Contrails are line-type ice-crystal clouds that trail behind jet aircraft when hot, humid engine exhaust mixes with cold ambient air of low vapor pressure and low temperature. The cloud forms because the water vapor in the exhaust condenses immediately, acquiring both natural aerosol and engine-emitted particles as freezing nuclei, generating a visible atmospheric cloud. In the right tropopause conditions, contrails are a normal effect of jet aircraft operations.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the exhaust from an airplane called?
The exhaust from an airplane is called contrails, which are line-type clouds. Contrails form at cruising altitudes from the condensation of airplane engine exhaust water vapor onto naturally occurring particles in the atmosphere or onto particles emitted from the engine.
The exhaust from an airplane is called a contrail, short for ‘condensation trail’, and sometimes a distrail, short for ‘dissipation trail’. Each jet engine has a single exhaust but if an airplane has four main engines, it will likely have four exhausts. The hot, humid exhaust from jet engines mixes with the atmosphere, the water vapor condenses and freezes, forming ice crystals. Scientists divide these trails into three types: short-lived, persistent non-spreading, and persistent spreading. Contrails linger several kilometres above the Earth's surface and evolve into contrail cirrus clouds. A distrail appears when existing cloud droplets evaporate, yielding a cloud-free corridor.
Why do planes leave exhaust trails?
Planes leave exhaust trails because aircraft typically fly at high altitudes where it is very cold - around -55°C (-67°F) at 10,000 m (32,808 ft) - so the warm, moist exhaust fumes mix with cold ambient air. This mixing produces ice-crystal clouds that are observed as white trails. Water vapour produced by the combustion of fuel in aircraft engines condenses and freezes, while impurities in fuel like soot act as a platform on which ice crystals form. Very hot exhaust reacts with the very cold air, and the larger temperature gradient results in rapid cooling of water vapour into visible ice crystals.
Where does the exhaust go? The newly formed ice particles quickly evaporate as exhaust gases and are completely mixed into the surrounding atmosphere. If the humidity is low, the contrail will be short-lived but if the air is relatively humid, contrails will grow and remain visible long after the aircraft has disappeared, sometimes persisting for hours to days under extremely humid, high-pressure, super-saturated conditions.
What is airplane exhaust made of?
Aircraft engine exhaust is composed of water vapor, carbon dioxide, and other emissions. The gases released include carbon dioxide, water vapour, carbon monoxide, unburned hydrocarbons, sulphur oxides, and nitrogen oxides. Engine exhaust includes unburned fuel, soot, metal particles, nitrogen oxides, and sulfur oxides. Trace elements consist of various metals and compounds that contribute to the overall composition. Aircraft exhaust is composed of particulate matter that comprises microscopic solid and liquid particles. Soot provides particles that serve as cloud condensation nuclei. Particulate matter consists of unburned fuel residues and metal particles emitted during combustion.
The high pressure turbine operates at very high temperatures with the combustion chamber, where fuel combustion occurs. High temperatures, moisture, and vibration create conditions for various chemical reactions and emissions formation. Water vapor condenses and freezes in certain atmospheric conditions, contributing to visible contrail formation.
Engine exhaust is predominantly made up of carbon dioxide and water vapor, but also contains amounts of other compounds. Aircraft exhaust is composed of gases including sulfur oxides and hydrocarbons. The composition varies based on engine type, fuel quality, and operating conditions, with modern engines designed to minimize certain harmful emissions.
What materials are used in the construction of aircraft exhaust systems?
Aircraft exhaust systems are built from materials that can survive gas streams up to 1,000°C (1,832°F) for thousands of flight hours. Stainless steel and Inconel dominate the design: stainless steel grades like 321 or duplex stainless provide strength, fatigue resistance, and corrosion protection, while Inconel - typically 80% nickel, 14% chromium, trace iron and other metals - retains both strength and corrosion resistance above 650°C (1202°F). These nickel-chromium superalloys are used for the hottest sections: exhaust nozzles, tail-pipes, collector rings, and engine mount flanges. Titanium alloys, corrosion-resistant and half the density of steel, appear in modern lightweight nozzles. In wartime, Staybrite exhaust collector rings were fitted to many piston-engine fighters as their very high tensile-strength-to-density ratio allowed thin-walled construction that keeps mass low and weldability high.
Carbon steel flanges were once welded to Inconel bodies, but field experience - the badly corroded carbon steel flange on a 1943 RAAF Mosquito stub - led designers to abandon the practice. Vintage systems therefore contain steel cast-iron or carbon-steel manifolds and risers, though most have now been replaced with stainless or Inconel parts. Ceramic matrix composites, an emerging option, withstand even higher temperatures and permit lighter, thinner sections, yet they remain confined to developmental or military engines for now. Throughout, all metals are kept weldable so that cracks can be repaired on the wing. Thin-walled tubing is employed to minimise weight, and the manifold is designed to guarantee an efficient, low-loss flow of combined exhaust gases toward the tail pipe.
We set nickel-based super alloys like Income for the exhaust spout and parts of the propulsion reverser descents. These substances withstand combustion under heat. A sample of the metal was exposed to periodic temperature change. I observed how it extended and constricted without developing micro-fractures.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What are the impacts of aircraft exhaust emissions?
Aircraft exhaust emissions contribute to climate change through the release of greenhouse gases like carbon dioxide, which trap heat in the Earth's atmosphere and accelerate global warming. Aviation accounts for around 2.5 percent of global CO2 emissions, yet the overall contribution of aviation to climate change is higher than this share alone because non-CO2 effects constitute more than half of the sector's net climate forcing. These non-CO2 impacts include the formation of persistent contrail cirrus clouds that retain Earth's heat, and the release of nitrogen oxides that increase ozone concentrations by about 6 percent in the region 30-60° N latitude and 9-13 km (29,527.56-42,650.92 ft) altitude. Ozone is a potent greenhouse gas, and its formation near the ground degrades air quality.
At cruise altitudes, aircraft emissions increase water vapor, soot, sulfur aerosols, and ultrafine particles smaller than 20 nanometers. These particles have longer atmospheric residence times and lower background concentrations than surface emissions, amplifying their radiative forcing effect. Soot particles contain polycyclic aromatic hydrocarbons and metals, while sulfur oxides lead to the formation of sulfate aerosols. Nitrogen oxides further form nitric acid at cruise altitudes, which produces ammonium nitrate particles in the boundary layer when ammonia is present, affecting surface air quality.
Near airports, approximately 10 percent of aircraft pollutant emissions are emitted close to the surface during taxi, takeoff, initial climb, and approach and landing. These ground and low-altitude operations add nitrogen oxides, sulfur oxides, hydrocarbons, and soot particulates to local air, degrading air quality particularly around densely populated areas. The release of particulate matter and trace elements induces pulmonary and systemic inflammation, contributing to respiratory disease, asthma, coronary heart disease, cancer, and an estimated 58,000 premature mortalities per year. Ground-idle emissions are potent: particulate matter from conventional fuel at ground-idle conditions is most potent for health impacts, increasing vulnerability of airway epithelia to secondary pollutants and pathogens, and disproportionately affecting children under 5, elderly above 65, and lower socioeconomic groups.
Aviation generates secondary particulate matter that is transported continent to continent, and its emissions affect the Earth's energy and water budgets by altering clouds and precipitation. Sustainable aviation fuels, like a 50:50 blend of conventional and synthetic jet fuel, reduce PM number and mass emissions by 50 to 70 percent, offering a pathway to mitigate both climate and health impacts.
