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Wirls or eddies exist at all length scales in
the atmosphere. |
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Microscale (2m) small turbulent eddies |
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Mesoscale (20km) thunderstorms, tornadoes,
land/sea breeze |
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Synoptic scale (2000km) hurricanes, weather
map features like highs and lows |
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Global scale (5000 km) long waves in the
westerlies |
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Why do speed skaters crouch down and put their
hands behind them? |
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The friction of fluid flow is called viscosity. |
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Molecular viscosity is due to the random motion
of air molecules. |
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Eddy viscosity is due to turbulent whirling
eddies. |
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As long as the relative speed between the two
air layers is small (small shear), there is no turbulence (laminar flow). |
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As the relative wind speed increases, the
boundary deforms and then waves and eddies appear. |
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This leads to clear air turbulence. |
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The size of the eddies increases as the wind
speed increases and as the atmosphere becomes unstable by, for example, the
solar heating of the ground. |
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Shown: mechanical turbulence and thermal
turbulence. |
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Because of
friction the air near the ground moves slower than the air above. |
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Friction layer |
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Planetary boundary layer |
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With more vertical mixing of the air (e.g.
unstable atmosphere) the difference in speed is smaller. |
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Surface heating air rises producing strong
thermal turbulence |
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High wind speeds produces strong mechanical
turbulence |
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Rough or hilly landscape produces strong
mechanical turbulence |
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Eddies form when air encounters an obstacle. |
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They occur on both large scales (mountains) and
small scales (house). |
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Eddies form on an objects leeward side. |
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Roll eddies or rotors can occur further downwind
of the obstacle. |
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Wind layers moving at difference speeds (wind
sheer) can lead to clear air turbulence, which is dangerous for flying. |
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Wind exerts force on objects. |
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This can move small particles like sand and dirt
and for stronger winds large objects (e.g. in hurricances). |
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Wind picks up tiny, loose particles (e.g. sand). |
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Removal of these particles, leaving only larger
gravel and pebbles can lead to desert pavement. |
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Constant sand blasting of rocks causes a
flattened and pitted windward surface. |
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Blowing sand comes to rest behind obstacles and
eventually becomes a sand dune. |
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On a dunes surface sand rolls and slides
producing sand ripples. |
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Wind blowing over snow can produce snow dunes
and snow ripples. |
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For a strong enough wind clumps of snow can be
rolled along (snow rollers). |
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Behind snow fences the wind speed is reduced
allowing the snow to settle to the ground. |
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This keeps a blanket of snow on crops and keeps
large drifts away from towns. |
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Wind can bend and break branches. |
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Wind also causes plants to loose water more
rapidly. |
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Rows of trees decrease the surface wind speed
and hence reduce erosion. |
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Waves forming by wind blowing over a water
surface are called wind waves. |
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They depend on wind speed, the length of time
the wind blows, and fetch (distance of deep water over which the wind
blows). |
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Above: microscale winds help waves grow taller. |
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Wind direction by characterized by N, E,
or by
an angle. |
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Other common terms are: |
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Onshore wind |
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Offshore wind |
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Upslope wind |
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Downslope wind. |
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The wind direction most often observed is called
the prevailing wind. |
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A wind rose represents the percent of the time
that wind blew in a given direction. |
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Viscosity is the friction of air flow. |
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There are two sources: molecular and eddy. |
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Turbulence can be created by obstacles
(mechanical) or air rising (thermal). |
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Near the surface the wind moves slower. |
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The wind strongly influences the surface of the
earth (dunes, waves,
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