The image above is a simulation (top) and camera (bottom) comparison (320 degree panorama) shown during evening twilight. Crepuscular rays are visible in the southwest in the camera image. The clouds casting these shadows are so far away they are outside the model domain and thus aren't showing up in the simulation. The twilight arch is visible with a gradient of intensity and colors as we near the western horizon. This August and September in Colorado I've noticed enhanced purple colors (and yellow lower arch) in the twilights that seem to be either from a recent volcano or some other type of sulfate pollution. One candidate would be Calbuco that erupted in Chile in April, 2015. This is reminiscent of the volcanic twilights of the 1980s and 1990s though more moderate in intensity. The simulated sky is modeled by considering an additional (mainly) stratospheric aerosol layer from 13-25km high having an optical depth of .026, uniformly distributed within this vertical layer. This reasonably matches my visual impression of recent sunsets, though the all-sky camera has a tendency to wash out the colors.
The photo below was taken on the evening of September 20, 2015 with a regular digital camera and shows the volcanic twilight colors more clearly. The mountains cut off the view within about 3 degrees of the horizon.
Below is a morning twilight view where we can render essentially all the way to the horizon. Even though the photograph immediately above was taken a few days earlier the color match is pretty close to the (360 degree panorama) rendering below.
The photograph below shows some purple crepuscular rays, taken at 0127UTC Sep 27, 2015. These are visible late enough in twilight so that the cirrus in the foreground (partly lit earlier) are already dark. If we also consider the solar depression angle at this time (35 minutes after sunset), it is evident that the altitude of the purple glow and crepuscular rays is quite a bit higher than that of the cirrus. This helps support the scenario of high altitude aerosols being present.
Beautiful twilights have continued with very bright yellow arches noted on October 11 and 12, 2015. The Oct 14 twilight featured purple crepuscular rays that showed up in the automated camera images, in the simulations, and in my visual observations. It would be worth following up with some simulations to quantify the expected brightness of the yellow arch with respect to upper troposphere/lower stratosphere (UTLS) aerosol optical depth. I suspect values could reach .01 or even up to .02. A connection with a recent lunar eclipse is mentioned here.
October 24, 2015 was clear with a very nice display. It started with a prominent Earth Shadow and Belt of Venus, a good sign. Then some purple anti-crepuscular rays started to develop. This all presaged a developing purple glow higher in the west along with a brilliant yellow lower arch. Large (still low to moderate contrast) purple crepuscular rays fanned in the west. A new observation was a subtle texturing of ultra-cirrus, right near the intersection of the purple crepusclar rays and yellow arch, about 5 degrees high in the west. This is my first ultra-cirrus sighting this millennium.
It might be noted that even at the time of sunset, I can generally see an initial stratospheric aerosol signature in the aureole around the sun. It is large and white and more concentrated in azimuth (compared to usual) in the west (as a large semicircle). This is at the same time a smaller orange aureole may still be visible much closer to the sun. In non-volcanic cases, the developing twilight glow isn't so large and white and is more uniform in brightness over the western range of azimuths. In other words, at sunset any orange sky glow is from lower aerosols and the still white component indicates it must be higher. Comparison of the developing twilight with occasional contrails (or isolated clouds) helps provide some insight as well.
Volcanic twilights, often with ultra-cirrus clouds continued in the end of October and beginning of November. The ultra-cirrus actually became more pronounced around the end of November, visible over a wider part of the sky and sooner after the moment of sunset.
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