This paper defines the meteorological state of the atmosphere which will give rise to the formation of condensation trails (contrails) as the exhaust from an aircraft engine mixes with and saturates the environment. Three basic assumptions were made with regard to the formation of visible contrails: (1) contrails are composed of ice crystals; (2) water vapor cannot be transformed into ice without first passing through the liquid phase, thus necessitating an intermediate state of saturation with respect to water; (3) a minimum visible water content of 0.004 gm/m3 is required for a faint trail and 0.01 gm/m3 for a distinct trail. This last requirement proved of no importance in determining whether or not a trail would form, but did affect its persistence.
Curves were constructed showing the critical temperature for the formation of a visible trail as a function of the pressure and relative humidity of the environment and the amount of air entrained into the exhaust. It is shown that these curves are applicable to any aircraft which has the same water to heat ratio in its exhaust as the case discussed in this report. In general this ratio is fairly constant regardless of the type of airplane, control settings, or fuel. The major exception occurs with aircraft powered by reciprocating engines in which case a considerable portion of the heat produced may be dissipated outside of the trail. A separate, but similar, study would be necessary for each aircraft with a significantly different proportion of such heat loss.
