Contrails in a Nutshell

The What, When, Where, and How of Contrails

Dec 17, 2024

Contrails have become a ‘hot’ topic in recent years, gaining recognition as a major contributor to aviation’s climate impact and a potential key to achieving the aviation industry’s target of achieving net-zero by the year 2050. As such, understanding and managing their impact on the climate is becoming a higher priority within sustainable aviation. To have a full picture of aviation and the environment, it is important to understand what contrails are, when and where they are created, and how they impact the climate.

What are Contrails?

Condensation trails, also known as contrails or their fancier moniker sublimation trails, are clouds created by jet aircraft engines under certain conditions, usually at cruising altitude in the upper atmosphere. Aircraft contrails form when water vapor and particles from aircraft engine exhaust condense to form ice. Research confirms that visible contrails form when exhaust plumes comprised of soot and other aerosol particles interact with the ambient conditions to form water droplets which freeze and expand rapidly with the uptake of water vapor.

Formation and radiative forcing of contrail cirrus | Nature Communications — Contrail Formation - Nature Communications - Kärcher 2018

Not all flights create contrails, however, due to the need for specific atmospheric conditions. Though each aircraft has the potential to form contrails, the air through which the flight cruises must be both cool and humid enough to mingle with the particles from the exhaust. If the air is too dry, the emitted water vapor will not condense around the particulate matter to form a contrail. That’s why most flights do not have those wispy tails trailing behind them – it’s also why there may be ten flights in a row that do have them.

When and Where are Contrails Created?

Contrails form when airplanes fly through particularly cold, humid regions in the sky, bearing no heed to any particular time of day or night. These areas of high humidity are known as Ice Super Saturated Regions (ISSRs). Generally, contrails are short-lived, fading into the ether as quickly as they are created. However, contrails created in ISSRs can persist and spread out, becoming persistent contrails. If enough of these persistent contrails are formed in a region, they can eventually merge and become contrail-induced cirrus, which act like natural cirrus clouds.

Contrails and Cirrus Clouds from Aviation - Carbon Offset Guide — Contrail Cirrus Clouds - Carbon Offset Guide

Due to the need for cold air for formation, contrails are more likely to form and persist during the winter months. A study measuring contrail climate effects from 2019 – 2021 found contrail formation occurs more frequently during the winter months and those formed have a longer mean lifetime than those created in the summer. As for geographical occurrence – because of air traffic density, more contrails are observed to form in the Northern Hemisphere with research showing the North Atlantic corridor to be particularly fertile pasture for contrail generation and their subsequent climate impact.

How do Contrails Impact the Climate?

Contrails, along with other non-CO2 effects, are responsible for a lion’s share of aviation’s climate impact. Like natural cirrus clouds, persistent contrails trap heat, prevent heat from leaving the Earth’s atmosphere, and reflect heat back to Earth. However, researchers have found, as contrails are made up of smaller ice particles than those found in natural cirrus clouds, contrails are more reflective than natural clouds, and increase the Earth’s radiative forcing.

What is Radiative Forcing? Put simply, radiative forcing occurs when the energy leaving the Earth’s atmosphere is different from the incoming energy. This is usually caused by things like greenhouse gases, aerosols, clouds, or… contrails.

Contrail radiative forcing (RF) can be comprised of both longwave (terrestrial) and shortwave (solar) mechanisms. Observations show that longwave RF can be positive (warming) during the day and the night, with the greatest observed impact coming from cold contrails hovering over warm, cloud-free surfaces such as a desert. Here, contrail cirrus are able to trap the heat being redirected off of the sizzling sands. Meanwhile, shortwave RF is found to be mainly negative (cooling), as darker, cloudless surfaces like oceans and forests, which absorb much of the radiation, prove to have the largest impact.

Visual of Radiative Forcing - ResearchGate - Tait et. al. 2022

Though clouds do indeed trap heat, their reflective qualities can divert the sun’s rays, invoking cooling properties (negative RF) for the atmosphere. The same principle applies for contrail cirrus clouds.

A paper published by ATAG states that some contrails (particularly those occurring during the day) have a cooling effect by reducing the solar radiation reaching the Earth’s surface, much like naturally occurring clouds. Some (particularly those occurring at night) have a warming effect by trapping heat in the atmosphere, rather than allowing it to escape.

These night contrails retain the heat that entered the atmosphere during the day and outweigh the slight cooling effects of the daytime contrails, ending up with a net-warming impact on the climate.

Contrail Avoidance - American Airlines — Day vs. Night Contrails - Google Research

The exact amount of radiative forcing that is caused directly by contrails, is yet to be ascertained, although researchers have concluded contrails have a negative effect on the Earth’s climate. Uncertainties persist as to the full scale of non-CO2 impacts as well as the choice of metrics for measurement of those impacts, but despite these uncertainties, there is a growing scientific consensus that contrails warm the climate as much as, if not more so than, the CO2 emitted by aircraft. Studies and reports like this address some of the uncertainties surrounding contrail research and mitigation efforts, calling for “better quantification of the actual effects, and further studies of the tradeoffs between non-CO₂ reductions vs. potential CO₂ increases.”

There is a great deal of work going into determining the true radiative effect of contrail cirrus as well as the most efficient ways to manage that effect. Projects such as these are seeking to improve the scientific understanding of contrails with the goal of introducing contrail management as a short-term goal for a major component of aviation’s contribution to climate change.