APOD: Plane Contrail and Sun Halo (2017 Apr 04)

[APOD Robot]

Plane Contrail and Sun Halo

Explanation: What's happened to the sky? Several common features of the daytime sky are interacting in uncommon ways. First, well behind the silhouetted hills, is the typically bright Sun. In front of the Sun are thin clouds, possibly the home to a layer of hexagonal ice crystals that together are creating the 22 degree halo of light surrounding the Sun. The unusual bent line that crosses the image is a contrail -- a type of cloud created by a passing airplane. Much of the contrail must actually be further away than the thin cloud because it casts a shadow onto the cloud, giving an unusual three-dimensional quality to the featured image. The featured image was taken in late January in the city of Patras in West Greece.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

[Astronymus]

Conspiracies in 3, 2, 1,...

[dolfen]

That's not a normal jet aircraft contrail nor is that haze layer normal altostratus clouds.

[Magellan10]

The original photo also shows an upper tangent arc at the top of the halo.

[Chris Peterson]

That's not a normal jet aircraft contrail nor is that haze layer normal altostratus clouds.

They both look pretty normal to me.

[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.

The visual contrast from the dark shadow right next to the contrail really makes it visually "pop" as well.

[neufer]

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.

<<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.>>

---

<<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.

<<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.>>

[jude]

I'm novice so I just cannot yet understand that if the contrail is higher than the cloud, how does the shadow appear above the contrail picture taken from the ground?

Plz be gentle.

jude

[neufer]

I'm novice so I just cannot yet understand that if the contrail is higher than the cloud, how does the shadow appear above the contrail picture taken from the ground?

An optical illusion?

Clearly the contrail sweep on the right appears much further to the right in the shadow.

[florid_snow]

Even though it's well below the "freezing temperature" water up there, contrails (like other high altitude clouds) can actually be made of mostly liquid droplets, "super-cooled", they might freeze eventually but for a while that cloud is made of liquid, multiply scattering light of all wavelengths all around including towards the camera, which is why contrails often look bright and puffy white like low altitude cumulus clouds.

Meanwhile the middle atmosphere cloud that is made out of ice particles scatters light totally differently, very wavelength and scattered at an angle light making a halo of colors. The bright white washes out the colors, but the dark shadow doesn't blot them out because their light is coming from an integration of scattered light coming off a significant angle (22 degrees?) from perpendicular to the toward camera direction.

Here's a nice write up for a very similar situation! http://www.atoptics.co.uk/atoptics/contr2.htm

Screen Shot 2017-04-04 at 2.25.03 PM.png (52.31 KiB) Viewed 2122 times