Contrails and Aviation-cirrus
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:https://www.grc.nasa.gov/
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.
--CECIL ADAMS firstname.lastname@example.org
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 126.96.36.199).
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
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.
3.4.5. 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).