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Contrails and Aviation-cirrus  

Clouds (02)

What Clouds Are Made Of

Both SO2 (which forms sulfate particles) and soot particles from aircraft exhaust are aerosols�microscopic particles suspended in air. They act like seeds. Water molecules can condense or freeze on them to form cloud particles.

Aircraft exhaust produces contrails�condensation trails in the atmosphere about 5 miles above the Earth's surface. At these high altitudes, contrails and cirrus clouds form depending on the quantity of water vapor and atmospheric conditions.

Contrails and cirrus clouds both reflect sunlight that would otherwise warm the Earth's surface. At the same time, they absorb heat from the ground instead of allowing it to escape. Do they contribute to global warming or global cooling? The scientific community is still trying to answer that question.

Contrails contribute to the phenomenon known as "global change." Right now this effect is small, but it is growing. Although scientists are uncertain about the impact of contrails on global change, they believe that persistent contrails, those that last longer than a few minutes, gradually develop into cirrus clouds. Over the past 40 years, cloudiness seems to have increased. If this is in fact true, then this continual increase in cloudiness may lead to global climate change because it will change the amount of radiation entering and leaving the Earth's atmosphere. This characteristic of aircraft engine exhaust may act in a way similar to the effects produced by greenhouse gases.

What effect, if any, do contrails have on weather? Answering this accurately is difficult because so many factors affect weather. However, many meteorologists believe increased jet traffic and the contrails it produces have altered the weather. They point out that areas of high jet traffic show the greatest change. Although circumstantial, evidence seems to indicate that contrails do affect climate. Glenn's contribution of a new generation of highly efficient, clean-burning aircraft engines will reduce the amount of aerosols in the upper atmosphere, which will in turn lessen the effect of increased air traffic on climate.


For more information, visit NASA Glenn's home page at:

Are jet contrails the latest threat?


[..] But there's an element of truth to what they say. In recent years scientists have become concerned about the effects of jet contrails on the environment. In research published in 1998, NASA scientists found that by circling a jet off the Pacific coast they were able to create contrails that eventually coalesced into a cirrus cloud covering 1,400 square miles. An examination of satellite photographs turned up an instance of jet contrails produced by commercial aircraft over New Mexico forming a cloud covering 13,000 square miles. NASA atmospheric scientist Patrick Minnis thinks the publicity surrounding these revelations may have been what set off the chemtrail nuts. Cirrus cloud cover is thought to have increased significantly over the U.S. since 1971, possibly due to jet travel. Given the steady rise in air traffic, these clouds may lead to local and eventually global warming and other meteorological effects--not necessarily angel hair filaments and Jell-O-like goop, but perhaps a lot fewer sunny days.


Contrail Occurrence and Persistence and Impact of Aircraft Exhaust on Cirrus


Aircraft cause visible changes in the atmosphere by forming contrails that represent artificially induced cirrus clouds. The conditions under which contrails form are discussed in Section 3.2.4. This section describes the formation, occurrence, and properties of persistent contrails and how they compare with natural cirrus.

3.4.1. Cirrus and Contrails

Cirrus clouds (Liou, 1986; Pruppacher and Klett, 1997) contain mainly ice crystals (Weickmann, 1945). The distinctive properties of cirrus and contrails derive from the physics of ice formation. Ice particle nucleation occurs either through homogeneous nucleation (when pure water droplets or liquid aerosol particles freeze) or through heterogeneous nucleation, when freezing of the liquid is triggered by a solid particle or surface that is in contact with the liquid or suspended within the liquid. Both processes depend strongly on temperature and relative humidity (Heymsfield and Miloshevich, 1995; see Section

Once ice crystals form and take up available water vapor, supersaturation declines and further nucleation of ice ceases. This selectivity causes ice crystals in cirrus to be larger relative to droplets in liquid water-containing clouds�apart from differences in saturation vapor pressures over ice compared to water, which causes more water vapor to be available for deposition on ice particles than on water droplets. Large ice particles may precipitate rapidly. Many cirrus clouds have a �fuzzy� appearance because rapid precipitation causes optically thin edges of clouds to be diffuse, and precipitation allows particles to spread in the wind, forming long tails of cloud. Ice crystal nucleation also depends on available aerosol in the upper troposphere, the properties of which are only poorly known (Str�m and Heintzenberg, 1994; Podzimek et al., 1995; Sassen et al., 1995; Schr�der and Str�m, 1997). In some locations, upper tropospheric particles are dominated by sulfates (Yamato and Ono, 1989; Sheridan et al., 1994). However, more recent data show that minerals, organic compounds, metals, and other substances may often be present in significant quantities (Chen et al., 1998; Talbot et al., 1998).

Cirrus clouds occur mainly in the upper troposphere. The mean tropopause altitude is about 16 km in the tropics and 10 km (250 hPa) north of 45�N latitude (Hoinka, 1998). The tropopause temperature at northern mid-latitudes varies typically between -40 and -65�C; it may reach below -80�C in the tropics. At mid-latitudes, the upper troposphere often is humid enough for cirrus and persistent contrails to form

Persistent contrail formation requires air that is ice-supersaturated (Brewer, 1946). Ice-supersaturated air is often free of visible clouds (Sassen, 1997) because the supersaturation is too small for ice particle nucleation to occur (Heymsfield et al., 1998b). Supersaturated regions are expected to be quite common in the upper troposphere (Ludlam, 1980). The presence of persistent contrails demonstrates that the upper troposphere contains air that is ice-supersaturated but will not form clouds unless initiated by aircraft exhaust (Jensen et al., 1998a). Aircraft initiate contrail formation by increasing the humidity within their exhaust trails, whereas local atmospheric conditions govern the subsequent evolution of contrail cirrus clouds. Indeed, the ice mass in long-lasting contrails originates almost completely from ambient water vapor (Knollenberg, 1972).

3.4.3. Contrail Occurrence

At plume ages between 1 min and 1 h, contrails grow much faster horizontally (to several km width) than vertically (200 to 400 m), especially in highly sheared environments (Freudenthaler et al., 1995, 1996; Sassen, 1997). Young contrails spread as a result of turbulence created by aircraft vortices (Lewellen and Lewellen, 1996; Gerz et al., 1998; Jensen et al., 1998a,b,c), shear in the ambient wind field (Freudenthaler et al., 1995; Schumann et al., 1995; D�rbeck and Gerz, 1996; Gierens, 1996), and possibly radiatively driven mixing (Jensen et al., 1998d).

Contrails often become wide and thick enough to induce radiative disturbances that are sufficient to be detectable in multispectral satellite observations. They have been observed at 1-km spatial resolution with instruments such as the Advanced Very High Resolution Radiometer (AVHRR) on board National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites (e.g., Lee, 1989) and at 4-km resolution in the infrared with the Geostationary Operational Environmental Satellite (GOES) (Minnis et al., 1998a). The AVHRR channels in the 11- to 12-�m range (4 and 5) are particularly suited to detect thin ice clouds because of the different emissivity of ice particles in this spectral range (King et al., 1992; Minnis et al., 1998c).

The figure shows that aircraft trigger contrail cirrus that evolves into cirrus clouds that are much more extensive in scale than the initial contrails. Such spread and deformed contrail cirrus can no longer be distinguished from naturally occurring cirrus. In Figure 3-12, contrails that still have a line-shaped appearance cover about 5% of the scene.

Aged contrails often cannot be distinguished from cirrus, which poses an observational problem in determining the frequency and area of coverage by contrails. An important example of the persistence of contrails and their evolution into more extensive cirrus is shown in Figure 3-13.

3.4.4. Contrail Properties
The relatively small particles present in newly formed contrails serve to distinguish contrail radiative properties from those of most natural cirrus (Grassl, 1970; Ackerman et al., 1998; see also Section 3.6).

Aircraft create a large number of new ice crystals that grow from ambient water vapor. As a contrail spreads, some evidence suggests that ice crystals are neither lost nor created in significant numbers. In one remote-sensing study, the total number of particles (about 2.6 x 109 ice crystals per cm of contrail length) in a contrail was found to remain constant as the contrail dispersed and the particle concentration (particles per unit volume) dropped (Spinhirne et al., 1998). In situ measurements (Schr�der et al., 1998b) reveal 108 to 109 particles larger than 4.5-mm diameter formed per cm of contrail length. In contrast, changes in particle number will occur when some particles eventually sediment out of the contrail and new particles are nucleated as a result of air motions induced by the contrail. The ice water content of new contrails increases with time to exceed the amount of water emitted by the aircraft by more than two orders of magnitude. However, the water content per unit volume remained approximately constant in the case observed by Spinhirne et al. (1998). The optical depth of contrails remains nearly constant during the first hour despite horizontal spreading (J�ger et al., 1998).

3.4.5. Impact of Aircraft Exhaust on Cirrus Clouds and Related Properties

Aircraft may perturb natural cirrus through the addition of water vapor, soot, and sulfate particles and by inducing vertical motions and turbulent mixing (Gierens and Str�m, 1998). Observations of cirrus coverage in certain regions have found perturbations from anthropogenic aerosol (Str�m et al., 1997). Persistent contrails are often associated with or embedded in natural cirrus (Minnis et al., 1997; Sassen, 1997).

Soot particles originating from aircraft exhaust may act as freezing nuclei. In an atmosphere with few freezing nuclei, this perturbation could lead to an expansion of cirrus cover, a change in average particle size, and related changes in cloud surface area and optical depth (Jensen and Toon, 1997)�hence have consequences for radiative forcing (see Section 3.6.5). For aircraft to alter cirrus properties, exhaust particles would need to be more efficient or more abundant than ice nuclei currently existing in the atmosphere. If the predominant particle that freezes to form ice contains sulfate, aircraft soot could serve as important ice nuclei because sulfate particles require a relatively high supersaturation before freezing occurs. Over the central United States of America, however, as many as 0.1 to 0.2 cm-3 freezing nuclei (effective at -35�C) have been found in the upper troposphere (DeMott et al., 1998), indicating that the number of freezing nuclei is not always small. Some observations show that aircraft exhaust does not contain large numbers of ice nuclei active at temperatures above -35�C (Rogers et al., 1998). Soot and metals were found to be significant, but not dominant, components of ice nuclei in contrails (Chen et al., 1998; Petzold et al., 1998; Twohy and Gandrud, 1998).

Aircraft measurements in and near clouds have indicated the presence of light-absorbing material contained inside ice crystals. The distribution pattern and the amount of measured absorbers suggest that the material is related to aircraft soot (Str�m and Ohlsson, 1998) (Figure 3-17). For the same abundance of aerosol particles, clouds perturbed by absorbing material contained 1.6 to 2.8 times more ice crystals than unperturbed portions of clouds. These observations suggest that aircraft-produced particles enhance cloud ice particle concentrations, although they have not revealed the physical mechanism involved. Specifically, exhaust soot particles may have been involved in ice crystal formation within the cirrus or formed contrail ice particles within the exhaust plume before being incorporated into the cloud. These observations are roughly consistent with calculations of a cirrus cloud forming in a region of recent exhaust trails (Jensen and Toon, 1997).

Sedimentation of large particles in persistent contrails may remove water vapor from the upper troposphere, possibly reducing radiative heating by water vapor, and cause seeding of lower level clouds (Murcray, 1970; Knollenberg, 1972). Sedimentation of ice crystals, which has been observed occasionally (Konrad and Howard, 1974; Schumann, 1994; Heymsfield et al., 1998a), becomes important only in strongly supersaturated air (Hauf and Alheit, 1997) when large ice crystals form and have the potential to fall through lower-lying saturated air in which they will not evaporate readily. Because no attempts have been made to quantify the precipitation rate from contrails, the significance of this precipitation has not been assessed.


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