Contrail

The combination of water vapor in aircraft engine exhaust and the low ambient temperatures at high altitudes causes the trails' formation.

[3] Depending on the temperature and humidity at the altitude where the contrails form, they may be visible for only a few seconds or minutes, or may persist for hours and spread to be several kilometres/miles wide, eventually resembling natural cirrus or altocumulus clouds.

[7] A 2013–2014 study jointly supported by NASA, the German aerospace center DLR, and Canada's National Research Council NRC, determined that biofuels could reduce contrail generation.

This reduction was explained by demonstrating that biofuels produce fewer soot particles, which are the nuclei around which the ice crystals form.

In these samples, the contrail-producing soot particle count was reduced by 50 to 70 percent, using a 50% blend of conventional Jet A1 fuel and HEFA (hydroprocessed esters and fatty acids) biofuel produced from camelina.

The reduction in pressure and temperature across each vortex can cause water to condense and make the cores of the wingtip vortices visible; this effect is more common on humid days.

At high-thrust settings the fan blades at the intake of a turbofan engine reach transonic speeds, causing a sudden drop in air pressure.

[11] In firearms, a vapor trail is sometimes observed when firing under rare conditions, due to condensation induced by changes in air pressure around the bullet.

[16] In 2009, its 2005 value was estimated at 12 mW/m2, based on the reanalysis data, climate models, and radiative transfer codes; with an uncertainty range of 5 to 26 mW/m2, and with a low level of scientific understanding.

[17] Contrail cirrus may be air traffic's largest radiative forcing component, larger than all CO2 accumulated from aviation, and could triple from a 2006 baseline to 160–180 mW/m2 by 2050 without intervention.

[22] Starting from the 1990s, it was suggested that contrails during daytime have a strong cooling effect, and when combined with the warming from night-time flights, this would lead to a substantial diurnal temperature variation (the difference in the day's highs and lows at a fixed station).

"[29] In 2011, a study of British meteorological records taken during World War II identified one event where the temperature was 0.8 °C (1.4 °F) higher than the day's average near airbases used by USAAF strategic bombers after they flew in a formation.

[33][34][35] An EU project launched in 2020 aims to assess the feasibility of minimising contrail effects by the operational choices in making flight plans.

The plane's warm engine exhaust and enhanced vertical mixing in the aircraft's wake can cause existing cloud droplets to evaporate.