Moon and Contrail
Photograph made with Olympus E3 of a military plane contrail drifting across the moon.
Contrails (short for “condensation trails”) or vapor trails are visible trails of condensed water vapor made by the exhaust of aircraft engines. As the hot exhaust gases cool in the surrounding air they may precipitate a cloud of microscopic water droplets. If the air is cold enough, this trail will comprise tiny ice crystals.
The main products of hydrocarbon fuel combustion are carbon dioxide and water vapor. At high altitudes this water vapor emerges into a cold environment, and the local increase in water vapor can push the water content of the air past saturation point. The vapor then condenses into tiny water droplets and/or deposits into ice. These millions of tiny water droplets and/or ice crystals form the vapor trail or contrails. The energy drop (and therefore, time and distance) the vapor needs to condense accounts for the contrail forming some way behind the aircraft’s engines. The majority of the cloud content comes from water trapped in the surrounding air. At high altitudes, supercooled water vapor requires a trigger to encourage deposition or condensation. The exhaust particles in the aircraft’s exhaust act as this trigger, causing the trapped vapor to rapidly turn to ice crystals. Exhaust vapor trails or contrails usually occur above 8000 metres (26,000 feet). where the temperature is below -40°C (-40°F).[2]
