Chapter 10 - Wind: Small Scale and Local Systems
Scales of motion:
Microscale- 2 m; seconds to minutes; small eddies
Mesoscale – 20 km; minutes to hours; tornadoes, thunderstorms, sea breezes
Synoptic (synopsis, overview)- 2000km; days to a week; hurricanes, weather fronts
Global – 5000km; week; westerlies
Friction:
Between solids – kinetic and static friction
In fluids, friction is called viscosity.
Molecular viscosity (like conduction)
• Random motion of air molecules mixes stationary air with moving air at a boundary
• This slows down moving air
• A very small effect
For:
1. Higher wind speeds
2. Surface heating- unstable air and thermals
3. Rough landscape
Laminar flow becomes turbulent and much more effective mixing occurs with higher
viscosity.
Eddy viscosity
Thermal turbulence
Mechanical turbulence
all like convection
800
Much
mixing
600
Altitude
Little
vertical
mixing
400
200
0
0
5
Wind speed
10
Fig 10.3
Wind blowing
against a barrier
creates low pressure
on leeward side and
reverse wind
direction
High P
Low P
High V
Low V
Wind blowing over a mountain ridge increases in speed because it is constrained in
space.
Same mass per
unit time must
pass through each
loop otherwise
pressure builds up.
Application to house:
Prevailing wind
4 passive features that utilize wind:
1. Windmill at peak of roof takes advantage of increased wind speed .
Energy produced ∝ V3
2. Open window on leeward side creates ventilation because of low pressure outside
window.
3. House up on pillars – wind speed increases with height.
4. Casement windows deflect wind inside.
Top view
You can tell the direction of prevailing winds by the ripples in sand and snow and the
shape of sand dunes.
Force, Pressure and Power in Wind:
Object of area A is
perpendicular to
wind direction
V
P
Wind
Area
P = pressure =
F
= 2mρV2
A
ρ = air density
V = wind speed
P ∝ V2
Power =
m = mass of air molecule
Energy
Fd
=
= FV
Unit Time
t
d = distance traveled by wind in time t
Power ∝ V3
Power can move trains or boats and turn wind turbines
Instruments:
Anemometer – measures wind speed
Wind Vane – measures wind direction
Aerovane – does both!
Pressure plate anemometer:
Side view
Wind
V
Plate swings
up in the wind.
Measure Θ
can calculate
Fw , hence V
Θ
Pivot
Fw
Fg
Local Wind Systems
Sea and Land Breezes – familiar in Florida
• A type of thermal circulation
• Mesoscale wind system
Thermal circulations are always cool
Winds over large lakes :
you get change in wind speed and direction
Less friction over lake,
wind speeds up,
Coriolis causes veer to right (NH).
On other edge of lake, reverse happens.
hot near surface
Lows and highs can form ( clouds)
L
H
divergence
convergence
Lake
At windward shore, air speeds up, which creates a surface low. This causes sinking air
above and stable conditions. Clear air.
Recall: subsidence inversion, p 168
At leeward shore, wind slows down and bunches up, a surface high is formed, air rises
and unstable conditions result. Clouds and Rain!
Chinook wind - same as orographic uplift, p 178, name given to warm, dry wind that
descends on leeward side of mountain.
No need to memorize local wind names in table 10.2