Purple light
The purple light is a twilight phenomenon through differently scattered light components, which appears bright purple in the sky .
Observation and Origin
The purple light appears about 15 minutes after sunset over the western horizon, the sun is then about 2 ° below the horizon. The phenomenon is caused by scattering and reflection from small dust particles and haze in the atmosphere. In the early twilight phase, the angles of incidence of light are favorable and the lower layers of the air can no longer be reached by direct sunlight. The scattered light is therefore less overexposed and therefore less visible.
The appearance, visibility and strength of the purple light are strongly dependent on the weather and observation conditions; In addition to air pressure and wind conditions in the stratosphere , the content and distribution of scattering particles in the air layers are decisive. The causes of noticeable purple light can be forest fires, volcanic eruptions and air pollution over large cities. Then the purple twilight that follows the purple light can also be impressive.
The physical prerequisites for the purple light are scattering of sunlight in layers of air with different proportions of scattered light, which reach the observer with both a long-wave (red) and a short-wave (blue) part. Because the color perception of a purple shade only arises when different color stimuli from both ends of the visible spectrum meet simultaneously on the retina of the eye , and is a qualitative peculiarity of human color perception (see also purple line ). In this case it is the perception of the superimposed parts of red scattered light from hazy layers and of blue scattered light from higher layers of the atmosphere. The blue parts reach the observer through Rayleigh scattering of sunlight on air molecules. The haze in the deeper layers cause Mie scattering . In rare cases there can be a reflection of the purple light on the opposite horizon compared to the set sun, during the (lower) counter- twilight .
The reflections on cloud formations in the troposphere , popularly known as dawn or evening red , are not the effect known as purple light. Not infrequently conspicuous twilight rays are observed in connection with purple light . Their appearance has nothing to do with the purple light; however, they are much more noticeable when projected onto a purple light than in a normal twilight with a blue sky background.
After the appearance of intense purple light over large parts of Western and Central Europe in February 2008, polar stratospheric clouds (PSC) are now suspected as a possible atmospheric condition. The scattering of particles such in the dawn-lit over 20 km of high clouds was probably the special circumstance for a spectacular purple light that on 6 December 2002 polar night on Spitsbergen enlightened almost as bright as day, as "Red Sky Event".
Individual evidence
- ↑ An extensive description of the chronological sequence of dusk can be found in Karl Mütze, Leonhard Foitzik, Wolfgang Krug, Günter Schreiber: ABC der Optik . VEB FABrockhaus Verlag, Leipzig 1961, keyword: twilight phenomena.
- ↑ Color magic in the twilight - The purple light
- ↑ C. Hinz, P. Krämer, GP Can: Polar Stratospheric Clouds over Western Europe ( Memento of the original from April 2, 2015 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Weather - April 2009, Vol. 64, No. 4 (English)
- ↑ F. Sigernes et al .: The Mysterious Red Sky on December 6 2002
