Contrails, short for condensation trails, are man-made clouds appearing as thin, white streaks behind an aircraft. They are line-shaped formations composed primarily of ice crystals, originating from the exhaust of jet engines at high altitudes. The visible trail marks the intersection of the aircraft’s hot engine emissions with the surrounding cold atmosphere. Understanding their formation requires looking closely at the atmospheric conditions found at typical cruising altitudes. Altitude is not the only factor; a precise balance of temperature and moisture is necessary for the trail to appear and linger.
The Physics of Contrail Formation
The formation of a contrail is a rapid thermodynamic process beginning with the jet engine’s exhaust. Burning jet fuel produces hot water vapor and tiny solid particles (soot and sulfate aerosols) that serve as condensation nuclei. When the exhaust plume is ejected, it mixes instantaneously with the extremely cold ambient air, causing the water vapor to cool rapidly. This cooling raises the localized relative humidity past the saturation point, leading to immediate condensation onto the nuclei.
The thermodynamic threshold is outlined by the Schmidt-Appleman criterion. For a contrail to form, the ambient temperature must be colder than approximately -36.5°C (-34°F). The condensed water droplets subsequently freeze into ice crystals due to the low temperature. If the temperature is too warm, the droplets will not freeze, and no visible trail will appear.
The Typical Altitude Range
Contrails are most commonly observed at the cruising altitudes of commercial jet traffic, where the atmosphere is consistently cold enough for trails to form. This range typically begins above 25,000 feet (7,600 meters), with the majority of formation occurring between 28,000 and 42,000 feet (8,500 to 12,800 meters).
This altitude band corresponds to the upper troposphere and lower stratosphere, where temperatures are routinely below the required -40°C (-40°F) threshold. Aircraft fly here for better fuel efficiency, and it is the coldest part of the atmosphere before temperatures warm again higher up. Below this level, the air is usually too warm for contrail formation.
The lowest altitude for contrail formation is variable, dictated by the temperature requirement. Not all flights at cruising altitude produce them, demonstrating dependency on atmospheric conditions. The presence of a contrail indicates the aircraft is flying through air colder than the formation threshold.
Atmospheric Conditions for Persistent Contrails
The initial formation of a contrail does not guarantee its longevity; the atmosphere must meet a second, more stringent condition for the trail to persist. Many trails are short-lived, dissipating within minutes because the ambient air is too dry, causing the ice crystals to sublimate quickly.
For a contrail to become persistent (lasting for hours), the air must be ice-supersaturated, meaning the relative humidity with respect to ice (RHi) is greater than 100%. When the air is supersaturated, the ice crystals are stable, attracting more water vapor, growing larger, and spreading out due to wind shear.
These persistent contrails can eventually spread out to become indistinguishable from natural high-altitude cirrus clouds, known as contrail cirrus. The narrow bands where ice supersaturation occurs are called ice supersaturated regions (ISSRs). Since these regions are often only a few thousand feet thick, a small change in altitude can determine whether a trail is short-lived or persistent.
Contrails and Climate Effects
The persistence of these man-made clouds is significant because of their effect on the Earth’s energy balance, known as radiative forcing. Contrail cirrus clouds behave like natural cirrus clouds, acting as a blanket in the upper atmosphere with a dual effect dependent on the time of day.
During the day, contrails reflect incoming solar radiation back into space, providing a slight cooling effect. However, both day and night, these ice clouds trap outgoing longwave radiation (heat radiating from the Earth’s surface) and re-radiate it downward, causing warming. Because the warming effect generally outweighs the cooling effect globally, the net impact of contrails is positive radiative forcing, contributing to planetary warming.
Research suggests the total warming contribution from contrail cirrus is a significant portion of aviation’s climate impact, comparable to the CO2 emissions from a year’s worth of flights. The impact is especially pronounced for contrails forming at night or in winter when the solar reflection effect is absent. Consequently, mitigation efforts focus on identifying and avoiding the ice supersaturated regions where persistent, warming contrails are most likely to form.