How high are contrails?

A contrail is a man-made cirrus cloud. Jet exhaust is hot and contains water vapour and soot particles; when it mixes with the surrounding air the mixture cools, and if the ambient air is cold enough the vapour condenses on the soot and freezes within a fraction of a second. The physics was worked out in the 1940s and 1950s by Ernst Schmidt and Herbert Appleman, and the Schmidt–Appleman criterion states the threshold: the ambient temperature must be below roughly −40 °C, with the exact value depending on pressure, humidity and the engine's efficiency.

Those temperatures are found in the upper troposphere. In the standard atmosphere the air passes −40 °C at about 8 km (26,000 ft) and reaches −56 °C at the tropopause, near 11 km (36,000 ft) in temperate latitudes. Jet airliners cruise between 9 and 12 km (30,000–40,000 ft) precisely because that thin, cold air is where they burn the least fuel, so most of the trails visible from the ground come from that band. In the tropics, where the air is warmer at a given height, the threshold shifts upward; in the polar winter contrails can form at much lower levels, occasionally even at the surface.

Whether a trail is a brief streak or a spreading sheet depends on the humidity of the surrounding air rather than on the aircraft. If the ambient air is dry, the ice crystals sublimate within seconds and the contrail is only a few hundred metres long. If the air is supersaturated with respect to ice, a condition that is common in the upper troposphere and invisible to the eye, the crystals grow, the trail persists for hours and wind shear spreads it into a diffuse contrail cirrus that can cover thousands of square kilometres.

Persistent contrails are the part that matters for climate. They reflect a little sunlight during the day but trap outgoing infrared radiation all the time, and assessments for the aviation sector estimate that contrail cirrus contributes a warming effect comparable to that of aviation's carbon dioxide. Because ice-supersaturated regions are thin layers, often only a few hundred metres deep, airlines and researchers are trialling small altitude changes to avoid them.

From the ground it is hard to estimate a contrail's altitude, but there are clues. A trail with a visible gap between engine and cloud, forming a few aircraft lengths behind, is typical of cruise altitude. Trails at holding or climbing altitudes below about 8 km are rare in temperate summers. Tracking apps that list altitude for a passing flight almost always show 10 to 12 km for a persistent trail, a useful check for anyone curious about what is passing overhead.