Wind

:''For the 1928 in film, see The Wind.'' Wind is the quasi-horizontal movement of air (as opposed to an air current) caused by a horizontal pressure gradient force. It occurs at all scales, from local breezes generated by heating of land surfaces and lasting tens of minutes to global winds resulting from solar heating of the Earth. The two major influences on the atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (Coriolis effect). Given a difference in barometer pressure between two air masses, a wind will arise between the two which tends to flow from the area of high pressure area to the area of low pressure area until the two air masses are at the same pressure, although these flows will be modified by the Coriolis effect in the extratropics. Winds can be classified either by their scale, the kinds of forces which cause them (according to the atmospheric equations of motion), or the geographic regions in which they exist. There are global winds, such as the wind belts which exist between the atmospheric circulation cells. There are upper-level winds, such as the jet streams. There are synoptic-scale winds that result from pressure differences in surface air masses in the middle latitudes, and there are winds that come about as a consequence of geography features such as the sea breeze. Scale (spatial) winds are those which act on a local scale, such as gust fronts. At the smallest scale are the winds which blow on a scale of only tens to hundreds of metres and are essentially unpredictable, such as dust devils and microbursts. Winds can also shape landforms, via a variety of eolian processes. ==Winds by spatial scale== ===Prevailing winds — the general circulation of the atmosphere=== Prevailing winds are winds which come about as a consequence of atmospheric circulation. These include the Trade wind, the Westerlies, the Polar Easterlies, and the jet streams. Because of differential heating and the fact that warm air rises and cool air falls, there arise circulations that (on a non-rotating planet) would lead to an equator-to-pole flow in the upper atmosphere and an pole-to-equator flow at lower levels. Because of the Earth's rotation, this simple situation is vastly modified in the real atmosphere. In almost all circumstances the horizontal component of the wind is much larger than the vertical — the exception being violent convection. The ''Trade Winds'' are the most familiar consistent and reliable winds on the planet, exceeded in constancy only by the katabatic winds of the major ice sheets of Antarctica and Greenland. It was these winds that early mariners relied upon to propel their ships from Europe to North and South America. Their name derives from the Old English 'trade', meaning "path" or "track," and thus the phrase "the wind blows trade," that is to say, on track. The Trades form under the Hadley cell, and are part of the return flow for this cell. The Hadley carries air aloft at the equator and transports it poleward north and south. At about 30°N/S latitude, the air cools and descends. It then begins its journey back to the equator, but with a noticeably westward shift as a result of the action of the Coriolis force. Along the east coast of North America, friction twists the flow of the Trades even further clockwise. The result is that the Trades feed into the Westerlies, and thus provide a continuous zone of wind for ships travelling between Europe and the Americas. The ''Westerlies'', which can be found at the mid-latitudes beneath the atmospheric circulation, likewise arise from the tendency of winds to move in a curved path on a rotating planet. Together with the airflow in the Ferrel cell, poleward at ground level and tending to equatorial aloft (though not clearly defined, particularly in the winter), this predisposes the formation of eddy currents which maintain a more-or-less continuous flow of westerly air. The upper-level polar jet stream assists by providing a path of least resistance under which low pressure areas may travel. The ''Polar Easterlies'' result from the outflow of the Polar high, a permanent body of descending cold air which makes up the poleward end of the atmospheric circulation. These winds, though persistent, are not deep. However, they are cool and strong, and can combine with warm, moist Gulf Stream air transported northward by weather systems to produce violent thunderstorms and tornadoes as far as 60°N on the North American continent. Records of tornadoes in northerly latitudes are spotty and incomplete because of the vast amount of uninhabited terrain and lack of monitoring, and it is certain that tornadoes have gone unseen and unreported. The deadly Edmonton tornado of 1987, which ranked as an F4 on the Fujita scale and killed 27 people, is evidence that powerful tornadoes can occur north of the 50th parallel. The ''jet streams'' are rapidly moving upper-level currents. Travelling generally eastward in the tropopause, the polar jets reside at the juncture of the Ferrel cell and the Polar cell and mark the location of the polar cold front. During winter, a second jet stream forms at about the 30th parallel, at the interface of the Hadley and Ferrel cells, as a result of the contrast in temperature between tropical air and continental polar air. The jet streams are not continuous, and fade in and out along their paths as they speed up and slow down. Though they move generally eastward, they may range significantly north and south. The polar jet stream also marks the presence of Rossby waves, long-scale (4000 - 6000 km in wavelength) harmonic waves which perpetuate around the globe. ===Seasonal winds=== Seasonal winds are winds that only exist during specific seasons, such as the India monsoon. ===Synoptic winds=== Synoptic winds are winds associated with large-scale events such as warm and cold Weather front, and are part of what makes up everyday weather. These include the geostrophic wind, the gradient wind, and the cyclostrophic wind. As a result of the Coriolis force, winds in the northern hemisphere always flow clockwise around a high pressure area and counterclockwise around a low pressure area (the reverse occurs in the southern hemisphere). At the same time, winds always flow from areas of high pressure to areas of low pressure. These two forces are opposite but not equal, and the path that results when the two forces cancel each other runs parallel to the isobars. Wind following this path is known as geostrophic wind. Winds are said to be truly geostrophic only when other forces (e.g. friction) acting on the air are negligible, a situation which is often a good approximation to the large-scale flow away from the tropics. In certain circumstances, the Coriolis force acting on moving air may be almost or entirely overwhelmed by the centripetal force. Such a wind is said to be cyclostrophic, and is characterized by rapid rotation over a relatively small area. Hurricanes, tornadoes, and typhoons are examples of this type of wind. ===Mesoscale winds=== Synoptic winds occupy the lower boundary of what is considered to be "forecastable" wind. Winds at the next lowest level of magnitude typically arise and fade over time periods too short and over geographic regions too narrow to predict with any long-range accuracy. These mesoscale winds include such phenomena as the cold outflow from thunderstorms. This wind frequently advances ahead of more intense thunderstorms and may be sufficiently energetic to generate local weather of its own. Many of the "special" winds, addressed in the last section of this article, are mesoscale winds. ===Microscale winds=== Microscale winds take place over very short durations of time - seconds to minutes - and spatially over only tens to hundreds of metres. The turbulence following the passage of an active front is composed of microscale winds, and it is microscale wind which produces convective events such as dust devils. Though small in scope, microscale winds can play a major role in human affairs. It was the crash of a fully loaded Lockheed L-1011 at Dallas-Fort Worth International Airport in the summer of 1985, and the subsequent loss of 133 lives, that introduced the term "microburst" to many people, and that was a factor in the installation of doppler radar in airports and weather installations worldwide. ==Winds by effect== In classical terminology,''Aeolian winds'', or winds producing ''Aeolian action'', are winds which produce geologic changes. Modern tornadoes and hurricanes might at times be considered to produce such changes. Largescale erosion, dune formation, and other geologic and topographic effects influenced by wind are still referred to as aeolian activity. ==Local winds that are tied to specific temperature distributions == Some local winds blow only under only certain circumstances, i.e. they require a certain temperature distribution. ''Differential heating'' is the motive force behind land breezes and sea breezes (or, in the case of larger bodies, lake breezes), also known as on- or off-shore winds. Water is a rapid absorber/radiator of heat, whereas land not only absorbs heat more slowly but releases it over a greater period of time. The result is that, in locations where sea and land meet, heat absorbed over the day will be released more quickly by the water. Air contacting water cools. Over the land, heat is still being released into the air, which rises. This convective motion draws the cool sea air in to replace the rising air, resulting in a sea breeze. During the day, the roles are reversed. The land, cooled from a night of radiation, continues to soak up heat long after the heat capacity of the water has been reached. Warm air over the water rises, pulling cool air from inland to replace it. And so it goes. Mountain breezes and valley breezes are due to a combination of differential heating and geometry. When the sun rises, it is the tops of the mountain peaks which receive first light, and as the day progresses, the mountain slopes take on a greater heat load than the valleys. This results in a temperature inequity between the two, and as warm air rises off the slopes, cool air moves up out of the valleys to replace it. This upslope wind is called a ''valley breeze''. The opposite effect takes place in the afternoon, as the valley radiates heat. The peaks, long since cooled, transport air into the valley in a process that is partly gravitational and partly convective and is called a ''mountain breeze''. Mountain breezes are one example of what is known more generally as a Katabatic wind. These are winds driven by cold air flowing down a slope, and occur on the largest scale in Greenland and Antarctica. Most often, this term refers to winds which form when air which has cooled over a high, cold plateau is set in motion and descends under the influence of gravity. Winds of this type are common in regions of Mongolia and in glaciated locations. Because ''katabatic'' refers specifically to the vertical motion of the wind, this group also includes winds which form on the lee side of mountains, and heat as a consequence of compression (f�hn winds). Such winds may undergo a temperature increase of 20°C or more, and many of the world's "named" winds (see list below) belong to this group. Among the most well-known of these winds are the chinook of Western Canada and the American Northwest, the Swiss foehn wind , California's infamous Santa Ana wind, and the Spanish mistral. The opposite of a katabatic wind is an anabatic wind, or an upward-moving wind. The above-described ''valley breeze'' is an anabatic wind. A widely-used term, though one not formally recognised by meteorologists, is ''orographic wind''. This refers to air which undergoes orographic lifting. Most often, this is in the context of winds such as the chinook or the foehn, which undergo lifting by mountain ranges before descending and warming on the lee side. == Winds that are defined by an equilibrium of physical forces == These winds are used in the decomposition and analysis of wind profiles. They are useful for simplifying the atmospheric equations of motion and for making qualitative arguments about the horizontal and vertical distribution of winds. Examples are: * geostrophic wind (wind that is a result of the balance between Coriolis force and pressure gradient force; flows parallel to isobars and approximates the flow above the atmospheric boundary layer in the midlatitudes if frictional effects are low) * thermal wind (not actually a wind but a wind ''difference'' between two levels; only exists in an atmosphere with horizontal temperature gradients, i.e. baroclinicity) * ageostropic wind (difference between actual and geostrophic wind; the wind component which is responsible for air "filling up" cyclones over time) * gradient wind (like geostrophic wind but also including centrifugal force) ==Names for specific winds in certain regions== ===Classical Greek and Roman wind names=== In ancient Greek mythology, the four winds were personified as gods. The Greeks also observed the seasonal change of the winds, as evidenced by the Tower of the Winds in Athens. Roman writers later gave them Latin names. Note: the "north wind", for example, is the wind that blows ''from'' the north, not towards it.

	Greek	Latin
north wind	''Boreas''	''Aquilo''
south wind	''Notus''	''Auster''
east wind	''Eurus''	''Eurus''
west wind	''Zephyrus''	''Favonius''
north-west wind	''Skiron'' or ''Skeiron''	''Caurus'' or ''Corus''
north-east wind	''Kaikias''	''Caecius''
south-east wind	''Eurus'' or ''Apeliotus''	''Volturnus'' or ''Vulturnus''
south-west wind	''Lips'' or ''Livos''	''Africus'' or ''Afer ventus''
north-north-west wind		''Thrascius''
west-south-west wind		''Libs''

===Modern wind names=== Many local wind systems have their own names. For example: *''Bora'' (northeasterly from eastern Europe to Italy) *''Chinook'' (easterly off the Rocky Mountains) *''Etesian'' (Greek name) or ''Meltemi'' (Turkish name) (northerly across Greece and Turkey) *''Foehn'' (off the northern side of the Alps) *''Gregale'' (northeasterly from Greece) *''Halny'' (in northern Carpathians) *''Khamsin'' (southeasterly from North Africa to the eastern Mediterranean) *''Levanter'' (easterly through Strait of Gibraltar) *''Libeccio'' (southwesterly towards Italy) *''Marin (wind)'' (south-easterly from Mediterranean to France) *''Mistral'' (northwesterly from central France to Mediterranean) *''Nor'easter'' (Eastern United States) *''Santa Ana winds'' (Southern California) *''Sirocco'' (southerly from North Africa to southern Europe) *''Vendavel'' (westerly through Strait of Gibraltar) *''Zonda wind'' (on the eastern slope of the Andes in Argentina) ==Meteorological instruments to measure wind speed and/or direction== *anemometer (measures wind speed, either directly, e.g. with rotating cups, or indirectly, e.g. via pressure differences or the propagation speed of ultrasound signals) *rawinsonde (GPS-based wind measurement is performed by the probe) *weather balloon (passive measurement, balloon position is tracked from the ground visually or via radar; wind profile is computed from drift rate and the theoretical speed of ascent) *weather vane (used to determine wind direction) == See also == * Beaufort scale * meteorology * climatology * Wind power == External links == * [http://www.lpi.usra.edu/publications/slidesets/winds.html The Winds of Mars: Aeolian Activity and Landforms], paper with slides that illustrate the wind activity on the planet Mars (planet). * [http://www.winddata.com Database of Wind Characteristics]. Wind data for wind (turbine) design and wind resource assessment and siting. * [http://www.windatlas.dk Wind Atlases of the World]. Lists of wind atlases and wind surveys from all over the world. * [http://ggweather.com/winds.html A List of Named Winds] * [http://www.animalu.com/pics/dd1.htm Dust Devil Movie] A short movie showing Dust Devils in action on a dry lakebed. Meteorology Atmosphere Weather Winds

Wind

What is the difference between the versions of September 1 and August 6? User:RickK 02:35, 2 Sep 2003 (UTC) I appreciate the effort made by the writer of the original page, but there were many problems with it. I have reorganized it to follow current terms and practices in meteorology/climatology, and have extended it considerably. User:Dwindrim 19:05, 2004 Jan 18 (UTC) ---- (User:William M. Connolley 16:35, 2004 Mar 10 (UTC)) I've made a load of minor hacks (and rewritten the intro). I completely removed: : In certain circumstances, the Coriolis force acting on moving air may be almost or entirely overwhelmed by the centripetal force. One such circumstance is at the equator, where, for all practical purposes, the Coriolis force is nonexistent. Such a wind is said to be cyclostrophic, and is characterized by rapid rotation over a relatively small area. Hurricanes, tornadoes, and typhoons are examples of this type of wind. because its badly flawed. There are no hurricanes on the equator. The orographic wind bit is dodgy. I didn't believe the origin of the phrase "trade wind". ---- While I cannot lay any right to this article, I nonetheless take some pride in it. : (User:William M. Connolley 22:39, 2004 Mar 11 (UTC)) Thats what I thought, so I've done my best to inform you rather than slip the changes in... Many of the changes you made were good ones, especially in those cases, such as the intro, where you added to existing material. However, I am a little concerned that material has simply been deleted without correction or modification - paragraphs on the cyclostrophic and geostrophic winds (while you quite correctly say that hurricanes do not form on the equator, they have been known to drift across it). : (User:William M. Connolley 22:39, 2004 Mar 11 (UTC)) I'm fairly sure that hurricanes (or as wikipedia calls them, trop cyclones) depend crucially on coriolis to form and maintain. I did check on some hurricane track pages to confirm this... I can't find the page I used now. But look at http://www.wunderground.com/hurricane/at2003.asp or 2002 or 2001. No tracks within 10 deg of the equator. If hurricanes do occaisionally cross the equator, I think this is the exception not norm. I would also expect that you would agree that disbelief is not a vaid reason on its own to change a point. "The wind blows trade," is legit - "trade" is an Old English word for path or track, which makes sense, because that is exactly how trade was (and still is) carried out. : I was unsure of that. I see you've restored it - fair enough. To me, "trade wind" means the wind used for trading ships, and needs no further explanation. But OK. What is it you find "dodgy" about ''orographic wind''? As I have indicated, it is not a formally-employed meteorological term. It nonetheless describes a real meteorological event, and I can vouch for that, living, as I do, less than 30 km from the east slope of the Canadian Rockies. : (User:William M. Connolley 22:39, 2004 Mar 11 (UTC)) I'm embarrassed to say I can't remember exactly why. Apologies for needless irritation. Trying to reconstruct, I think my feeling was that an orogrpahic wind often meant a wind affected by local orography - in the sense of being turned perhaps rather than lifted. And it does seem to be a recognised term: see e.g. http://amsglossary.allenpress.com/glossary/browse?s=o&p=15 The Internet is our friend. Doing an advanced search on trade or on geostrophic/cyclostrophic wind will show you that if my facts are incorrect, so are those of meteorology professors. User:Dwindrim 19:10, 2004 Mar 11 (UTC) : Re cyclostrophic... I felt your para was implying that these winds happened *particularly* over the equator. I plead that others might be similarly mislead. I've restored the para, with the equator bit removed. ---- I appreciate your reply, William. I'm not interested in any turf war here; this is, after all, a place of collaborative effort. We serve each other best by keeping each other honest. I'd like to keep a dialog going - I'm the first to admit that my knowledge is not complete and so I'm always open to other views. I also hate getting called out (as you might have noted), and therefore make every effort to ensure my facts are correct. Those I am not absolutely certain of, I research until I'm satisfied with their integrity. Still, things slip by; see further on the Talk:Atmospheric circulation page. User:Dwindrim 18:04, 2004 Mar 12 (UTC) == No 'third force' involved in geostrophic flow == The following passage in the article is unintelligable: :In nature, isobars are almost always curved. The result is that a wind moving parallel to the isobars encounters a third force, the centripetal force. This is the force which tends to keep a body in motion moving in the same direction. The effect of this force, though not a force in itself, is called the centrifugal force, and acts to counteract the Coriolis force (coincidentally also the effect of a force rather than a force in itself) and decrease the wind speed. This much more common situation results in what is known as a gradient wind. :In certain circumstances, the Coriolis force acting on moving air may be almost or entirely overwhelmed by the centripetal force. Such a wind is said to be cyclostrophic, and is characterized by rapid rotation over a relatively small area. Hurricanes, tornadoes, and typhoons are examples of this type of wind. Here is how I understand it.
Winds are categorized as geostrophic if in the dynamics the following two factors are vastly dominant: the pressure gradient force, and the coriolis effect. In the case of northern hemisphere winds circling a low pressure area, the pressure gradient acts in centripetal direction. (I will call the full circle of winds around the low pressure area 'the geostrophic flow') The coriolis effect tends to deflect any flow to the right. When there is no contraction of the geostrophic flow then there is an region of dynamic equilibrium, where the pressure gradient force is enough to maintain the same deflection to the left, but not strong enough to cause contraction of the geostrophic flow. In regions where there is a surplus of pressure gradient force there will be a surplus of deflection to the left, which is a contraction of the geostrophic flow. This contraction is deflected to the right due to the coriolis effect, increasing the velocity of the wind. At small diameters of the geostrophic flow the coriolis effect becomes negligable, but contraction down a pressure gradient will still increase the angular velocity in the center. There is no third force!
The coriolis effect deflects all flows to the right, therefore the direction in which it acts varies with the direction of the flow. In the absence of contraction the coriolis effect is what counteracts flow down the pressure gradient. As soon as there is contraction it is deflected to the right. --User:Cleon Teunissen | User talk:Cleon_Teunissen 20:05, 13 Mar 2005 (UTC) : (User:William M. Connolley 13:29, 15 Mar 2005 (UTC)) As I understand it (Holton, An Intro to Dynamic Meteorology) geostrophic approx is pressure-coriolis balance. This can only occur in the real world if the height contours are latitude circles. Gradient approx is flow parallel to the height contours, a balance between coriolis, centrifugal force and pressure gradient. It is usually a better approx to the actual wind than the geostrophic approx. ---- Another remark: in the case of geostrophic flow, the pressure gradient force can in particular regions be stronger than a centrifugally acting coriolis effect, the strengh of the coriolis effect is proportional to the velocity of the wind, and if there is not enough velocity then that's it. But I think the opposite will occur in very special circumstances only. It is pressure gradient force that starts air moving in the first place, so only input from outside could possibly whip up winds to a velocity where a centrifugally acting coriolis effect is stronger than the pressure gradient force. The engine of the process is the pressure gradient force; when the geostrophic flow loses kinetic energy through friction, the geostrophic flow will contract, releasing energy that is instantly converted to kinetic energy. I learned the use of flow-of-energy to keep track of the causal chain from an article by Anders Persson. [http://www.astro.columbia.edu/~dave/papers/coriolis.pdf A 374 KB article on the coriolis force by Anders Persson]
The pressure gradient force and the coriolis effect act in a fundamentally different way. The pressure gradient force is the engine, the coriolis effect converts energy (and friction dissipates the energy). Using the expression 'coriolis force' has the disadvantage of disguising the difference between force and coriolis effect. --User:Cleon Teunissen | User talk:Cleon_Teunissen 21:37, 13 Mar 2005 (UTC) ---- Third remark. In the article it is stated:
:This is the force which tends to keep a body in motion moving in the same direction. In a non-rotating environmont objects will move in a straight line when there is no force being exerted, the well-known inertia. However, the solid Earth plus atmosphere is a rotating environment, and because of that the inertial properties of the air masses of the atmosphere are governed by orbital dynamics, the dynamics of mass orbiting the axis of the Earth. Inertial motion of atmospheric air masses is a curved motion with respect to the coordinate system that is co-rotating with earth. Air masses that do not move are in dynamic equilibrium, but air masses that are in motion curve away to the right (on the northern hemisphere).
So the usual thinking that mass tends to move in straight lines does not apply in the case of atmospheric motions on the rotating Earth. Geostrophic flow away from a high pressure area is a very interesting situation. Being deflected to the right, and not being free to move outward indefinately, it will start to flow clockwise. The clockwise flow curves to the right, the local direction of inertial motion.
Any flow that rotates clockwise with a period of 12 hours will be entirely inertial motion. A clockwise atmospheric circulation with a period of 12 hours will be stable on its own; any pressure gradient would disrupt it. A clockwise circulation with a period of 12 hours is in dynamic equilibrium with respect to the inertial system it is orbiting in. == To much explanatory dynamics == (User:William M. Connolley 13:21, 15 Mar 2005 (UTC)) I reverted CT's changes. Sorry. The reason was that it seemed to be introducing too much explanatory dynamics into a general purpose article. I have cut out the para he didn't like though. In general there seems to be a confusion here between the actual winds in the world and the various mathematical approximations to the equations. The math approx is dealt with lower down (in Winds that are defined by an equilibrium of physical forces) and that seems to make sense. I don't think the section about "synoptic winds" should be doing this again, but worse... it should be about actual winds. I also didn't like ''The primary motion of air is to move from higher to lower pressure. Since there is nothing to stop the air from curving to the right it curves to the right as it moves.'' This seems too much like the handy-wavy version of the dynamics... it starts to do this, then it does that. Atmos flow is almost always near-balanced. The geostrophic wind page seems better. :OK, to much in-depth in a general purpose article.
When I wrote, 'the primary motion is' I did not mean to imply a sequential ordering, a sequence in time. My intention was to emphasize a direction of energy flow through the system. The pressure gradient as the supplier of the energy of wind, (and friction dissipating energy). Of course, low pressure areas form gradually, so the wind patterns change gradually.
Language is sequential, it's pretty hard to convey the interconnectedness of the dynamics of wind. If only I could use infinitisimals in everyday language... --User:Cleon Teunissen | User talk:Cleon_Teunissen 14:24, 15 Mar 2005 (UTC) == A third force == There is no third force! --User:Cleon Teunissen | User talk:Cleon_Teunissen 20:05, 13 Mar 2005 (UTC) :(User:William M. Connolley 13:29, 15 Mar 2005 (UTC)) As I understand it (Holton, An Intro to Dynamic Meteorology) geostrophic approx is pressure-coriolis balance. This can only occur in the real world if the height contours are latitude circles. Gradient approx is flow parallel to the height contours, a balance between coriolis, centrifugal force and pressure gradient. It is usually a better approx to the actual wind than the geostrophic approx. (User:William M. Connolley 13:29, 15 Mar 2005 (UTC)) Yes, in the gradient wind approx there are three forces in the equations. At first, I didn't quite see the implications of that.
Gradient wind approx covers both the situations where coriolis effect and centrifugal force act in the same direction, and situations where they act in opposite directions. Of course, coriolis effect and centrifugal effect cannot actually oppose each other since both are manifestation of inertia. If they oppose each other in the calculation, then in the atmosphere there will be either centrifugal force or coriolis force, ~~depending on the direction of the pressure gradient force.~~ more likely depending on magnitude of pressure gradient force and velocity of the wind. --User:Cleon Teunissen | User talk:Cleon_Teunissen 20:02, 15 Mar 2005 (UTC) A special case of gradient wind is inertial wind. In inertial wind there is no force involved. External link: [http://vortex.plymouth.edu/winds/webpage/inertial.html Inertial flow - Balanced flow tutorial by Ryan Turkington] On that webpage the period of inertial wind is given: 12 divided by the sine of the angle of the latitude gives the number of hours of the period of rotation. Close to the poles the period is nearly 12 hours. --User:Cleon Teunissen | User talk:Cleon_Teunissen 15:33, 15 Mar 2005 (UTC)

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