9th September 2005
Contrails
Source: Excerpt from The Book " Weather "
Distribution: Worldwide. Height: Above 16,500 feet. Cause: Aircraft operations at high levels combined with significant moisture in the surrounding air. Associated Weather: If long-lasting, can indicate an approaching frontal system. This is an exceptional formation in that it does not occur naturally. Rather, it is an artificial cirrus cloud produced by high-level aircraft operations. The term contrails is short for condensation trails.
All aircraft engines emit water droplets from their exhausts. When an airplane flies through the upper levels of the troposphere, where temperatures are normally well below 32° F (0° C), these droplets immediately freeze to form ice crystals, creating an artificial cloud.
Often, the surrounding air mass will contain little moisture, and the resulting cloud will be thin, short-lived, and invisible to an observer on the ground. However, if the surrounding air mass is close to saturation, the cloud produced will be much broader and longer and may last for half an hour. It is this visible formation that is known as a contrail. For the weather- matcher, a long-lasting contrail can be a useful sign, as it reveals the presence of significant high-level moisture. This, in turn, may indicate the approach of a frontal system.
Contrails also have a military and strategic significance in that they reveal the presence and location of high-level aircraft that would normally be invisible to the naked eye. The enduring memory of many who witnessed the Battle of Britain, during the Second World War, is the criss-crossing spider's web of contrails that occurred day after day, high above England, as the Luftwaffe and the Royal Air Force engaged in combat in the upper levels of the troposphere.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
Check out Skyscapes for cloud photos taken from the aircraft.
