How condensation trails are formed
Aircraft engines release various emissions, including water vapour, carbon dioxide (CO2), traces of nitrogen oxides (NOx), hydrocarbons, carbon monoxide, sulphur compounds, as well as soot and metal particles. Among these emissions, water vapor is the only significant component for contrail formation.
The formation of extensive contrails trailing behind aircraft during flight requires specific temperature and humidity conditions that allow the condensation of water vapor released by the engines. While sulfur gases can assist in this process by promoting the creation of small particles that function as condensation nuclei, it is generally the case that there are enough particles in the atmosphere to fulfill this function. Other gases and particles emitted by aircraft engines do not affect the development of condensation contrails.
As the gases released by the aircraft interact with the ambient air, they undergo a rapid cooling process. If atmospheric humidity is adequate for the mixture to reach saturation, condensation of water vapor will occur. The degree of humidity in the mixture, which determines whether saturation is reached, is determined by the amount of air in the mixture. influenced by the temperature and humidity of the surrounding air, in addition to the amount of water vapor and the temperature of the aircraft's emissions.
Categories of condensation trails
The evolution of a contrail, once formed, is influenced by atmospheric conditions. Consequently, the three types of contrails shown in the poster can be observed.
Transient contrails are the brief white streaks visible behind an aircraft, which disappear almost as quickly as the aircraft itself moves. These formations arise under conditions where atmospheric water vapor is minimal, causing the ice particles that make up the contrail to quickly revert to their gaseous form.
The persistent condensation trails that They do not extend are elongated white streaks that remain in the atmosphere after an aircraft has passed through the area, without changing their size.These phenomena arise when atmospheric humidity levels are high, which prevents the condensation trail from dissipating, and can last for several hours.
Condensation contrails that are persistent and extend manifest as lines that increase in thickness, width and irregularity as the cloud expands. This phenomenon arises when atmospheric humidity approaches the condensation threshold, which facilitates the condensation of water vapor on the ice particles present in the condensation contrail. In addition, If there is instability and turbulence in the atmosphere, the condensation trails assume an irregular configuration. These condensation trails can be influenced by the movement of the wind.
Is it possible to make predictions?
The first mentions of condensation trails date back to the conclusion of the First World War, a period when aircraft began to operate at altitudes conducive to their formation. Before the start of the Second World War, these phenomena were considered little more than a curiosity. However, During the conflict, contrails attracted significant attention due to their potential to reveal the presence of an aircraft.. Consequently, several nations initiated research focused on understanding the causes and conditions that lead to their formation.
Should we be concerned about the impact of aircraft contrails?
In addition to their strategic importance for military aviation, persistent contrails have become increasingly relevant in the field of forecasting, especially with regard to long-term climate predictions. A scientific study conducted in 1998 estimated that cloud cover produced by condensation trails Condensation trails from man-made aircraft accounted for 0,1% of the Earth's surface, not taking into account the cirrus clouds that develop from the longer-lasting contrails. Moreover, projections indicate that the expansion of air traffic, coupled with advances in engine technology, will likely drive this percentage higher.
If you're ever curious about the phenomenon of contrails that crisscross and sometimes seem to create a mesh pattern in the sky, take a look at ENAIRE's map of upper airspace airways and you'll see why.
A 1999 IPCC report on the atmospheric effects of aviation, which can be accessed here, indicates that research has identified a correlation between the increase in contrail frequency and aircraft emissions and the increase in cirrus cloud cover. Cirrus clouds typically cover about 30% of the Earth's surface.An increase in cirrus cloud cover is likely to contribute to an increase in global surface temperatures. The potential influence of contrails on the escalation of global temperatures is therefore a matter of concern.
On the contrary, this report also assesses that emissions from engines linked air traffic contributes 3,5% of the total impact of human activities on climate change. For more details on how contrails form on airplanes, you can check out our dedicated section on How condensation trails form on airplanes.