| 词条 | Upper and lower tangent arcs |
| 释义 |
DescriptionUpper arcThe shape of an upper tangent arc varies with the elevation of the sun; while the sun is low (less than 29–32°) it appears as an arc over the sun forming a sharp angle. As the sun rises, the curved wings of the arc lower towards the 22° halo while gradually becoming longer. As the sun rises over 29–32°, the upper tangent arc unites with the lower tangent arc to form the circumscribed halo.[3] Lower arcThe lower tangent arc is rarely observable, appearing under and tangent to a 22° halo centred on the sun. Just like upper tangent arcs, the shape of a lower arc is dependent on the altitude of the sun. As the sun slips over the horizon the lower tangent arc forms a sharp, wing-shaped angle below the sun. As the sun rises above over the horizon, the arc first folds upon itself and then takes the shape of a wide arc. As the sun reaches 29-32° over the horizon, it finally begins to widen and merge with the upper tangent arc to form the circumscribed halo.[4] Since by definition, the sun elevation must exceed 22° above the horizon, most observations are from elevated observation points such as mountains and planes.[4] OriginBoth the upper and lower tangent arc form when hexagonal rod-shaped ice crystals in cirrus clouds have their long axis oriented horizontally. Each crystal can have its long axis oriented in a different horizontal direction, and can rotate around the long axis. Such a crystal configuration also produces other halos, including 22° halos and sun dogs; a predominant horizontal orientation is required to produce a crisp upper tangent arc. Like all colored halos, tangent arcs grade from red towards the Sun (i.e., downwards) to blue away from it, because red light is refracted less strongly than blue light.[2] See also{{Commons category|Tangent arcs}}
References1. ^{{cite web|url=http://www.photolib.noaa.gov/historic/nws/wea00195.htm |title=A magnificent halo |publisher=NOAA |date=1980-12-21 |accessdate=2007-04-14 |archiveurl=https://web.archive.org/web/20061213123117/http://www.photolib.noaa.gov/historic/nws/wea00195.htm |archivedate=2006-12-13 |deadurl=yes |df= }} 2. ^1 {{cite web | url = http://www.atoptics.co.uk/halo/column.htm | title = Tangent Arcs | publisher = Atmospheric Optics | accessdate = 2015-04-07 | author = Les Cowley (?)}} 3. ^{{cite web | url = http://www.meteoros.de/arten/ee05e.htm | title = Upper Tangent Arc | publisher = Arbeitskreis Meteore e.V. | accessdate = 2007-04-15 | author = Les Cowley}} 4. ^1 {{cite web | url = http://www.meteoros.de/arten/ee06e.htm | title = Lower Tangent Arc | publisher = Arbeitskreis Meteore e.V. | accessdate = 2015-04-07 | author = Les Cowley}} External links
1 : Atmospheric optical phenomena |
| 随便看 |
|
开放百科全书收录14589846条英语、德语、日语等多语种百科知识，基本涵盖了大多数领域的百科知识，是一部内容自由、开放的电子版国际百科全书。