by neufer » Tue Apr 04, 2017 6:07 pm
florid_snow wrote:
Very interesting! I think the contrail is at higher altitude than the ice layer the whole time, and it looks reasonably fresh, probably made of mostly small liquid cloud droplets, which do multiple scattering of the sunlight, bouncing it all around so there's no wavelength dependence and it appears white and is adding light along a path to the camera. This washes out the ice halo colors, making it seem like it is on top of them, but really it is behind.
Contrail ice crystals may simply not be the right kind (bullet-shaped ice columns?) to produce a 22° halo.
The length of the contrail shadow indicates that it is well above the (altostratus?) cloud layer.
https://en.wikipedia.org/wiki/Contrail wrote:
<<Contrails (short for "condensation trails") are line-shaped clouds sometimes produced by aircraft engine exhaust, typically at aircraft cruise altitudes (
25,000 to 40,000 ft). Contrails are composed primarily of water, in the form of ice crystals. Impurities in the engine exhaust from the fuel, including sulfur compounds (0.05% by weight in jet fuel) provide some of the particles that 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.>>
https://en.wikipedia.org/wiki/22%C2%B0_halo wrote:
<<A 22° halo is an optical phenomenon that belongs to the family of ice crystal halos, in the form of a ring with a radius of approximately 22° around the Sun or Moon. It forms as the sun- or moonlight is refracted in millions of hexagonal ice crystals suspended in the atmosphere. Like other ice halos, 22° halos appear when the sky is covered by thin cirrus or cirrostratus clouds (
above 18,000 ft).
The exact shape and orientation of the ice crystals responsible for the 22° halo are still the topic of debate. Hexagonal, randomly oriented columns are usually put forward as the most likely candidate, but this explanation posits problems, such as the fact that the aerodynamic properties of such crystals leads them to be orientated horizontally rather than randomly.
Alternative explanations include the involvement of clusters of bullet-shaped ice columns.
https://en.wikipedia.org/wiki/Altostratus_cloud wrote:
<<Altostratus is a middle altitude (
6,500–24,000 ft) cloud genus belonging to the stratiform physical category characterized by a generally uniform gray to bluish-green and sheet or layer. Altostratus is formed by the lifting of a large mostly stable air mass that causes invisible water vapor to condense into cloud. Altostratus may be composed of ice crystals. In some ice crystal altostratus, very thin, rapidly disappearing horizontal sheets of water droplets appear at random. The sizes of the ice crystals in the cloud tended to increase as altitude decreased.
During the sampling of one cloud, the scientists noted a halo while flying near the top of the cloud.>>
[quote="florid_snow"]
Very interesting! I think the contrail is at higher altitude than the ice layer the whole time, and it looks reasonably fresh, probably made of mostly small liquid cloud droplets, which do multiple scattering of the sunlight, bouncing it all around so there's no wavelength dependence and it appears white and is adding light along a path to the camera. This washes out the ice halo colors, making it seem like it is on top of them, but really it is behind.[/quote]
Contrail ice crystals may simply not be the right kind (bullet-shaped ice columns?) to produce a 22° halo.
The length of the contrail shadow indicates that it is well above the (altostratus?) cloud layer.
[quote=" https://en.wikipedia.org/wiki/Contrail"]
<<Contrails (short for "condensation trails") are line-shaped clouds sometimes produced by aircraft engine exhaust, typically at aircraft cruise altitudes ([b][color=#FF00FF]25,000 to 40,000 ft[/color][/b]). Contrails are composed primarily of water, in the form of ice crystals. Impurities in the engine exhaust from the fuel, including sulfur compounds (0.05% by weight in jet fuel) provide some of the particles that 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.>>[/quote][quote=" https://en.wikipedia.org/wiki/22%C2%B0_halo"]
[float=left][img3=""]https://upload.wikimedia.org/wikipedia/commons/thumb/5/57/Path_of_rays_in_a_hexagonal_prism.png/220px-Path_of_rays_in_a_hexagonal_prism.png[/img3][/float]<<A 22° halo is an optical phenomenon that belongs to the family of ice crystal halos, in the form of a ring with a radius of approximately 22° around the Sun or Moon. It forms as the sun- or moonlight is refracted in millions of hexagonal ice crystals suspended in the atmosphere. Like other ice halos, 22° halos appear when the sky is covered by thin cirrus or cirrostratus clouds ([b][color=#FF00FF]above 18,000 ft[/color][/b]). [b][u]The exact shape and orientation of the ice crystals responsible for the 22° halo are still the topic of debate[/u][/b]. Hexagonal, randomly oriented columns are usually put forward as the most likely candidate, but this explanation posits problems, such as the fact that the aerodynamic properties of such crystals leads them to be orientated horizontally rather than randomly. [b][u]Alternative explanations include the involvement of clusters of bullet-shaped ice columns.[/u][/b][/quote][quote=" https://en.wikipedia.org/wiki/Altostratus_cloud"]
<<Altostratus is a middle altitude ([b][color=#FF00FF]6,500–24,000 ft[/color][/b]) cloud genus belonging to the stratiform physical category characterized by a generally uniform gray to bluish-green and sheet or layer. Altostratus is formed by the lifting of a large mostly stable air mass that causes invisible water vapor to condense into cloud. Altostratus may be composed of ice crystals. In some ice crystal altostratus, very thin, rapidly disappearing horizontal sheets of water droplets appear at random. The sizes of the ice crystals in the cloud tended to increase as altitude decreased. [b][u]During the sampling of one cloud, the scientists noted a halo while flying near the top of the cloud.[/u][/b]>>[/quote]