Portal:Atmospheric sciences/09-2006

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September 2006

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Brocken Spectre of a Caravelle aircraft, photographed from an altitude of 35000 feet over France. A circular rainbow surrounds the shadow of the aircraft. The Caravelle's wing appears in the left foreground.

A Brocken spectre, also called Brocken bow or mountain spectre, observed and described by Johann Silberschlag in 1780, is the apparently enormously magnified shadow of an observer cast, when the Sun is low, upon the upper surfaces of clouds that are below the mountain upon which he stands. The apparent magnification of size of the shadow is an optical illusion that occurs when the observer judges his shadow on relatively nearby clouds to be at the same distance as faraway land objects seen through gaps in the clouds. The phenomenon is often observed on mountain peaks but is recorded in literature with special reference to The Brocken, a peak in the Harz Mountains in Germany where the Brocken spectre sometimes produces spectacular effects.

Did you know ...

Calima is fine dust or sand blown over the Atlantic Ocean from the Moroccan Sahara Desert to the Canary Islands. This phenomenon can happen at any time of year but is usually associated with the hot air found over the islands during the summer months, ranging in time scale from a few hours up to a week. The Calima is caused by a duststorm that is stirred up by high winds in the Sahara and is then driven over the Canary Islands by south easterly winds. The fine sand particles cause the air to become thick and visibility becomes rather like that experienced during a thick fog, depending on the severity. During the Calima, every surface will be covered in fine reddish brown dust. Many Canarian inhabitants experience allergy-like sinus and respiratory problems similar to those of hayfever during the Calima.


A stratocumulus cloud belongs to a class characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumuli, and the whole being at a lower altitude, usually below 2,400 m (8,000 ft). Weak convective currents create shallow cloud layers because of drier, stable air above preventing continued vertical development.

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Meteorology and history

The frieze of the tower showing the Greek wind gods Boreas (north wind, on the left) and Skiron (northwesterly wind, on the right).

The Tower of the Winds, also called horologion (timepiece), is an octagonal Pentelic marble tower on the Roman agora in Athens. It was supposedly built by Andronicus of Cyrrhus around 50 BC, but according to other sources might have been constructed in the 2nd century BC before the rest of the forum.


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Picture of the month

Noctilucent clouds over Lake Saimaa

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