Katabatic Wind

the sinking and downward movement of cold, dense air beneath warmer, lighter air. The air is cooled by radiation, usually at night. It occurs over ice-covered surfaces and in the fjords of Norway, and in many cases can be gale force.

Taken from Dictionary of Science

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A katabatic wind, from the Greek word katabatikos meaning "going downhill", is the technical name for a drainage wind, a wind that carries high density air from a higher elevation down a slope under the force of gravity. Such winds are sometimes also called fall winds.

Not all downslope winds are katabatic. For instance, winds such as the Foehn, Chinook or Bergwind, are rain shadow winds where air driven upslope on the windward side of a mountain range drops its moisture and descends leeward drier and warmer.

Katabatic winds can rush down elevated slopes at hurricane speeds, but most are not that intense and many are on the order of 10 knots or less.

Examples of true katabatic winds include the Mistral in the Mediterranean, the Bora (or Bura) in the Adriatic, the Santa Ana in southern California, and the Oroshi in Japan. Another example is "The Barber," an enhanced katabatic wind that blows over the town of Greymouth in New Zealand when there is a southeast flow over the South Island. It is a wind that is known in the area for its coldness.

A katabatic wind originates from the cooling by radiation of air atop a plateau, a mountain, glacier, or even a hill. Since the density of air is inversely proportional to temperature, the air will flow downwards, warming adiabatically as it descends. The temperature of the wind depends on the temperature in the source region and the amount of descent. In the case of the Santa Ana, for example, the wind can (but does not always) become hot by the time it reaches sea level. In the case of Antarctica, by contrast, the wind is still intensely cold.

Katabatic winds are most commonly found blowing out from the large and elevated ice sheets of Antarctica and Greenland. The buildup of high density cold air over the ice sheets and the elevation of the ice sheets brings into play enormous gravitational energy, propelling the winds well over hurricane force. In Greenland these winds are called Piteraq and are most intense whenever a low pressure area approaches the coast.

In the Fuegian Archipelago (or Tierra del Fuego ) in South America as well as in Alaska, a wind known as a williwaw is a particular danger to harbouring vessels. It originates in the snow and ice fields of the coastal mountains. Williwaws commonly blow as high as 100 knots, and 200 knot williwaws have been reported.

Taken from Wikipedia

Dark Nebulae

clouds of thick gas and dust common throughout the MILKY WAY and other GALAXIES.

Taken from Dictionary of Science

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A dark nebula is a type of interstellar cloud that is so dense that it obscures the light from the background emission or reflection nebula (e.g., the Horsehead Nebula) or that it blocks out background stars (e.g., the Coalsack Nebula). The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of larger molecular clouds. Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds. Isolated small dark nebulae are called Bok globules. Like other interstellar dust/material, things it obscures are only visible using radio waves in radio astronomy.

Dark clouds appear so because of submicrometre-sized dust particles, coated with frozen carbon monoxide and nitrogen, which effectively block the passage of light at visible wavelengths. Also present are molecular hydrogen, atomic helium, C¹⁸O, CS, NH₃ (ammonia), H₂CO (formaldehyde), c-C₃H₂ (cyclopropenylidene) and a molecular ion N₂H⁺ (diazenylium), all of which are relatively transparent. These clouds are the spawning grounds of stars and planets, and understanding their development is essential to understanding star formation.

The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes. The largest dark nebulae are visible to the naked eye, appearing as dark patches against the brighter background of the Milky Way.

In the inner regions of dark nebulae important events take place, such as the formation of stars and masers.

Taken from Wikipedia