When the weather is clear, why do planes leave traces after flying in the sky?

When a jet plane flies high in the clear sky, people often see one or several long "white smoke" behind the fuselage. In fact, this is not the smoke from a jet plane, but a special cloud system formed by the condensation of water vapor after the exhaust gas from the plane is mixed with the surrounding air. In aviation flight and aviation meteorology, it is called aircraft wake, which is also commonly known as "aircraft smoke". So, how did it come into being?

According to the formation reasons, aircraft wake can be divided into exhaust wake, aerodynamic wake and convection wake. Exhaust wake can be divided into exhaust condensation wake and exhaust evaporation wake. The most common one is the tail of tail gas condensation, which is also the tail of aircraft condensation discussed in this paper.

The aircraft consumes a lot of fuel when flying, and the generated water vapor and some heat are discharged from the aircraft with the exhaust gas, enter the atmosphere, and quickly mix with the surrounding air to form a condensation trail. Its formation process is different from dew on the ground, frost and clouds in the air. The exhaust gas from high-altitude aircraft is mixed with ambient air, and the saturation of this mixed gas depends on the net effect of heat and water vapor increment. When the humidification effect is dominant and exceeds the given critical value, the condensation trail will be formed; When the thermal effect is dominant, condensation will not occur and there will be no condensation traces. Because the heating and humidification of waste gas is certain, whether condensation will occur in this mixed gas depends on the temperature, humidity and atmospheric pressure of the ambient air itself. In short, when the ambient air temperature is high, it is not conducive to the formation of condensation trails. Only when the ambient temperature is quite low (usually below -40℃) can the condensation wake of the aircraft appear. According to relevant data, at present, the temperature of aircraft tail outflow is mostly -4 1℃ ~-60℃, accounting for about 86% of aircraft wake. If the temperature is higher than -40℃ or lower than -60℃, the aircraft wake rarely appears.

The seasonal variation of aircraft wake height is not obvious. Generally speaking, winter appears more often than summer. The average thickness of wake layer is 1 ~ 2km, and the lower limit height is the lowest in winter and the highest in summer. In the thick tail flow of aircraft condensation, the length and concentration of wake formed at different heights are also different. Usually, the tail with short length and light concentration flows out at its bottom, gradually becomes longer and thicker upward, and becomes an intermittent light-colored wake when it reaches a certain height.

Aircraft wake has important military value and has been paid attention to by military strategists all over the world since modern times. Jet fighters are most likely to produce aircraft wakes when flying at appropriate altitudes. In aviation operations, the appearance of aircraft wake can easily expose the flight trajectory and position of aircraft. In order to avoid exposing the target, it is necessary to find out the altitude layer where the aircraft wake appears and choose the flight altitude far away from the aircraft wake, so as to achieve the purpose of hiding itself.