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| '''Roughness length''' (<math>z_0</math>), is a [[parameter]] of some vertical wind profile equations that model the horizontal mean wind speed near the ground; in the [[log wind profile]], it is equivalent to the height at which the wind speed theoretically becomes zero. In reality the wind at this height no longer follows a mathematical logarithm. It is so named because it is typically related to the height of terrain roughness elements. Whilst it is not a physical length, it can be considered as a length-scale a representation of the roughness of the surface.
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| As an approximation, the roughness length is approximately one-tenth of the height of the surface roughness elements. For example, short grass of height 0.01m has a roughness length of approximately 0.001m. Surfaces are rougher if they have more protrusions. Forests have much larger roughness lengths than tundra, for example. Roughness length is an important concept in [[urban meteorology]] as the building of tall structures, such as skyscrapers, has an effect on roughness length and wind patterns.
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| {|
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| |Terrain description
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| |<math>z_0</math> (''m'')
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| |-
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| |Open sea, [[Fetch (geography)|Fetch]] at least 5 km
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| |0.0002
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| |-
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| |Mud flats, snow; no vegetation, no obstacles
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| |0.005
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| |-
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| |Open flat terrain; grass, few isolated obstacles
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| |0.03
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| |-
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| |Low crops; occasional large obstacles, ''x/H'' > 20
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| |0.10
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| |-
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| |High crops; scattered obstacles, 15 < ''x/H'' < 20
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| |0.25
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| |-
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| |parkland, bushes; numerous obstacles, ''x/H'' ≈ 10
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| |0.5
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| |-
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| |Regular large obstacle coverage (suburb, forest)
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| |1.0
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| |-
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| |City centre with high- and low-rise buildings
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| |≥ 2
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| |}<ref>WMO Guide to Meteorological Instruments and Methods of Observation WMO-No. 8 page I.5-12</ref>
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| ==References==
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| {{reflist}}
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| ==External links==
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| * [http://amsglossary.allenpress.com/glossary/search?id=aerodynamic-roughness-length1 Aerodynamic Roughness (Length AMS Glossary)]
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| * [http://www.webmet.com/met_monitoring/663.html Surface Roughness Length]
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| * [http://www-das.uwyo.edu/~geerts/cwx/notes/chap14/roughness.html Roughness]
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| * [http://amsglossary.allenpress.com/glossary/search?id=roughness-length1 Roughness (AMS Glossary)]
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| ==See also==
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| *[[Von Kármán constant]]
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| *[[Monin-Obukhov length]]
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| *[[Wind profile power law]]
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| *[[Log wind profile]]
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| *[[Displacement height]]
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| [[Category:Atmospheric dispersion modeling]]
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| [[Category:Fluid dynamics]]
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| {{climate-stub}}
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