into clear air the contrail will not persist and evaporates where the air there is drier and not suitable for the development of ice crystals. Enhanced cloud development within the contrail can also be linked to the copious amounts of microscopic nuclei available from the aircraft exhaust, vorticity within the exhaust and to the rear of the plane is another benefiting contribution. The vortex core is an area of lower pressure and this aids in production of ice crystals under favourable conditions..

So what about Distrails?, next time you look at the sky for contrails look for other oddities associated with the passage of high flying aircraft, a distrail is a clearing of cloud particles by the passage of the airliner through the high level cloud deck, this may be caused by heating and evaporation or by drier air from above the aircraft being ingested into the vorticity area to the rear of the aircraft. On other occasions a clear dissecting line may appear with faint ice crystal fallout appearing below the original cloud base (more common in high based alto cumulus in a thin moist layer). At localities of high latitude airports such as Fairbanks Alaska, temperatures occasionally fall below -40c, on these occasion aircraft exhaust can form trailing lines of ice crystals at ground level, I would like to see that!.... At other times a keen observer may be able to see contrail shadow on a lower deck of clouds often appearing as a thin dark line, I have seen such phenomena mainly associated with thin stratocumulus as the high level contrail extends its shadow from high above onto the lower cloud layer.

When satellites first started to photograph the Earths cloud systems a few odd lines were found in thin stratocumulus over oceans, careful study revealed them to be 'ship trails', at some locations over the larger oceans the air is very clean near to the surface, and under the favourable conditions especially near to anticyclones the exhaust from the ships engines would rise up into a thin stratocumulus layer, the addition of flue particles enhanced the condensation process and thickened the stratocumulus layer in a narrow line behind the ship, sometimes ship trails can be many hundreds of kilometres long!.... So next time your out in the garden, keep an eye skyward for the telltale appearance of white lines across the sky watch them for awhile and be amazed at what you can deduce about the conditions of the upper atmosphere..... regards Clyve Herbert.  and happy skywatching.....

Acknowledgments....Richard Scorer: Clouds of the World ..  A Complete Colour Encyclopaedia 1972

and the Sky..............

Clyve Herbert 2002

Contrails over Melbourne sketch: Clyve Herbert
Images: C Herbert (ASWA Archive), J ONeill (ASWA  Archive), Robert Goler