Monday, November 23, 2009

Crepuscular rays in desert dust

After the twilights had been getting normal through the past three weeks, where hardly any volcanic aerosoles from Sarychev volcano had been measured, I was very astonished when I saw an intense purple light with crepuscular rays about half an hour before sunrise (sun elevation at -6°) in the morning of November 17. The crepuscular rays crossed the whole sky near the horizon, converging at the antisolar point (1 - 2 - 3).

Of course I immediately asked my colleagues from the Hohenpeissenberg observatory about the phenomenon. And I got a very surprising answer:
At that moment there were two different layers of dust from the Sahara desert above us, a lower one at an altitude of about 8.5 kms with dust from the western parts of the Sahara, and a higher one at about 11 kms, which contained dust from the eastern part of the Sahara. There were two different currents of air at higher levels which overlapped each other above the Alps.
It is new for me to learn that such twilights are also possible in desert dust, just as this dust up to now only caused a kind of certain dimness in the air. But at that moment there was no desert dust directly above us; I only looked into the layers of dust.

However, there was an extra bonus on the next morning. Unfortunately could only watch it from the valley:

Thursday, November 19, 2009

Twinned rainbow

Mark Worme observed this rainbow today at around 3.15 pm Atlantic Standard Time in central Barbados facing east. At first he only noticed the double rainbow, but then he realized the lower one was split.

During a heavy rainshower, a twinning of the upper part of a rainbow can sometimes be observed, which often lasts from a few seconds up to several minutes. As for a long time there were only few observations of this phenomenon available, only speculations could be made about its origin. Only in the past few years, this twinning could be registered more often by continuous observations, and due to some detailed descriptions, new theories could be advanced. As in all observations both bows are of equal brightness, light refraction on ice particles can be ruled out. Most probably is that raindrops of a non-spherical shape produce one of these bows or even both of them. Due to surface-tension, small rain droplets hardly change their shape when falling, but large drops can be flattened by the air resistance. The more flattened they become, the smaller is their refractive index. So the sunlight has to fall upon water drops of different size at the same time to make the twinning appear. As this twinning was up to now observed under big shower or thunder clouds which formed in hot air, it can be supposed that the small, not flattened raindrops evaporate at a short distance below the cloud basis. This would explain why the twinning can only be seen for a short time and exclusively in the upper part of the rainbow. It should be important to determine the radius of the rainbow when the twinning is observed, and to record the weather situation at the time the twinning appears as exactly as possible.

Sunday, November 15, 2009

Shadows and lights of a flight

Many people find flights boring - but not all! If you are lucky to be seated by a window you can always find something interesting in the air beside or below the airplane. David Lukacs from Hungary took this picture on 2nd November 2009 on a flight from Rome to Budapest, about 15 minutes after the departure. A thin layer of haze was between the plane and the sea so the sun shining on the right side above the plane could cast radial shadows on the left below. The beams of shadow and light join at the antisolar point.

A bit later when the plane travelled above a cloud layer David also noticed a nice glory below them:

Even the shadow of the airplane appeared in the middle: