Chapter 10, Part 1

Small Scale Winds

Scales of Motion

Wirls or eddies exist at all length scales in the atmosphere.

Examples of Wind at Different Scales

Microscale (2m) – small turbulent eddies

Mesoscale (20km) – thunderstorms, tornadoes, land/sea breeze

Synoptic scale (2000km) – hurricanes, weather map features like highs and lows

Global scale (5000 km) – long waves in the westerlies

Speed Skating

Why do speed skaters crouch down and put their hands behind them?

Viscosity

The friction of fluid flow is called viscosity.

Molecular viscosity is due to the random motion of air molecules.

Eddy viscosity is due to turbulent whirling eddies.

Development of Turbulent Flow

As long as the relative speed between the two air layers is small (small shear), there is no turbulence (laminar flow).

As the relative wind speed increases, the boundary deforms and then waves and eddies appear.

This leads to clear air turbulence.

Development of Turbulence

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.

Shown: mechanical turbulence and thermal turbulence.

Planetary Boundary Layer

Because of friction the air near the ground moves slower than the air above.

Friction layer

Planetary boundary layer

With more vertical mixing of the air (e.g. unstable atmosphere) the difference in speed is smaller.

Factors which Increase Turbulence

Surface heating – air rises producing strong thermal turbulence

High wind speeds – produces strong mechanical turbulence

Rough or hilly landscape – produces strong mechanical turbulence

Large and Small Eddies

Eddies form when air encounters an obstacle.

They occur on both large scales (mountains) and small scales (house).

Eddy Formation

Eddies form on an objects leeward side.

Roll eddies or rotors can occur further downwind of the obstacle.

Clear Air Turbulence

Wind layers moving at difference speeds (wind sheer) can lead to clear air turbulence, which is dangerous for flying.

Force of the Wind

Wind exerts force on objects.

This can move small particles like sand and dirt and for stronger winds large objects (e.g. in hurricances).

Wind and Exposed Soil

Wind picks up tiny, loose particles (e.g. sand).

Removal of these particles, leaving only larger gravel and pebbles can lead to “desert pavement.”

Constant sand blasting of rocks causes a flattened and pitted windward surface.

Blowing sand comes to rest behind obstacles and eventually becomes a sand dune.

On a dune’s surface sand rolls and slides producing sand ripples.

Wind and Snow Surfaces

Wind blowing over snow can produce snow dunes and snow ripples.

For a strong enough wind clumps of snow can be rolled along (snow rollers).

Snow Fences

Behind snow fences the wind speed is reduced allowing the snow to settle to the ground.

This keeps a blanket of snow on crops and keeps large drifts away from towns.

Wind and Vegetation

Wind can bend and break branches.

Wind also causes plants to loose water more rapidly.

Shelter Belts

Rows of trees decrease the surface wind speed and hence reduce erosion.

Wind and Waves

Waves forming by wind blowing over a water surface are called wind waves.

They depend on wind speed, the length of time the wind blows, and fetch (distance of deep water over which the wind blows).

Above: microscale winds help waves grow taller.

Wind Direction

Wind direction by characterized by N, E, … or by an angle.

Other common terms are:

Onshore wind

Offshore wind

Upslope wind

Downslope wind.

Prevailing Wind

The wind direction most often observed is called the prevailing wind.

A wind rose represents the percent of the time that wind blew in a given direction.

Measuring Wind Speed and Direction

Summary

Viscosity is the friction of air flow.

There are two sources: molecular and eddy.

Turbulence can be created by obstacles (mechanical) or air rising (thermal).

Near the surface the wind moves slower.

The wind strongly influences the surface of the earth (dunes, waves, …).


	Wirls or eddies exist at all length scales in the atmosphere.


	Microscale (2m) – small turbulent eddies
	Mesoscale (20km) – thunderstorms, tornadoes, land/sea breeze
	Synoptic scale (2000km) – hurricanes, weather map features like highs and lows
	Global scale (5000 km) – long waves in the westerlies


	Why do speed skaters crouch down and put their hands behind them?


	The friction of fluid flow is called viscosity.
	Molecular viscosity is due to the random motion of air molecules.
	Eddy viscosity is due to turbulent whirling eddies.


	As long as the relative speed between the two air layers is small (small shear), there is no turbulence (laminar flow).
	As the relative wind speed increases, the boundary deforms and then waves and eddies appear.
	This leads to clear air turbulence.


	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.
	Shown: mechanical turbulence and thermal turbulence.


	Because of friction the air near the ground moves slower than the air above.
		Friction layer
		Planetary boundary layer
	With more vertical mixing of the air (e.g. unstable atmosphere) the difference in speed is smaller.


	Surface heating – air rises producing strong thermal turbulence
	High wind speeds – produces strong mechanical turbulence
	Rough or hilly landscape – produces strong mechanical turbulence


	Eddies form when air encounters an obstacle.
	They occur on both large scales (mountains) and small scales (house).


	Eddies form on an objects leeward side.
	Roll eddies or rotors can occur further downwind of the obstacle.


	Wind layers moving at difference speeds (wind sheer) can lead to clear air turbulence, which is dangerous for flying.


	Wind exerts force on objects.
	This can move small particles like sand and dirt and for stronger winds large objects (e.g. in hurricances).


	Wind picks up tiny, loose particles (e.g. sand).
	Removal of these particles, leaving only larger gravel and pebbles can lead to “desert pavement.”
	Constant sand blasting of rocks causes a flattened and pitted windward surface.
	Blowing sand comes to rest behind obstacles and eventually becomes a sand dune.
	On a dune’s surface sand rolls and slides producing sand ripples.


	Wind blowing over snow can produce snow dunes and snow ripples.
	For a strong enough wind clumps of snow can be rolled along (snow rollers).


	Behind snow fences the wind speed is reduced allowing the snow to settle to the ground.
	This keeps a blanket of snow on crops and keeps large drifts away from towns.


	Wind can bend and break branches.
	Wind also causes plants to loose water more rapidly.


	Rows of trees decrease the surface wind speed and hence reduce erosion.


	Waves forming by wind blowing over a water surface are called wind waves.
	They depend on wind speed, the length of time the wind blows, and fetch (distance of deep water over which the wind blows).
	Above: microscale winds help waves grow taller.


	Wind direction by characterized by N, E, … or by an angle.
	Other common terms are:
		Onshore wind
		Offshore wind
		Upslope wind
		Downslope wind.


	The wind direction most often observed is called the prevailing wind.
	A wind rose represents the percent of the time that wind blew in a given direction.


	Viscosity is the friction of air flow.
	There are two sources: molecular and eddy.
	Turbulence can be created by obstacles (mechanical) or air rising (thermal).
	Near the surface the wind moves slower.
	The wind strongly influences the surface of the earth (dunes, waves, …).