Contrail

A C-141 Starlifter leaves vapour trails over Antarctica

Boeing 747 in cruise at roughly 35000 feet

Multiple contrails in an area with high airline traffic.

A satellite photo of Nova Scotia showing numerous contrails of jets travelling between the Eastern Seaboard and Europe.

Contrails are condensation trails (sometimes vapour trails): artificial cirrus clouds made by the exhaust of aircraft engines or wingtip vortices which precipitate a stream of tiny ice crystals in moist, frigid upper air.

Contrail creation

Contrails are created in one of two ways:

1. First, the airplane's exhaust increases the amount of moisture in the air, which can push the water content of the air past saturation point. This causes condensation to occur, and the contrail to form.

Aviation fuel such as petrol/ gasoline (piston engines) or paraffin/ kerosene (jet engines) consists primarily of hydrocarbons. When the fuel is burned, the carbon combines with oxygen to form carbon dioxide; the hydrogen also combines with oxygen to form water, which emerges as steam in the exhaust. For every gallon of fuel burned, approximately one gallon of water is produced, in addition to the water already present as humidity in the air used to burn the fuel. At high altitudes this steam emerges into a cold environment, (as altitude increases, the atmospheric temperature drops) which lowers the temperature of the steam until it condenses into tiny water droplets and/or desublimates into ice. These millions of tiny water droplets and/or ice crystals form the contrails. The temperature drop (and therefore, time and distance) the steam needs to condense accounts for the contrail forming some way behind the aircraft's engines.

2. The wings of an airplane cause a drop in air pressure in the vicinity of the wing (this is partly what allows a plane to fly). This drop in air pressure brings with it a drop in temperature, which can cause water to condense out of the air and form a contrail.

Exhaust contrails tend to be more stable and long-lasting than wing-tip contrails, which are often disrupted by the aircraft's wake and are commonly very short-lived.

Contrails and climate

Contrails, by affecting cloud formation, can act as a radiative forcing. Perhaps surprisingly, various studies find that the overall effect is a warming (Ponater et al., GRL, 32 (10): L10706 2005). However, the size of the forcing is not well known: globally (for 1992 air traffic conditions), values range from 3.5 mW/m2 to 17 mW/m2.

Sept. 11th, 2001 Climate Impact "Experiment"

It had been hypothesized that in regions such as the United States with heavy air traffic, contrails affected the weather, reducing solar heating during the day and radiation of heat during the night by increasing the albedo. The suspension of air travel for three days in the United States after September 11, 2001 provided an opportunity to test this hypothesis. Measurements did in fact show that without contrails the local diurnal temperature range (difference of day and night temperatures) was about 1 degree Celsius higher than immediately before (Travis et al., J. Climate, 17, 1123-1134, 2004); however, it has also been suggested that this was due to unusually clear weather during the period (Kalkstein and Balling Jr., Climate Research, 26, 1-4, 2004).