Monday, December 15, 2014

Another Classroom Session From Our Trip!

My bloggin' buddy Ken Spencer and I both love to watch the country slide past our windows while flying.  We're both amazed more people don't watch - on our trip last week to the Midwest, most even had their shades down!  Besides illustrations of fluid dynamics, shown in the last post, it was a classroom in other effects, even though it was cloudy for most of the trip. 

On a trip just over a year ago, I caught my first "subsun", documented in another travel post about this time last year. Les Cowley, who runs the "Optics Picture of the Day", a great website of atmospheric phenomenon, even devoted a page to the image last Summer!  While evidently pretty common, subsuns appear low with the sun high in the sky, so are likely normally unobserved.  This trip, about an hour before our arrival in Chicago, likely somewhere near Kansas City if we kept to our normal route, I spotted one, this time without "subsun-dogs".  It appeared as a very good, though dimmed, image of the sun without much scatter to blur the suns reflection from aligned flat ice crystals.  From my first image to the last, 6 frames were obtained in 90 seconds - the extent of the subsun data collection this trip.

The clouds were dense and constant from central New Mexico to our arrival in O'Hare, and as we banked for final approach, were still above the cloud deck.  I hoped-for and watched, but didn't see the Willis Tower (second-tallest building in the country) poking above the clouds.  We usually have a fine view of the skyline during that final turn.

This time, however, we had an even more interesting view. Now headed west towards O'Hare, and us on the right side of the plane, we were treated to seeing our shadow. If you had been looking, you would have seen a shadow of the plane, yes, but something extra - circular bands - a glory! The photo at left was taken with the camera at full zoom of 85mm focal length, with minimal adjustments of brightness, contrast, and a slight boost of color saturation. I'll let you go to Les Cowley's page about glories to learn more rather than me wave my arms here to explain. Make sure you look at the several pages of illustrations and explanations and other examples. Now what is really cool is that you can infer the water drop size from the diameter of the glory! The subtended diameter is inversely proportional to the droplet diameter. For grins, I took the full-frame image at left, and cranked the color saturation, and made some measurements, shown at right. The sensor size is 24mm long, and the number I've carried in my head from college is that 1 degree for any lens is .01744 X the focal length, in this case, 1.48mm.  Knowing the scale, the red rim of the first ring is 6.5 degrees.  With Les' equation from one of those pages, the droplet size works out to 19 microns - pretty cool!

Two minutes later, we approached the cloud deck and I shot again, this time with the lens set much wider to get in the whole shadow.   Note that the glory indicates where we were sitting, between the wing and the rear-mounted engines near the back of the plane.  As Les points out, every observer sees their own personal glory - the one the pilot sees would be centered on the front tip of the plane...  The calculation of the droplet size should be independent of the lens focal length, so I repeated the calculations on another image with the saturation cranked for this image's 33mm focal length.  This time I got 21 micron diameter droplets, a difference of 10% from the first calculation - good enough for me!  Whether the difference indicates a real change in droplet size, or an uncertainty in the or measurement remains to be seen...

The last science demonstration occurred minutes later as we descended below the clouds.  I saw what looked like a flash of "white smoke", but was actually water vapor condensing in the pressure difference caused by the turbulence between the flaps and ailerons.  The flaps, fully extended, with the ailerons in their neutral position, caused enough of a density change to cause moisture to condense into a stream (arrowed at left).  Again, the world is a science laboratory with much to observe if you only look!

No comments: