Flying across the county, what are some phenomena you're more likely to observe than when travelling by train or car?

If you're travelling long distances, such as from coast to coast, flying has obvious advantages over driving or taking the train. In today's fast-paced world, time is one of the most important considerations involved in any decision making process. More than ever, time is linked to money, and any delay winds up in the debit column. Perhaps this is one reason why there are so many grouchy travelers today.

Anyway, aside from saving time, travelling aloft sometimes provides less obvious rewards too. The key to reaping these rewards is to get a window seat. Not just any old window seat will do. Even though we've figured out how to lift tons of steel and aluminum off the ground, we apparently still need wings to do so. Unfortunately, these wings are pretty big, and they're positioned near where most passengers are seated. If we had to pay more for a window seat, we'd have something to gripe about, especially since the windows aren't much bigger than a piece of paper and you have to bend your head to look out. Despite the airlines burgeoning debt, they haven't stooped this low just yet.

For the best views, you'll usually need to sit in the very front or in the back of the plane. If you're on a 747, stay away from the wing by selecting row 40 or beyond. In a 737, avoid rows 8 -17, and in a 727, reserve a seat in one of the first 10 rows or the final 5. Also, if you're on an Airbus, 757, 767, or 777, take any of the final 15 rows. Now you have an idea where to sit, but you should also give a little though to what side of the plane you should be on -- this is related to your destination, the time of day you're flying and what you want to see. For instance, if you're flying at night from Boston to Seattle, in the late fall, take a seat on the right hand side of the plane (facing the cockpit), if you're hoping to catch a glimpse of the northern lights.

While some of the things you'll see can be fascinating, you're not likely to win any friends by pointing them out. These are just for you to enjoy -- the guy in the next seat, the purser and the pilot are busy with other things.

As you gain altitude and happen to pass through a cloud deck while the Sun is on the opposite side of the plane, you may be able to see a glory -- an image of the jet you're riding on ringed by subdued colors. Glories are quite often seen by passengers as an aircraft moves between clouds and Sun. This optical phenomena results from diffraction, in this case, the back-scattering of light by small droplets of water. The radius of the glory is dependent on the size of the drops - the smaller the drops, the bigger the glory. If you're earthbound, while you won't be able to see the glory, under special circumstances you can see its first cousin, the Brocken Spectre.

Rainbows can be spied from above the Earth as well as on the surface. However, from above, rainbows look somewhat different inasmuch as they're not the familiar graceful arcs we're accustomed to seeing but rather often inscribe a complete circle. In essence, from an airplane there's no horizon to block the bow. Rainbows are actually circles of about 42º radius centered on a point directly opposite the Sun (antisolar point). Since on the surface you can only detect rainbows if the Sun is less than 42 degrees above the horizon, you're never able to view a truly circular bow, except perhaps from a mountain precipice or looking down upon a waterfall. From a flat surface, rainbows may arch high in the sky, but the fact that rain stops when it hits the ground insures that the term rain-circles never became part of our vocabulary.

Once you've achieved cruising altitude, you're essentially above the weather and any weather-related clouds. This affords you the best views of phenomena such as the aurora, from early fall through early spring, and perhaps even noctilucent clouds, in spring and summer. Noctilucent clouds form at very high altitudes (between 75-90 km above the Earth’s surface) in the polar mesosphere and are quite separate phenomenon from tropospheric (weather) clouds. Thought to be ice crystals condensing on meteoritic dust, these shiny looking clouds appear somewhat similar to cirrus clouds, but they can only be observed during twilight hours, when the Sun is below the horizon and illuminates the highest levels of the atmosphere. You'll have a much better chance to observe them if you're flying across Canada or the northern tier of the U.S. Of course, you can see the northern lights and noctilucent clouds just fine from terra-firma, but on those precious occasions when they're visible in the mid latitudes, clouds often interfere with your view.

If the sky is clear all the way to the surface, you should be able to detect the opposition effect. This is a brightened, fuzzy patch on the ground (about 1-2 degrees across) at the antisolar point. It's especially noticeable if the landscape is uniform, a pine forest, for instance. These patches have several causes, including "shadow hiding." Basically, when the Sun is directly opposite where you're looking, shadows are minimized. The same thing occurs when you look at the full Moon. It's about 9 times brighter than the Moon when at 1/4 phase not only because more of its surface is illuminated but because shadows are nearly absent.

If you happen to fly over cirrus clouds, which are composed of hexagonal ice crystals, it's possible to see a feature known as the subsun. This is a colorless, elliptical spot seen directly below the Sun. At times, it can be extremely bright. The subsun is essentially an image of the Sun reflected from the upper side of any ice crystal having a horizontal face. Keep an eye out too for the shadow of the contrail from your jet as it flickers off a cloud deck below.

Another feature that can only be viewed when aloft is a cloud contrast bow. When looking down at a cloud deck, near the antisolar point, on occasion, a notion of circularity exists. While there may be minimal brightening, these features are devoid of color. The motion of the aircraft acts to enhance the visibility of these subtle bows. They certainly won't knock your socks off or entertain you as much as watching "Meet the Fockers" for the third time, but they might stimulate your brain ever so slightly. However, it may not be advisable to wake up someone you care about from a sound sleep in order to describe them. Save this phenomenon for repartee at a party, but don't be surprised if you're not invited back.

To see a photo of a glory, click on
http://epod.usra.edu/archive/epodviewer.php3?oid=86784
To see a photo of a Brocken Spectre, click on
http://epod.usra.edu/archive/epodviewer.php3?oid=184384
To see a photo of Noctilucent Clouds, click on
http://epod.usra.edu/archive/epodviewer.php3?oid=157248

Also, check out Bob Berman's book Strange Universe, Chapter 6 (Times Books) and Color and Light in Nature by D. Lynch and W. Livingston (Cambridge Press).

This week's question is provided by Dr. James Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for EIGHT YEARS! This year Dr. Foster has decided to share the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis.