SkyStef's weather page
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SkyStef's weather page |
Weather picture of the month July 2005
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| Picture taken at Kampenhout on June 14 2005 around 2350 hours. |
| Clouds may form in clear air and
their characteristic forms can
be broadly grouped in a classification in terms of "genera", "species" and
"varieties", eventually accompanied by "supplementary
features or accessory clouds". They may also form or grow from other
clouds, called "mother-clouds". There is a group of clouds that don't fit in this classification: the special clouds. They include nacreous clouds, noctilucent clouds, condensation trails, clouds from waterfalls, clouds from fires, clouds from volcanic eruptions, clouds resulting from explosions and clouds resulting from industry. The definition of a noctilucent cloud is: clouds resembling thin Cirrus, but usually with a bluish or silvery, or sometimes orange to red color; they stand out against the dark night sky. The physical constitution is still unknown. According to Störmer and Vestine (1932) there is some evidence that they are composed of very fine cosmic dust on which ice crystals form. Noctilucent clouds are seldom seen and then only in the northern parts of the middle latitude zone of the northern hemisphere during the summer, when the sun was 5 to 13 degrees below the horizon. Measurements have shown that their altitude ranges between 75 and 90 kilometers. They become visible at about the same time as first magnitude stars. They are at first greyish, then more and more brilliant and, as time advances, they appear bluish white like tarnished silver. This sequence of changes is then repeated in reverse order. Sometimes they appear reddish in the immediate vicinity of the horizon. The clouds are more frequently observed and appear more brilliant after midnight than before midnight. They have been observed to move with speeds ranging from about 100 knots to more than 500 knots, usually from the north-east or east.
The picture shows bright noctilucent clouds which were visible during the night of June 14-15 2005. The spots in the sky are stars. More special clouds via this link |
