Blog Archives
Wake vortices caused by planes
When air humidity is high, sometimes wake vortices can be observed on the wings of a plane. These are an accessory phenomenon of the ascending force, which needs a certain underpressure to be effective. This underpressure makes the air flow from beneath the wing to its surface for pressurization. As in these vortices there is an area of especially low pressure, the air cools down adiabatically here, often reaching temperatures below the dew point. This makes the water vapour in the air condensate to steam or fog, making the wakes visible.
In the morning of October 8, 2012, Renate Possiel could take a photograph of this phenomenon from the control tower of Munich airport. That day there were wafts of mist with different ranges of sight on the runways. More photographs: 1-2-3
Another reason for wake vortices to form is the downward acceleration of the air along the wings when the plane is ascending. At low temperatures and high humidity, also here visible condensation can occur. When a plane passes near the sun, sometimes an iridescence of the wakes can be observed, as showed in this photograph taken by Gabor Metzger.
More often iridescence can be observed on contrails, especially when they are very “young”. Also these photographs (1-2-3) taken by Gabor Metzger.
Another articles to this topic:
Norway iridescent foehn clouds
On February 6, 2011, Dirk Steinborn observed these beautiful iridescent foehn clouds in Favang (about 1-hour-drive north of Lillehammer, Norway). The photographs were taken between 14 and 15 hours CET. The observer supposes: “Probably there weren´t any mother-of-pearl clouds. It is cirrus, but also some medium level clouds had a reddish colour.”
Ice particle iridescence?
Normally, iridescence shows rather faint colours which can only be seen by covering the sun. On December 6, 2010, however, iridescence was visible in such a brightness and colourfulness in high level clouds that at first sight it rather looked like halo fragments than like iridescence. (1-2-3)
Manfred Nehonsky also observed extremely bright iridescence on high level foehn clouds over Upper Austria the same day. This iridescence looked like bright mother-of-pearl-clouds.
Another observation made the same day, but by mistake entitled as a halo, can be found here.
I think this is iridescence on globular ice particles as Paul J. Neiman and Joseph A. Shaw suggested in their article “Coronas and Iridescence in Mountain Wave Clouds Over Northeastern Colorado“.
Author: Claudia Hinz, Germany
Iridescent Fireworks Smoke
Like everywhere around the world, New Years Day was also welcomed with fireworks around Mt. Wendelstein. It is always a special highlight to watch the fireworks from the top of the mountain at 1838 meters above sea level. In the Leitzach Valley, about 1000 meters lower, there was a fireworks display.
Even when watching it with the naked eye, the smoke and fog of the fireworks seemed to show iridescent colours. The photographs (photo) show the iridescence more obviously. It was probably caused by the condensation nuclei from the fireworks smoke, on which small droplets condensated. As short time before an area of precipitation had passed, air humidity was still very high.
Additionally, the fireworks caused a thin layer of fog over almost the whole alpine foothills area (photo), and the big city of Munich with more than a million inhabitants, was covered by a thick layer of clouds (photo).
Similar things were reported by other observers. In Bochum, Peter Krämer observed that light graupel turned into snow during the fireworks, leaving about a centimetre of snow. On the weather radar it could be seen that a precipitation area formed right over the Ruhr area just after midnight.
Two years ago, thin fog with visibilities around 300 meters thickended after the New Years Fireworks, forming a dense layer of fog with visibilities which were less than 10 meters in some places.
Posted by Claudia Hinz
Aircraft Iridescence
While taking wildlife pictures in a nature reserve close to the Vienna, Austria, airport, Franz Kerschbaum noticed this 747 jet aircraft in a landing approach with a huge condensation cloud behind its wings. As it moved closer to the runway, he was evidently at just the right position for this shocking but beautiful artificial iridescent cloud to come into view. Iridescence and coronas are diffraction phenomena. The pastel or metallic colors result from deflection of sunlight about minute water droplets.
Photo details: Canon EOS 30D camera, 100-400 mm/4.5-5.6L lens at 400 mm and f/8.0
Extremely bright iridiscence in Spain
On Saturday, February 10, 2007, Ramón Baylina shot these photographs of incredibly bright and colouful iridescent clouds at the Port of Tarragona (Spain). The pictures were taken at about 5 pm with a 200-mm-zoom lens.”
More pictures are here
Shock wave refraction and iridescence over airplane wing
The observation of Monika with the beautiful colored contrail pictures made me remember to post the following photos. On a flight in a 747-400 from Beijing to San Francisco on August 18th I was seated right over the wing on port side of the aircraft. Initially I was disappointed with this window seat but I did get to see some interesting things.
The first of these was a condensation cloud over the wing. The airplane was still ascending around 20,000 ft, when we flew through (super)saturated air. The faster airflow over the top surface of the wing and associated drop in pressure caused condensation. The cloud showed beautiful iridescent colors. The water droplets in the cloud were so small at this stage and all so uniform in size that they diffracted the light with constructive interference over large angles.
On the second photo this condensation cloud extends behind the wing. This is because it takes energy for a water droplet to form in supersaturated air, but once the droplet forms it quickly grows (or it quickly evaporates if the air is not supersaturated, as in the first photo). In other words, when the air is supersaturated, all that is needed is a trigger to initiate droplet formation, after which growth occurs because the air is supersaturated.
This droplet growth is responsible for the colored trail between the exhaust contrails that can sometimes be seen, as in the photos posted earlier. But most often, the air is either not supersaturated, or already condensed into a cloud. Then, no long trail forms (only the engine contrails perhaps), or it is not visible (within cloud). Therefore it is not common to see this effect, especially from ground.
At times a huge and bright corona formed around the sun. In this photo the airplane was banking right and the sun appeared low over the wing. My apologies for the low quality photo – it is very hard to photograph through an airplane window with the sun in view.
The other interesting thing, which I had observed before, but only now could photograph well, is the vertical standing shock wave that sometimes can be seen dancing above the wing of a commercial jet. Here, because of the condensation cloud, this shock wave actually became visible. Usually it can only be seen due to the feeble refraction and miraging of structures on the wing (you won””t notice this unless you look very carefully and at the right moment when the shock is positioned so that you are in its plane).
The normal shock occurs because air flowing over the wing has to flow faster than below it, and this flow can briefly reach the speed of sound even if the jet airplane is flying slower than the speed of sound. Because the Mach number is about 1, the shock wave is almost perfectly vertically oriented, normal to the airflow.
The shock plane in the photo can be seen as the vertical plane of enhanced condensation. I believe this is the rarefaction shock (as opposed to the compression shock) because condensation is enhanced behind the plane, indicating lower air pressure. Presumably the compression shock was somewhere ahead of this, invisible to me since I was not positioned in its plane.
The sharp density gradient at the shock wave causes a lateral mirage, but any miraging can only be seen if you are looking near grazing incidence along the shock plane (which is usually not perfectly planar but a little curved). The last two photos better show the miraging along the shock plane. Note the effect on the pylon of engine nr. 1.
Author: Harald Edens
Alder corona with iridescence and cloud rays
The 27th of March was in a manner of speaking one of our first warm days this year. Owing to the long and cold winter together with the abrupt change the plants exploded into life and produced a very strong pollen concentration in the air. The humidity was very low and so the resulting pollen coronae stood out against the dry sky. The coronae originated from from alders which are very common in the area (Barsinghausen, near Hannover). Hazel were also flowering at that time but would have made a different form of corona.
That day Reinhard Nitze took very unusual pictures of the alder coronae. On the one hand the coronae were intense but appeared together with another phenomena, e.g. with iridescence clouds or cloud rays. Sometimes, however, Reinhard saw them “undecorated”.
Condensation Trail Iridescence
Jurgen de Boer (site) imaged this aircraft and contrail near to sunset on 2nd May. The aircraft was 10-20° from the sun. The structured iridescence is interesting. Seven or more colour bands are visible. The water droplets or ice crystals of the condensation trail will have had similar formation histories. Perhaps this produced rather uniform sizes and mean size variation along the trail thus accounting for the uniform colour bands?
Posted by Les Cowley
