A condensation trail (contrail) is a visible trail of small water droplets or ice crystals formed under certain conditions in the wake of an aircraft.

The formation of contrails is considered to decrease significantly the effectiveness of opera-tional aircraft under combat conditions. In daylight when clear weather prevails, the problem of detecting the enemy is practically solved. When the aircraft produces contrails at night, the contrails greatly simplify the enemy-interception problem.

To remove the detection hazard during combat missions, predicting the altitudes at which contrail formation is probable is necessary so that flight at those altitudes may be avoided. Such prediction is a function of the weather office that provides the briefing for the flight.

The instability contrail is produced by the passage of an aircraft through an otherwise undisturbed layer of unstable air with a higher relative humidity. Conditions conducive to such formations exist only rarely.

The most prevalent of contrails is the engine exhaust contrail, and it is with this type that you as an Aerographers Mate will be concerned, since it is the only one that must be forecast.

When an aircraft passes through the at-mosphere, the engine releases a certain quantity of water vapor and heat as a result of combustion processes in the engine. When the exhaust mixes with the air, the water vapor introduced tends to increase relative humidity and to bring the air closer to saturation, and the heat released tends to decrease relative humidity. The formation of a contrail depends, therefore, upon such factors as the amount of heat and water produced by fuel combustion, the wake or entrainment characteristics of the aircraft, and the original temperature and humidity condition of the air.

Entrained air is that which is drawn into and mixed with the exhaust gases of the aircraft. The amount of air entrained into the exhaust trail varies continuously from near zero immediately behind the aircraft to an extremely large amount far behind it. The rate at which air is entrained varies with the type of aircraft, power setting, speed, and the density and stability of the at-mosphere.

The ratio of entrained air to exhaust gas at a given distance behind the aircraft is greatest when the aircraft is operating at its most efficient speed and altitude.

If the temperature, pressure, and humidity are suitable, the water vapor of the exhaust may produce supersaturation with consequent contrail formation. The more humid the air at a given temperature and pressure, the greater the tendency for contrails to form.

NOTE: Only in the case of jet aircraft can a definite relationship be established between pressure, temperature, and relative humidity. In propeller-driven aircraft, energy losses are variable. Since all of the energy is not contributed to the wake, only an estimate of the limiting temperatures for contrail formation can be given.

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