How condensation trails are formed
Aircraft engines release various emissions, including water vapor, carbon dioxide (CO2), traces of nitrogen oxides (NOx), hydrocarbons, carbon monoxide, sulfur compounds, as well as soot and metallic 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 the atmospheric humidity is sufficient for the mixture to reach saturation, condensation of the water vapor will occur. The degree of humidity in the mixture, which determines whether saturation is reached, is influenced by the temperature and humidity of the surrounding air , as well as 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.
Non-extending contrails are long, white streaks that remain in the atmosphere after an aircraft has passed through the area, without changing in size . These phenomena occur when atmospheric humidity levels are high, preventing the contrail from dissipating, and can last for several hours.
Persistent and extending contrails appear as lines that increase in thickness, width, and irregularity as the cloud expands. This phenomenon occurs when atmospheric humidity approaches the condensation threshold, facilitating the condensation of water vapor onto ice particles present in the contrail. Furthermore, atmospheric instability and turbulence cause contrails to take on an irregular shape. These contrails can also be influenced by wind movement.
Is it possible to make predictions?
The first mentions of contrails date back to the end of World War I, a period when aircraft began operating at altitudes conducive to their formation. Before the start of World War II, 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?
Besides their strategic importance to military aviation, persistent contrails have become increasingly relevant in the field of forecasting, especially with regard to long-range climate predictions. A scientific study conducted in 1998 estimated that cloud cover produced by contrails from human-made aircraft represented 0,1% of the Earth's surface , not including cirrus clouds that develop from the longest-lasting contrails. Furthermore, projections indicate that the expansion of air traffic, along with advances in engine technology, will likely increase this percentage.
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, accessible here, indicates that research has identified a correlation between the increased frequency of contrails and aircraft emissions and the increased coverage of cirrus clouds. Cirrus clouds typically cover approximately 30% of the Earth's surface . Increased cirrus cloud cover is likely to contribute to a rise in global surface temperatures. Consequently, the potential influence of contrails on rising global temperatures is a cause for concern.
Conversely, this report also estimates that emissions from aircraft engines contribute to 3,5% of the total impact of human activities on climate change. For more details on how contrails form on airplanes, you can consult our dedicated section on how contrails form on airplanes.