A contrail is the condensation trail that is left behind by a
passing jet plane. Contrails form when hot humid air from jet exhaust mixes
with environmental air of low vapor pressure and low temperature. Vapor
pressure is just a fancy term for the amount of pressure that is exerted by
water vapor itself (as opposed to atmospheric, or barometric, pressure which
is due to the weight of the entire atmosphere above you). The mixing occurs
directly behind the plane due to the turbulence generated by the engine. If
condensation (conversion from a gas to a liquid) occurs, then a contrail
becomes visible. Since air temperatures at these high atmospheric levels are
very cold (generally colder than -40 F), only a small amount of liquid is
necessary for condensation to occur. Water is a normal byproduct of
combustion in engines.
Image courtesy NASA Langley Research Center
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 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.
Present commercial aircraft fly at altitudes of 8-13 km. The emissions from
such air traffic can change the atmospheric composition: Directly: by
emitting carbon dioxide (CO2), nitrogen oxides (NOx = NO + NO2), water
vapor, hydrocarbons, soot, and sulfate particles. Indirectly: by a
chemical reaction chain similar to smog-formation the greenhouse gas ozone
(O3) can be formed. In this reaction chain nitrogen oxides act as a catalyst
under the influence of sunlight. As a result of these chemical reactions
also the concentration of methane (CH4), another greenhouse gas, decreases.
These changes can have effects on climate: Ozone, CO2, and water vapor are
greenhouse gases and their increase has a warming effect. Methane is also a
greenhouse gas and its decrease has a cooling effect. Aerosols (sulfate
particles, soot) could have a cooling effect. Contrails formed due to the
emission of particles and water vapor can increase the cloud cover in the
upper troposphere. This may result in a cooling or heating depending on the
size and optical depth of the ice crystals of which the contrails consist.
Presently it is believed that contrails lead to a net warming effect. There
may be changes in (non-contrail) upper level clouds: Most contrails decay
after minutes to hours, but some continue to exist and are then not
distinguishable from natural cirrus clouds .
Schematic based on DLR German Aerospace Center graphic and text
Schematic of aerosol and contrail formation processes in an aircraft plume and
wake as a function of plume age and temperature. Reactive sulfur gases, water
vapor, chemi-ions, soot aerosols, and metal particles are emitted from the
nozzle exit planes at high temperatures. H2SO4 increases as a result of
gas-phase oxidation processes. Soot particles become chemically activated by
adsorption and binary heterogeneous nucleation of SO3 and H2SO4 in the presence
of H2O, leading to the formation of a partial liquid H2SO4/H2O coating. Upon
further cooling, volatile liquid H2SO4/H2O droplets are formed by binary
homogeneous nucleation, whereby the chemi-ions act as preferred nucleation
centers. These aerosols grow in size by condensation and coagulation processes.
Coagulation between volatile particles and soot enhances the coating and forms a
mixed H2SO4/H2O-soot aerosol, which is eventually scavenged by background
aerosol particles at longer times. If liquid H2O saturation is reached in the
plume, a contrail forms. Ice particles are created in the contrail mainly by
freezing of exhaust aerosols. Scavenging of exhaust particles and further
deposition of H2O leads to an increase of the ice mass. The contrail persists in
ice-supersaturated air and may develop into a cirrus cloud. Short-lived and
persistent contrails return residual particles into the atmosphere upon
evaporation. The scavenging timescales are highly variable and depend on the
exhaust and background aerosol size distributions and abundances, as well as on
wake mixing rates
Types of Contrails
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 a
few 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 return again to a
vapor state.
Persistent (non-spreading) contrailslook like long white lines
that remain visible after the airplane has disappeared. This shows that the
air where the airplane is flying is quite humid, 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 contrailslook
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.
Contrail cousinsare things that look
like contrails but actually arise from a different physical process. For
example, under the right conditions you will see vapor trails form from the
wingtips of a jet on takeoff or landing. This phenomenon results from a
decrease in pressure and temperature in the wingtip vortex. If conditions
are right, liquid water drops form inside the vortex and make it visible.
These evaporate very quickly after they form.
Contributing to Climate Change and
Ozone Destruction
1 round trip from NY to LA or Trans Atlantic round trip = 2,000 pounds of CO2
In a year air travel releases 600 million tons of carbon dioxide into the
atmosphere
NASA Graphic from The TERRA Program
Clouds play a complex role in the Earth's radiation budget. Low Clouds
reflect much of the sunlight that falls on them, but have little Effect on
the emitted energy. Thus, low clouds act to cool the Current climate. High
clouds reflect less energy, but trap more of The energy emitted by the
surface.
Aircraft engine emissions
affect climate change in three ways that are expected to increase in
concern as aviation grows:
From the burning of
fossil fuels, aircraft produce about 3 percent of annual global
emissions of carbon dioxide (CO2), the most important greenhouse gas.
There is good scientific understanding of the impact of these emissions,
which is the same as for CO2 at the earth's surface, such as from autos
or power plants.
At high altitudes
(25,000 to 50,000 feet), nitrogen oxide (NOx) emissions affect the
production of ozone and the concentration of methane, both potent
greenhouse gases for which a fair scientific understanding has
developed.
The third effect results
from emissions of aerosol and particulate matter at high altitudes, and
can be observed by the apparent increased incidence of cirrus clouds and
the persistence of contrails, which influence the radiative character of
the atmosphere. There is increasing knowledge about these effects, but
poor scientific understanding.
EPA Aircraft
Contrails Factsheet
Summary
This fact sheet describes the formation, occurrence, and effects of
“condensation trails” or “contrails.” It was developed by scientific and
regulatory experts at the Environmental Protection Agency (EPA), the Federal
Aviation Administration (FAA), the National Aeronautics and Space
Administration (NASA), and the National Oceanic and Atmospheric
Administration (NOAA) in response to public inquiries regarding aircraft
contrails. Contrails are line-shaped clouds sometimes produced by aircraft
engine exhaust, typically at aircraft cruise altitudes several miles above
the Earth’s surface. The combination of water vapor in aircraft engine
exhaust and the low ambient temperatures that often exists at these high
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. They do affect the cloudiness of the Earth’s atmosphere,
however, and therefore might affect atmospheric temperature and climate. The
basic processes of contrail formation described in this fact sheet apply to
both civil and military aircraft.
What are contrails?
Contrails are line-shaped clouds or “condensation trails,” composed of ice
particles, that are visible behind jet aircraft engines, typically at cruise
altitudes in the upper atmosphere1. Contrails have been a normal effect of
jet aviation since its earliest days. Depending on the temperature and the
amount of moisture in the air at the aircraft altitude, contrails evaporate
quickly (if the humidity is low) or persist and grow (if the humidity is
high). Jet engine exhaust provides 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.
How are aircraft emissions linked to contrail formation?
Aircraft engines emit water vapor, carbon dioxide (CO2), small amounts of
nitrogen oxides (NO), 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. All other engine emissions are considered
nonessential to contrail formation.
How do contrails form?
For a contrail to form, suitable conditions must occur immediately behind a
jet engine in the expanding engine exhaust plume. A contrail will form if,
as exhaust gases cool and mix with surrounding air, the humidity becomes
high enough (or, equivalently, the air temperature becomes low enough) for
liquid water condensation to occur. The level of humidity reached depends on
the amount of water present in the surrounding air, the temperature of the
surrounding air, and the amount of water and heat emitted in the exhaust.
Atmospheric temperature and humidity at any given location undergo natural
daily and seasonal variations and hence, are not always suitable for the
formation of contrails.
If sufficient humidity occurs in the exhaust plume, water condenses on
particles to form liquid droplets. As the exhaust air cools due to mixing
with the cold local air, the newly formed droplets rapidly freeze and form
ice particles that make up a contrail . Thus, the surrounding atmosphere’s
conditions determine to a large extent whether or not a contrail will form
after an aircraft’s passage. Because the basic processes are very well
understood, contrail formation for a given aircraft flight can be accurately
predicted if atmospheric temperature and humidity conditions are known.
After the initial formation of ice, a contrail evolves in one of two ways,
again depending on the surrounding atmosphere’s humidity. If the humidity is
low (below the conditions for ice condensation to occur), the contrail will
be short-lived. Newly formed ice particles will quickly evaporate as exhaust
gases are completely mixed into the surrounding atmosphere. The resulting
line-shaped contrail will extend only a short distance behind the aircraft.
If the humidity is high
(greater than that needed for ice condensation to occur), the contrail will
be persistent. Newly formed ice particles will continue to grow in size by
taking water from the surrounding atmosphere. The resulting line-shaped
contrail extends for large distances behind an aircraft . Persistent
contrails can last for hours while growing to several kilometers in width
and 200 to 400 meters in height. Contrails spread because of air turbulence
created by the passage of aircraft, differences in wind speed along the
flight track, and possibly through effects of solar heating.
What are the ingredients of
jet fuel, and are they important to contrail formation?
All jet fuel is a hydrocarbon mixture containing small amounts of impurities
and additives. All aircraft jet fuel is analyzed for strict impurity limits
before use. The hydrocarbon content of jet fuel produces water vapor as a
by-product of combustion. Contrails would not form behind aircraft engines
without the water vapor by-product present in exhaust. A common impurity in
jet fuel is sulfur (~0.05% by weight), which contributes to the formation of
small particles containing various sulfur species. These particles can serve
as sites for water droplet growth in the exhaust and, if water droplets
form, they might freeze to form ice particles that compose a contrail.
Enough particles are present in the surrounding atmosphere, however, that
particles from the engine are not required for contrail formation. There are
no lead or ethylene dibromide additives in jet fuel. Additives currently
used in jet fuels are all organic compounds that may also contain a small
fraction of sulfur or nitrogen.
Why are persistent contrails
of interest to scientists?
Persistent contrails are of
interest to scientists because they increase the cloudiness of the
atmosphere. The increase happens in two ways. First, persistent contrails
are line-shaped clouds that would not have formed in the atmosphere without
the passage of an aircraft. Secondly, persistent contrails often evolve and
spread into extensive cirrus cloud cover that is indistinguishable from
naturally occurring cloudiness (See Figure 3). At present, it is unknown how
much of this more extensive cloudiness would have occurred without the
passage of an aircraft. Not enough is known about how natural clouds form in
the atmosphere to answer this question.
Changes in cloudiness are important because clouds help control the
temperature of the Earth’s atmosphere. Changes in cloudiness resulting from
human activities are important because they might contribute to long-term
changes in the Earth’s climate. Many other human activities also have the
potential of contributing to climate change. Our climate involves important
parameters such as air temperature, weather patterns, and rainfall. Changes
in climate may have important impacts on natural resources and human health.
Contrails’ possible climate effects are one component of aviation’s expected
encing climate was recently assessed to be approximately 3.5 percent of the
potential from all human activities (See Box 1).
Persistent line-shaped contrails are estimated to cover, on average, about
0.1 percent of the Earth’s surface The estimate uses:
•meteorological analysis of atmospheric humidity to specify the global cover
of air masses that are sufficiently humid (low enough atmospheric
temperature) for persistent contrails to form
•data from 1992 reported aircraft operations to specify when and where
aircraft fly
•an estimated average for aircraft engine characteristics that affect
contrail formation
•satellite images of certain regions of the Earth in which contrail
cover can be accurately measured . The highest percentages of cover occur in
regions with the highest volume of air traffic, namely over Europe and the
United States . This estimate of contrail cloudiness cover does not include
extensive cirrus cloudiness that often evolves from persistent line-shaped
contrails. Some evidence suggests that this additional cirrus cloudiness
might actually exceed that of line-shaped cloudiness.
