1. Wind: small scale and local systems
Chapter 9
Wind: small scale and local systems

2. Small-scale winds interacting with the environment
Scales of motion
Micro, meso, synoptic
Friction and Turbulence in Boundary Layer
Laminar flow
Irregular turbulent motion
Planetary boundary layer (PBL)
Depth of PBL

3. FIGURE 9. 1 Scales of atmospheric motion
FIGURE 9.1 Scales of atmospheric motion. The tiny microscale motions constitute a part of the larger mesoscale motions, which,
in turn, are part of the much larger synoptic scale. Notice that as the scale becomes larger, motions observed at the smaller scale are no
longer visible.

4. FIGURE 9.2 The scales of atmospheric motion with the phenomenon’s average size and life span. (Because the actual size
of certain features may vary, some of the features fall into more than one category.)

5. FIGURE 9. 3 Winds fl owing past an obstacle
FIGURE 9.3 Winds fl owing past an obstacle. (a) In stable air, light winds produce small eddies and little vertical mixing.
(b) Greater winds in unstable air create deep, vertically mixing eddies that produce strong, gusty surface winds.

6. FIGURE 9.5 Satellite image of eddies forming on the leeward
(downwind) side of the Cape Verde Islands during April, As the
air moves past the islands, it breaks into a variety of swirls as indicated
by the cloud pattern. (The islands are situated in the Atlantic Ocean, off
Africa’s western coast.)

7. FIGURE 9.6 Under stable conditions, air flowing past a mountain
range can create eddies many kilometers downwind of the mountain
itself.

8. Small-scale winds interacting with the environment
Observations: Eddies & Air Pockets
Eddies on leeward side of solid object
Roll eddies, mountain wave eddy (clear air turbulence)
Increase wind speed/shear deforms layer into wave and air pocket.
Force of the Wind
Bridges and hills or rises can modify wind, increasing the force at specific locations.

9. FIGURE 9.9 The shape of this sand dune reveals that the wind
was blowing from left to right when it formed. Note also the shape of
the sand ripples on the dune.

10. FIGURE 9.11 Snow drifts accumulating behind snow fences in
Wyoming.

11. Determining wind speed and direction
Wind characterized by direction, speed, and gustiness
Wind direction describes the direction from which it is blowing
Topic: Pedaling in the Wind
As wind blows against an object it exerts force (V2 or P x A)

12. FIGURE 9.14 An onshore wind blows from water to land; whereas an offshore wind blows from land to water.

13. FIGURE 9.15 Wind direction can be expressed in degrees
about a circle or as compass points.

14. Determining wind speed and direction
Influence of Prevailing Winds
Prevailing most frequently observed direction during a given time period
Impact human and natural landscape
Wind rose
Topic: Wind Power
Turbines need moderate, steady winds
Only a few percent of nation’s total energy needs generated by wind power in the first half of the century

15. Local Winds Thermal circulation
Heating and cooling of the atmosphere above the ground create cold, core high and warm, core low pressure cells.
Wind travels from high to low and rises until it cools and begins to sink.

16. Local Winds Sea and Land Breeze Local Winds and Water
Uneven heating of land and water
Day: land hot, water cold = sea breeze
Night: water hot, land cold = land breeze
Sea breeze front, sea breeze convergence
Local Winds and Water
Local winds will change speed and direction as they cross a large body of water due to less friction, greater speed and greater Coriolis effect

17. FIGURE 9.24 Development of a sea
breeze and a land breeze. (a) At the surface, a sea
breeze blows from the water onto the land,
whereas (b) the land breeze blows from the land
out over the water. Notice that the pressure at
the surface changes more rapidly with the sea
breeze. This situation indicates a stronger
pressure gradient force and higher winds with a
sea breeze.

18. FIGURE 9.25 Typically, during the summer over Florida,
converging sea breezes in the afternoon produce uplift that enhances
thunderstorm development and rainfall. However, when westerly
surface winds dominate and a ridge of high pressure forms over the
area, thunderstorm activity diminishes, and dry conditions prevail.

19. FIGURE 9.26 Surface heating and lifting of air along a converging sea breeze combine to form thunderstorms almost daily during the summer
in southern Florida.

20. FIGURE 9.27 The convergence of two lake breezes and their in-
fluence on the maximum temperature during July in upper Michigan.

21. FIGURE 9.29 Sinking air develops where surface winds move
offshore, speed up, and diverge. Rising air develops as surface winds
move onshore, slow down, and converge.

22. Local Winds Seasonally Changing Winds: The Monsoon Arabic for seasonal
Winds change direction seasonably causing extreme dry and wet season
Eastern and southern Asia, North America

23. FIGURE 9.30 Changing annual wind-flow patterns associated with the winter and summer Asian monsoon.

24. FIGURE 9.31 Average annual precipitation for Cherrapunji,
India. Note the abundant rainfall during the summer monsoon (April
through October) with the lack of rainfall during the winter monsoon
(November through March).

25. Local Winds Mountain and Valley Breeze Katabatic winds
On mountain slopes, warm air rises during the day creating a valley breeze; during night nocturnal drainage of cool air creating a mountain breeze
Associated with cumulus clouds in the afternoon
Katabatic winds
Cold wind rushes down elevated slopes, usually 10 kts or less but can reach hurricane strength

26. FIGURE 9.34 Valley breezes blow uphill during the day; mountain breezes blow downhill at night. (The L’s and H’s represent pressure,
whereas the purple lines represent surfaces of constant pressure.)

27. FIGURE 9.36 Strong katabatic winds can form where cold winds
rush downhill from an elevated plateau covered with snow.

28. Local Winds Chinook/Foehn Winds Focus: Snow Eaters
Dry warm descending on the leeward side of a orographic barrier
Eastern slope of Rockies (chinook), Europe (foehn), Argentina (zonda)
Focus: Snow Eaters
Thirsty wind on east side of Rockies that eat or melt snow due to rapid change in temperature

29. FIGURE 9.37 (a) A chinook wind can be enhanced when clouds form on the mountain’s windward side. Heat added and moisture
lost on the upwind side produce warmer and drier air on the downwind sides. (b) A graphic representation of the rising and sinking air
as it moves over the mountain.

30. Local Winds Santa Anna Winds Desert winds
Warm dry that blows from east or northeast don canyons into S. California
Very fast, desiccates vegetation, providing fuel for fires
Desert winds
Dust storms, sand storms, dust devil, haboob

31. FIGURE 9.39 Surface weather map showing Santa Ana conditions
in January. Maximum temperatures for this particular day are
given in oF. Observe that the downslope winds blowing into southern
California raised temperatures into the upper 80s, while elsewhere temperature
readings were much lower.

32. FIGURE 9.40 Strong northeasterly Santa Ana winds on October
23, 2007, blew the smoke from massive wild fires (red dots) across
southern California out over the Pacific Ocean.

33. TABLE 9.1 Local Winds of the World