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Why Do Planes Leave White Trails in the Sky?

AviationAtmospheric Science

Quick Answer

Contrails (condensation trails) form when hot, humid jet exhaust mixes with cold ambient air at high altitude, causing water vapour to condense and freeze into ice crystals.

The Full Story

Jet engines burn fuel and expel exhaust containing water vapour and CO2 at temperatures around 600 degrees Celsius. At cruising altitude (8-12 km), the ambient air is typically minus 40 to minus 60 degrees Celsius. When the hot, moist exhaust mixes with this frigid air, the water vapour condenses on soot particles and freezes into tiny ice crystals, forming a visible white trail. Whether a contrail forms and how long it persists depends on ambient temperature and humidity. In dry air, contrails evaporate within seconds. In humid air near ice-supersaturation, they can persist for hours, spreading into cirrus-like cloud sheets that may affect local climate. Studies suggest persistent contrails contribute to aviation warming effect, potentially rivalling its CO2 emissions in climate impact.

Key Facts

1.Contrails typically form above 8,000 metres altitude where temperatures drop below minus 40 degrees Celsius.
2.A study estimated that contrail cirrus clouds may be responsible for about 57% of aviation total climate warming effect — more than the CO2 from burning jet fuel.
3.During the three-day US flight ban after September 11, 2001, scientists observed a 1.1 degree Celsius increase in the daily temperature range across the US, attributed partly to the absence of contrails.

YouTube Angle

Suggested video title for this topic:

"Contrails Explained — Why Planes Paint White Lines Across the Sky"