Trailspotter
Senior Member.
From a plane cruising at high altitudes, contrails behind other planes may sometimes appear dark while the planes themselves being sunlit.
from http://www.nasa.gov/topics/aeronautics/features/access_fuel.html
Like cirrus clouds, contrails are aerosols of tiny ice crystals, which themselves are colourless. The white colour of a contrail is due to the scattering of sunlight on these crystals. For the same reason, sunrise and sunset contrails acquire red hues. A contrail will appear dark, if the amount of light it scatters toward the observer is less than the amount of light coming from its background. At twilight, for example, the contrails in the Earth shadow are dark, but so the planes are. To appear bright the plane is ought to reflect more light than comes from the background. How can a right balance between the amounts of light scattered by the contrail, reflected by the plane and coming from the background be achieved?
The two types give most of the scattering from the atmosphere: Rayleigh scattering by gas molecules and Mie scattering by aerosol particles.
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html
Because aerosols scatter much more light forward than backward, backlit contrails are brighter than frontlit ones. In contrast, the planes are the brightest if they are illuminated from the front, in this case they also outshine their contrails:
Left images are of backlit contrails and planes; right images are of frontlit ones.
Therefore, if the Sun is behind the observer, it is possible in principle to have the background light brighter than the contrail but dimmer than the plane. However, it is (almost) impossible to have such lighting conditions for the ground observations. This is because contrails are normally formed at altitudes where the atmospheric pressure (about 250 kPa) is only one fourth of that at the sea level.
This means that the bulk (three quarters) of the air lies below the contrails, that is, in the foreground. Diffuse light scattered by the foreground air makes contrails dimmer but not darker. In contrast, for the contrails observed from about the same high altitudes, there is much more air behind them than in front of them. The intensity of light scattered by the huge mass of background air can in principle exceed the intensity of light scattered back by a frontlit contrail. There may be additional factors that can tip the balance toward "dark contrails". The presence of a high altitude haze will dim the contrail by reducing the direct sunlight and increase the background light at the same time, whereas light reflected from the ground or low clouds will add to the background illumination.
Ground level contrails also have all light-scattering atmosphere in the background and therefore may appear dark against the sky:
from http://www.esrl.noaa.gov/gmd/Photo_Gallery/Field_Sites/SPO/Emrys_Hall_Album/index.html
from http://www.nasa.gov/topics/aeronautics/features/access_fuel.html
Several previous Metabunk threads have been discussing this phenomenon in connection with the claims about the contrails' dark colour being due to a deliberate injection of chemicals into the engine exhaust or its contamination by incompletely burnt fuel (L1, L2, L3). These were rebutted by the notion that perfectly normal contrails might appear dark at certain lighting conditions. However, a proper physical explanation of why and how these conditions occur was lacking.The modified HU-25 Falcon probes the exhaust contrails from NASA's DC-8 flying laboratory as both aircraft enter a turn at about 35,000 feet altitude during the first data-collection flight in restricted test airspace over California's high desert.
Image Credit: NASA / Lori Losey
Like cirrus clouds, contrails are aerosols of tiny ice crystals, which themselves are colourless. The white colour of a contrail is due to the scattering of sunlight on these crystals. For the same reason, sunrise and sunset contrails acquire red hues. A contrail will appear dark, if the amount of light it scatters toward the observer is less than the amount of light coming from its background. At twilight, for example, the contrails in the Earth shadow are dark, but so the planes are. To appear bright the plane is ought to reflect more light than comes from the background. How can a right balance between the amounts of light scattered by the contrail, reflected by the plane and coming from the background be achieved?
The two types give most of the scattering from the atmosphere: Rayleigh scattering by gas molecules and Mie scattering by aerosol particles.
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html
Because aerosols scatter much more light forward than backward, backlit contrails are brighter than frontlit ones. In contrast, the planes are the brightest if they are illuminated from the front, in this case they also outshine their contrails:
Left images are of backlit contrails and planes; right images are of frontlit ones.
Therefore, if the Sun is behind the observer, it is possible in principle to have the background light brighter than the contrail but dimmer than the plane. However, it is (almost) impossible to have such lighting conditions for the ground observations. This is because contrails are normally formed at altitudes where the atmospheric pressure (about 250 kPa) is only one fourth of that at the sea level.
This means that the bulk (three quarters) of the air lies below the contrails, that is, in the foreground. Diffuse light scattered by the foreground air makes contrails dimmer but not darker. In contrast, for the contrails observed from about the same high altitudes, there is much more air behind them than in front of them. The intensity of light scattered by the huge mass of background air can in principle exceed the intensity of light scattered back by a frontlit contrail. There may be additional factors that can tip the balance toward "dark contrails". The presence of a high altitude haze will dim the contrail by reducing the direct sunlight and increase the background light at the same time, whereas light reflected from the ground or low clouds will add to the background illumination.
Ground level contrails also have all light-scattering atmosphere in the background and therefore may appear dark against the sky:
from http://www.esrl.noaa.gov/gmd/Photo_Gallery/Field_Sites/SPO/Emrys_Hall_Album/index.html
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