Some good research references for the future . . .
"Abstract. Condensation trails (contrails) are aircraft induced cirrus clouds, which may persist and grow to large cirrus cover in ice-supersaturated air, and may cause a warming of the atmosphere. This paper describes the formation, occurrence, properties and climatic effects of contrails. The global cover by lined-shaped contrails and the radiative impact of line-shaped contrails is smaller than as- sessed in an international assessment in 1999. Contrails trigger contrail cirrus with far larger coverage than observed for line-shaped contrails, but still unknown radiative properties. Some model simula- tions indicate an impact of particles and particle precursors emitted from aircraft engines on cirrus clouds properties. However, the magnitude of this effect cannot yet be assessed. Contrail formation can be avoided only by flying in sufficiently warm and dry air. The formation of contrail cirrus can be reduced by avoiding flights in ice-supersaturated regions of the atmosphere, e.g. by raising the flight level into the lower-most stratosphere."
[link to aero-net.info
"The introduction of more effi- cient aircraft engines does not solve the problem of contrail formation; just the opposite is true, be- cause the threshold temperature increases with increased overall efficiency η of the engines . An increased efficiency η is desirable of course for economic and climate reasons by reducing the fuel consumption and reducing emissions of carbon dioxide (CO2)."
"Contrail-cirrus could generally be avoided if flying slightly higher in the extratropical lowermost stratosphere. Flying higher generally would cause more contrails at lower latitudes and in the tropics [57, 129]. However, for long range traffic in the extratropics, flying a little higher than today, in the lower-most stratosphere just above the tropopause, may be recommendable both under climate and aviation aspects. This option should be considered if contrail cirrus dominates the climate impact by aviation and if the side-effects such as ozone changes are sufficiently small. The stratosphere is usu- ally dry so that formation of contrail cirrus in this region is avoided. Aviation can be performed effi- ciently at such altitudes with the potential for reduced fuel consumption and nitrogen oxides emission. The mid-latitude lower-most stratosphere appears to be only weakly sensitive to ozone changes from nitrogen oxides emitted by aircraft [3, 130]. Some authors point to the potential of ozone loss by chlo- rine activation at contrail and cirrus ice particles in the lowermost stratosphere by heterogeneous chemistry [4, 6, 133, 134]. However, high humidity in the lower-most stratosphere results from trans- port of humid tropospheric air masses into the lower stratosphere  containing low amounts of inorganic chlorine . Three-dimensional model studies suggest ozone to be reduced by not more than a few percent from heterogeneous reactions on subvisible cirrus from all sources in the tropo- pause region and this decrease may be of comparable magnitude as ozone formation by nitrogen ox- ides emissions from commercial air traffic . Since aviation causes only a small fraction of all cirrus, the contrail cirrus impact on ozone may also be small."
[link to www.maht0x0r.net
Box 3B Transport times in the Northern Hemisphere
A schematic showing a latitudinal profile of the tropopause in the Northern Hemisphere and giving transport time- scales. The tropopause is typically near 9 km in altitude in polar latitudes and 16 km in equatorial regions. On any particular day at any particular longitude its position could be as shown, with a structure determined by the weather systems. The likely position of ‘jet streams’, westerly wind maxima, is indicated. The range of cruise heights for subsonic and supersonic aircraft are shown. Arrows indicate typical directions for transport of gases and time-scales for these transports are given.
When the moist, high temperature air from a jet engine mixes with the ambient cold air, saturation can occur and the moisture can condense onto particles in the atmosphere, and in particular those present in the exhaust. The result is a condensation trail, or contrail.
On about 10 - 15% of occasions in the upper troposphere in middle latitudes, the ambient air is already supersaturated with respect to ice. In supersaturated conditions contrails will persist and tend to spread. Sometimes they can spread to form or initiate a cirrus cloud, though the amount of such cirrus cloud formed by aviation is currently unknown. The extent of the contrail and cirrus cloud can be striking, those shown in the lower left corner of the plate to the left persist out over the Atlantic.
Contrails and cirrus clouds reflect some solar radiation and therefore act to cool the surface. They also absorb some upwelling thermal radiation, re-emitting it both downwards,
which acts to warm the surface, and upwards. On average the latter warming effect is thought to dominate.