Wind:
Small
Scale
and
Local
Systems
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.
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.)
Winds flowing 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.
Satellite image of
eddies forming on the
leeward (downwind)
side of the Cape Verde
Islands during April,
2004.
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.)
Under stable conditions, air flowing past a mountain range can
create eddies many kilometers downwind of the mountain itself.
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.
Determining wind speed and
direction
Wind characterized by direction, speed,
and gustiness
 Wind direction describes the direction
from which it is blowing

An onshore wind blows from water to land; whereas an offshore
wind blows from land to water.
Wind
direction
can be
expressed
in degrees
about a
circle or
as
compass
points.
Determining wind speed and
direction

Wind Measurements
 Wind vane
 Pressure plate anemometer
 Cup anemometer
 Aerovane
 Rawinsonde
 Wind soundings
Determining wind speed and
direction

Influence of Prevailing Winds
 Prevailing most frequently observed
direction during a given time period
 Impact human and natural landscape
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.
Local Winds

Sea and Land Breeze
 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
Development of a
sea breeze & 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.
Local Winds

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
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.
Stepped Art
Fig. 9-25, p. 241
The convergence of two lake breezes and their influence on the
maximum temperature during July in upper Michigan.
Sinking air develops where surface winds move offshore, speed
up, and diverge. Rising air develops as surface winds move
onshore, slow down, and converge.
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
Changing annual wind-flow patterns associated with the
winter and summer Asian monsoon.
Local Winds

Mountain and Valley Breeze
 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
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.)
Strong katabatic winds can form where cold winds rush
downhill from an elevated plateau covered with snow.
Local Winds

Chinook/Foehn Winds
 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
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.
Local Winds

Santa Anna Winds
 Warm dry that blows from east or northeast
down canyons into S. California
 Very fast, desiccates vegetation, providing
fuel for fires

Desert winds
 Dust storms, sand storms, dust devil,
haboob