Contrails
Jet planes leave trails in the sky called contrails, short for condensation trails, and sometimes
vapour trails. They are 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. Contrary to
appearances, they are not pollution.
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| Caption: Contrail - photographed on 2006/08/15 around 1800 looking west from Bellevue, WA by K Gibson.
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There are two ways that contrails are formed. First, the plane'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. The other way is the wings of a plane cause a drop in air
pressure near 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 longer lasting than wing-tip contrails, which are often disrupted
by the aircraft's wake and are commonly very short-lived.
Adapted from a contribution by Mr Neil Cutten to the Burning Questions column,
West Australian newspaper.
Contrails are rarely seen over Perth and therefore are relatively unfamiliar
and often form the basis of reports of 'unidentified flying objects'.
This rarity of contrails arises because Perth is a very isolated city and
most jet planes seen over Perth are at low altitude and in the process of either
arriving or departing. The images shown here were probably created by a US
military jet undergoing a training exercise high over Perth. At this time
a US Navy aircraft carrier group was berthed in Fremantle harbour.
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| Caption: Contrail close up - photographed on 2006/08/15 around 1800 looking
west from Bellevue, WA by K Gibson. |
We observe contrails by the sunlight they reflect. The close up photograph
on this page clearly shows the contrail as well as the aircraft that created
the contrail. Many people contacted the Observatory reporting their observations
of this contrail. One detail that isn't clear from the photograph is the red
hue of the contrail. This red colour is caused by the low altitude of the
Sun (sunset occurred at 1749 for Perth on this day) and in this situation red
light is preferentially transferred through the atmosphere (it's
the reason why the rising or setting Sun is red). The red hue is essentially
lost in the images because the contrail is overexposed.
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| Caption: Contrail photographed on 2008/07/06 around 1728 looking west from Ferguson Valley, WA by C Jago. |
