George Wallace said:
OK
You are seriously turning into a "Conspiracy Theorist". Those vapour trails are natural. You will see them behind any aircraft under the correct weather conditions. There is a very scientific explanation for this occurance; one that I will leave to an expert........................but rest assured, IT IS NOT CHEMICAL SPRAYING!
Ok, I'll give the explanation a go.
Every engine that burns hydro carbons (ie fossil fuels) creates a multitude of byproducts, two major by products are Carbon Dioxide (CO2) and water. The water is in gas form due to the heat of the combustion.
The temperature drops with altitude at what's called the adiabatic lapse rate, there are two lapse rates, dry and saturated. The dry adiabatic lapse rate is 3 degrees Celsius per 1000 ft, once the air reaches saturation (the point at which clouds form), the rate drops to 1.5 degrees Celsius per 1000 ft. The average lapse rate is 2 degrees per 1000 ft. So if it's 15 degrees on the surface, it will be 12 degrees at 1000 ft, 9 degrees at 2000 ft and so on until the temperature/dew point spread reaches zero and the air becomes saturated, but using the average lapse rate, we can deduce that if the surface temp is 25 degrees, then at 25,000 ft, the temp will be approx 50 degrees cooler, or -25 degrees.
Since cold air cannot hold as much moisture as warm air, the water vapour from the exhaust condenses and forms clouds and condensation trails (contrails). Another phenomenon that happens is that the exhaust provides condensation nuclei (ie, a solid for the water vapour to condense on), so with the combo of cold air, high water vapour content and condensation nuclei, you will see contrails.
As for the Herc's black tails, as was explained earlier, turbine engines are not the most efficient and they often leave a black sooty exhaust hanging in the air, not unlike the black exhaust you see from diesel engined trucks.
