Aircraft Contrails
 | | Do aircraft contrails contribute to climate change? |
Could aircraft flying the busy South East air corridor be wrecking
the weather and leaving a grim legacy for future generations? On board
the Discovery space mission in 2006, the British astronaut, Piers Sellers from
Crowborough, was able to take a walk in space and look back towards home. What
he saw was the South East region blanketed not in clouds, but condensation trails
from aircraft. What are contrails and how do they form?Contrails
are vapour trails, artificial cirrus clouds made by the exhaust of aircraft engines
or wingtip vortices that precipitate a stream of tiny ice crystals in moist, frigid
upper air. They are not pollutants. Contrail creationContrails
are created in one of two ways: Firstly, 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 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, one gallon of water is produced.  | | Contrails are visible from satellites orbiting the South
East |
At high altitudes the water vapour faces low temperatures,
which cools the vapour until it condenses into tiny water droplets. Myriads
of water droplets and/or ice crystals form the contrails. At high altitudes,
super cooled water vapour requires a trigger to encourage desublimation. The
exhaust particles in the aircraft's exhaust act as this trigger, causing the trapped
vapour to rapidly turn to ice crystals. Contrails only occur when the outside
air temperature around the aircraft is at or below minus 57 degrees C. Drop
in pressure聟Airplane wings cause a drop in air pressure in the vicinity
of the wing. The reduction in air pressure brings with it a drop in temperature,
which can cause water to condense and form a contrail at high altitudes. At
lower altitudes, this phenomenon is known as Ectoplasm. Ectoplasm is more
commonly seen during high-energy manoeuvres. E.g. fighter jets, jet liners during
takeoff and landing, or around turbo-fan intakes on takeoff. Exhaust contrails
stableExhaust contrails tend to be more stable and long lasting than wing-tip
contrails, which are often disrupted by the aircraft's wake and are short-lived. Cambridge
University astronomer, Dr. Gerry Gilmore, has predicted that by 2050, the telescope
will be a redundant tool because the skies will be blanketed with clouds formed
by contrails. His claim has been disputed by some, but it seems inevitable
that the skies will get even busier聟 and that could mean a rather a grey
and cloudy future. Contrails and climateContrails, by affecting
cloud formation, can act as a warming factor. Various studies have found
that contrails trap outgoing heat emitted by the Earth and atmosphere at a greater
rate than they reflect incoming solar radiation. Therefore, the overall
effect of contrails is a warming. Night flightsSome studies have
shown that night flights are most responsible for the warming effect. Accounting
for only 25% of daily air traffic, they contribute 60 to 80% of contrail radiative
forcing. Similarly, winter flights account for only 22% of annual air traffic,
but contribute 50%of the warming. 9/11 effectThe first clue came
from an unlikely and tragic event. In the days after 9/11, aircraft across
the USA were grounded for three whole days. Surprisingly, there appeared
to be a change in the weather. Scientists found the difference between
day and night temperatures was one degree greater than usual. It appeared
that the lack of contrails had led to clear blue skies and an increase in daytime
temperatures. It seemed to confirm that contrails are contributing to a
phenomenon known as "global dimming", blocking out radiation from the
sun and cooling the planet. Global warmingBut that is not the end
of the story. The same clouds that block out the sun are also trapping in the
earth's heat and, as a result, contributing to global warming. So which
is it? Are contrails cooling us or heating us? In Herstmonceax, East Sussex,
researchers from Reading University have begun to measure which effect is the
greater. Being on one of Europe's busiest air corridors made it the ideal
place to test the effect of contrails.
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听听Inside
Out - South East: Monday September 18, 2006
