Contrails and Aviation cirrus
General and Definitions (01)
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The condensation trail left behind jet aircrafts are called contrails. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing is a result of turbulence generated by the engine exhaust. Cloud formation by a mixing process is similar to the cloud you see when you exhale and "see your breath". The figure below represents how saturation vapor pressure varies as a function of temperature. The blue line is the saturation vapor pressure for ice as a function of temperature (in degrees Kelvin). Air parcels in the region labeled saturated will form a cloud. Imagine two parcels of air, A and B as located on the diagram. Both parcels are unsaturated. If B represents the engine exhaust, then as it mixes with the environment (parcel A) its temperature and corresponding vapor pressure will follow the dotted line. Where this dotted line intersects the blue line is were the parcel becomes saturated.
If you are attentive to contrail formation and duration, you will notice that they can rapidly dissipate or spread horizontally into an extensive thin cirrus layer. How long a contrail remains intact, depends on the humidity structure and winds of the upper troposphere. If the atmosphere is near saturation, the contrail may exist for sometime. On the other hand, if the atmosphere is dry then as the contrail mixes with the environment it dissipates. Contrails are a concern in climate studies as increased jet aircraft traffic may result in an increase in cloud cover. It has been estimated that in certain heavy air-traffic corridors, cloud cover has increased by as much as 20%. An increase in cloud amount changes the region's radiation balance. For example, solar energy reaching the surface may be reduced, resulting in surface cooling. They also reduce the terrestrial energy losses of the planet, resulting in a warming. Jet exhaust also plays a role in modifying the chemistry of the upper troposphere and lower stratosphere. NASA and the DOE are sponsoring a research program to study the impact contrails have on atmospheric chemistry, weather and climate. In this series of satellite images we will investigate the duration of contrails.Please refer questions to Dr. Steve Ackermanfirstname.lastname@example.org
Contrails are human-induced clouds that only form at very high altitudes (usually above 8 km) where the air is extremely cold (less than -40�C). If the air is very dry, they do not form behind the plane. If the air is somewhat moist, a contrail will form immediately behind the aircraft and make a bright white line that lasts for a short while. Persistent contrails form immediately behind the airplane in very moist air. Persistent contrails can exist long after the airplane that made them has left the area. They can last for a few minutes or longer than a day. However, because they form at high altitudes where the winds are usually very strong, they will move away from the area where they were born. Persistent contrails are those most likely to affect climate. [..]
A research team of American and German scientists, headed by Patrick Minnis of the NASA Langley Research Center in Hampton, Virginia, reports evidence that contrails cause a warming of the Earth�s atmosphere. Currently their impact is currently small as compared to other greenhouse effects. They predict, however, that it may grow by a factor of six over the next 50 years. The researchers emphasize that these are conservative estimates, which take into account only the thicker contrails that can be readily observed. [..]
Air traffic and, therefore, contrails, are not evenly distributed around the globe. They are concentrated over parts of the United States and Europe, where local warming reaches up to 0.7 watts per square meter, or 35 times the global average. The ghostly white trails following airplanes and rockets through the sky, called contrails, are probably adding to global warming, according to scientists at NASA�s Langley Research Center, Hampton, Va. The contrails often turn into cirrus clouds, a thin, wispy type of cloud made of ice crystals. The most common form of high-level clouds are thin and often wispy cirrus clouds. Typically found at heights greater than 20,000 feet (6,000 meters), cirrus clouds are composed of ice crystals that originate from the freezing of super cooled water droplets. Cirrus generally occur in fair weather and point in the direction of air movement at their elevation. While some clouds tend to help cool the globe and negate the affects of global warming, thin cirrus clouds are heat trappers, holding in more heat than they reflect back into space. [..]
Contrails (short for "condensation trails") are line-shaped clouds sometimes produced by aircraft engine exhaust. The combination of high humidity and low temperatures that often exists at aircraft cruise altitudes allows the formation of contrails. Contrails are composed primarily of water (in the form of ice crystals) and do not pose health risks to humans. Contrails have been a normal effect of aviation since its earliest days. Depending on the temperature and the amount of moisture in the air at the aircraft altitude, contrails can either evaporate quickly or they can persist and grow. Engine exhaust produces only a small portion of the water that forms ice in persistent contrails. Persistent contrails are mainly composed of water naturally present along the aircraft flight path.
Aircraft engines emit
water vapor, carbon dioxide (CO2), small amounts of nitrogen oxides (NOx),
hydrocarbons, carbon monoxide, sulfur gases, and soot and metal particles formed
by the high-temperature combustion of jet fuel during flight. Of these
emittants, only water vapor is necessary for contrail formation. Sulfur gases
are also of potential interest because they lead to the formation of small
particles. Particles suitable for water droplet formation are necessary for
contrail formation. Initial contrail particles, however, can either be already
present in the atmosphere or formed in the exhaust gas.
Miriam Katz in Danish national newspaper Berlingske Tidende
Contrails are human-induced clouds that only form at very high altitudes (usually above 8 km) where the air is extremely cold (less than -40�C). If the air is very dry, they do not form behind the plane. If the air is somewhat moist, a contrail will form immediately behind the aircraft and make a bright white line that lasts for a short while. Persistent contrails form immediately behind the airplane in very moist air. Persistent contrails can exist long after the airplane that made them has left the area. They can last for a few minutes or longer than a day. However, because they form at high altitudes where the winds are usually very strong, they will move away from the area where they were born. Persistent contrails are those most likely to affect climate.
Although most persistent contrails are associated with cirrus, the frequency
of occurrence of persistent contrails when no cirrus was observed is ranging
from a few to 15% over the Langley Air Force Base and from 1 to 6% averaged over
the 19 surface sites of the Minnis et al. (1997) analysis. Therefore there is
some potential for an increase in high-level contrail-cirrus occurrence due to
A potential source of recent climatic change in the north-eastern and midwestern United States is the artificial cloudiness generated by jet aircraft, known as contrails. Central Pennsylvania, like central Ohio, Indiana, and Illinois, lies beneath the flight route connecting major cities of the east and west coasts. At least for areas in the Midwest located beneath the flight route, there is evidence of changes in surface temperature consistent with an increase in higher-level cloudiness since the 1960s. The same may also be happening in the north-eastern U.S. In the fall season especially, the lack of lower-level cloud cover often permits jet contrails to be seen from the ground. It is not known how the regional climatic effects of jet contrails may modfy "global warming" arising from enhanced trace gas emissions.
Source no longer available:
Contrail Education https://asd-www.larc.nasa.gov/GLOBE/science.html
What are contrails?
Contrails are clouds formed when water vapor condenses and freezes around small particles (aerosols) that exist in aircraft exhaust. Some of that water vapor comes from the air around the plane; and, some is added by the exhaust of the aircraft.
Are there different types of contrails?Contrails are all made of the same materials and are formed in the same way, but exist for different lengths of time. Because of the differences in contrail "life-spans", contrails can be divided into three groups: short-lived, persistent, and persistent spreading.
Short-lived contrails look like short white lines following along behind the plane, disappearing almost as fast as the airplane goes across the sky, perhaps lasting only thirty minutes or less. The air that the airplane is passing through is somewhat moist, and there is only a small amount of water vapor available to form a contrail. The ice particles that do form quickly evaporate again.
Persistent contrails look like long white lines that remain visible after the airplane has disappeared. This shows that the air where the airplane is flying is full of moisture, and there is a large amount of water vapor available to form a contrail. Persistent contrails can be further divided into two classes: those that spread and those that don't. Persistent contrails look like long, narrow white pencil-lines across the sky.
Persistent spreading contrails look like long, broad, fuzzy white lines. This is the type most likely to affect climate because they cover a larger area and last longer than short-lived or persistent contrails.
Can contrails move, or do they stay in the location where they were formed?Because contrails are formed at high altitudes where the winds are usually very strong, they will move away from the area where they originated. Often, when we look up into the sky, we will see old persistent contrails that formed far away but moved overhead because of the wind.
How are contrails different from other clouds?Contrails are "human-induced" clouds since they are formed by water vapor condensing on particles from airplane exhaust. Other types of clouds can be formed by water vapor that condenses on particles which are present in the atmosphere due to many sources, such as from volcanoes or dust storms, not specifically from aircraft exhaust.
Contrails only form at very high altitudes (usually above 8 km) where the air is extremely cold (less than -40 degrees C). Other clouds can form at a range of altitudes, from very close to the ground, such as fog, to very high off the ground, such as cirrus clouds.
When were contrails first observed?Contrails were first noticed during high-altitude flights in the 1920's. However, interest in contrails really blossomed during WWII when bombers could be sighted from miles away. In fact, numerous WWII veteran accounts tell of problems to aviation due to massive contrails formations. Planes could not find their targets, and sometimes collided with each other.
Frequently Asked Questions
Q: Why are we able to see contrails on some days but not on other
Q: Do contrails drastically affect weather patterns?
Robert C. van Waning
Tel +31 (0)20 441 76 73