Many years ago a grizzled old captain (Ex R.A.F C130) explained to me how he could calculate,at preflight, at what Flight Levels the aircraft would be contrailing by using various met charts. This was important on certain missions to avoid visual detection from the ground.
Being somewhat old and grizzled myself now I have completely forgotten how this was done. Can anyone help me out?
According to my copy of AP3340, the Handbook of Aviation Meteorology, contrails are likely to be visible when condensed water or ice particles are present in sufficient concentration to be seen. This in turn depends upon illumination, background contrast and distance.
For the formation of a contrail, the air temperature needs to be below a critical value which varies almost linearly from about -24 degrees celsius at sea level to about -45 deg C at 50,000 feet.
The critical temperatures are indicated by a particular line drawn on Metform 2810, the tephigram. This line is marked: "MINTRA," and will apply to an aircraft flying at cruising speed in an atmosphere just saturated with respect to ice. The corresponding temperatures for saturation with respect to water are lower by about two or three degrees C.
Trails may form at any altitude provided that the temperature is suitable. At 1000 millibars the critical temperature is about -30 deg C, so trails can occur at ground level in high latitudes!! Over southern England the average altitude at which trails would be expected to form in the winter ranges from about 26,000 feet to 70,000 feet. In summer, when the stratosphere is warmer, the range is about 30,000 to 45,000 feet.
So on the tephigram, a MINTRA would be that temperature above which a trail will not form even if the ambient air is already saturated. And a DRYTRA would be a line joining temperatures at which trails must occur even if the ambient air is completely dry. Between these two limits, corresponding with saturation and complete dryness, trail formation may be expected to depend primarily on the relative humidity of the environment and, to some extent, engine thrust.
On the Kipper Fleet in the 1980s, each sortie saw a specific briefing to the whole crew by the duty meteorologist. Such luxury, eh?
No doubt, for you in Transport, it can't have been too much different. Anyway, a tephigram could, and probably would have been specifically prepared for the types of sortie to which you allude in your post.
I've made contrails at 50 feet agl during an low pass to inspect a landing site in a ski equipped Garrett TPE331 powered turboprop in Antarctica. I did not actually see them as I was flying but the FO commented on the shadows they were making behind the aircraft. Noone on the ground either to get a photo not because it was too cold, but as noone had ever been there before.
As Dai has posted, MINTRA is the key, and of relevance to aircraft in combat zones.
There is a picture SOMEWHERE of an engine run somewhere well 'up north' of a 4 piston ground engine run with 4 trails. I.E Mintra level = ground elevation.
takes you to one claiming that the United States Air Force produced contrail prediction charts. It says: "Background: Military planners have been interested in condensation trail (contrail) forecasts since World War II. Contrails can make any aircraft easy to locate by enemy forces, and no amount of modern stealth technology can hide an aircraft if it leaves a persistent contrail in its wake. In 1953, a scientist named H. Appleman published a chart that can be used to determine when a jet airplane would or would not produce a contrail. For many years, the US Air Force Global Weather Center used a similar chart to make contrail forecasts."
The page illustrates such charts and gets you to compile your very own contrail forecast. There's also a student version!!
Now, BOAC, your image must be out there somewhere...
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not because it was too cold, but as noone had ever been there before. 
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