What are contrails behind airplanes

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Last updated: April 8, 2026

Quick Answer: Contrails are line-shaped clouds formed when hot, humid exhaust from aircraft engines mixes with cold, high-altitude air, typically above 26,000 feet where temperatures are below -40°C. They consist of ice crystals that form around engine soot particles and can persist for hours, spreading into cirrus-like clouds. Contrails were first scientifically described in 1919 and have been studied extensively since the 1950s for their climate impacts. They contribute to atmospheric warming by trapping outgoing infrared radiation, with aviation contrails estimated to account for about 35% of aviation's total climate impact.

Key Facts

Contrails form at altitudes above 26,000 feet where temperatures are typically below -40°C
Contrails can persist for hours and spread to cover thousands of square kilometers
Aviation contrails contribute approximately 35% of aviation's total climate impact
Contrails were first scientifically described in 1919 by meteorologist William J. Humphreys
Contrails consist primarily of ice crystals that form around engine soot particles measuring 0.1-1.0 micrometers in diameter

Overview

Contrails, short for condensation trails, are artificial clouds that form behind aircraft when water vapor from engine exhaust condenses and freezes in cold, high-altitude air. First scientifically documented in 1919 by American meteorologist William J. Humphreys, contrails became more common with the advent of high-altitude jet aircraft in the 1950s. These linear clouds typically form at cruising altitudes between 26,000 and 40,000 feet where temperatures drop below -40°C (-40°F). During World War II, contrails posed tactical challenges as they revealed aircraft positions, leading to military research into their formation. The environmental significance of contrails gained scientific attention in the 1990s when researchers began quantifying their climate impacts. Today, contrails are studied as part of aviation's environmental footprint, with NASA and other agencies conducting research flights specifically to measure contrail properties and effects.

How It Works

Contrail formation occurs through a precise physical process involving aircraft exhaust and atmospheric conditions. When aircraft engines burn fuel, they release hot, humid exhaust containing water vapor, carbon dioxide, sulfur compounds, and soot particles. As this exhaust mixes with the cold surrounding air at high altitudes, the water vapor rapidly cools and condenses onto the soot particles, which serve as condensation nuclei. If the ambient air is sufficiently cold (typically below -40°C) and humid enough, these water droplets instantly freeze into ice crystals, creating the visible trail. The persistence and spread of contrails depend on atmospheric humidity: in dry air, contrails dissipate quickly (within seconds or minutes), while in humid conditions they can persist for hours and spread into extensive cirrus-like clouds covering thousands of square kilometers. The specific temperature and humidity thresholds for contrail formation are described by the Schmidt-Appleman criterion, which predicts contrail formation when the mixing of exhaust and ambient air reaches water saturation.

Why It Matters

Contrails have significant environmental and climate implications that extend beyond their visual appearance. Scientifically, they contribute to atmospheric warming through a net positive radiative forcing effect—while they reflect some incoming solar radiation (cooling effect), they trap more outgoing infrared radiation (warming effect), with studies estimating contrails account for approximately 35% of aviation's total climate impact. This warming effect is particularly pronounced at night when there's no solar reflection to offset the infrared trapping. Environmentally, persistent contrails can develop into extensive cirrus cloud decks that modify regional weather patterns and potentially affect precipitation. The aviation industry is researching mitigation strategies, including altitude adjustments and alternative fuels, to reduce contrail formation. Understanding contrails also helps improve climate models and informs policy decisions about aviation's environmental footprint as air travel continues to grow globally.