Scales of motion
– Sea and land breezes
• Seasonally changing winds
• Mountain and valley breezes
– Katabatic winds
– Chinook (Foehn) winds
– Santa Ana winds
• Desert winds
• Global winds
– Three cell model
– Average surface winds and pressure
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.
Scales of Atmospheric Motion
• The hierarchy of motion from tiny gusts to giant storms
• Microscale – smallest scale of motion- tiny eddies within
smoke Typical size 2 m
• Mesoscale – (middle scale) – the circulation of city air –
range in size from a few km to about 100 km. (Local
winds, thunderstorms, tornadoes) Typical size 20 km
• Synoptic Scale – typical weather map scale that shows
features such as high/low pressure areas, fronts etc.
Typical size 2000 km
• Planetary scale – largest scale of motion – sometimes
called global scale (Longwaves in the atmosphere)
Typical size 5000 km
• When the wind encounters a solid object,
a whirl of air or eddy forms on the object’s
downwind side. The size and shape of the
eddy often depends upon the size and
shape of the obstacle and the speed of the
Winds flowing past an obstacle.
(a) In stable air, light winds produce small eddies and little vertical mixing.
Greater winds in unstable air create deep, vertically mixing eddies that
produce strong, gusty surface winds.
As the wind blows over the mountain peak, the direction of air flow is disturbed
and circular eddies form on the mountain's leeward (downwind) side. The
shallow cloud pattern indicates that some eddies are circulating clockwise while
others are circulating counterclockwise. Eddies such as these are commonly
called von Karman vortices
Mountain Waves and Rotors
Under stable conditions, air flowing past a mountain range can create
eddies many kilometers downwind from the mountain itself.
• A thermal circulation is produced by the heating and
cooling of the atmosphere near the ground. The lines
represent surfaces of constant pressure (isobaric
surfaces). In this example, the isobars are parallel to the
earth’s surface- there is no horizontal variation in
pressure or temperature- no PGF and therefore no wind
• A thermal circulation produced by the heating and
cooling of the atmosphere near the ground. The H's and
L's refer to atmospheric pressure. The lines represent
surfaces of constant pressure (isobaric surfaces).
Suppose the air is cooled north and warmed south. PGF
causes air to move from High to Low pressure
• A thermal circulation produced by the heating and cooling of the
atmosphere near the ground. The H's and L's refer to atmospheric
pressure. The lines represent surfaces of constant pressure
(isobaric surfaces). Air aloft moves from south to north, air leaves
the southern area and “piles up” above northern area. PGF is
established at surface and winds flow from north to south at the
surface. We now have a thermal circulation- air flow resulting
primarily from the uneven heating and cooling of air.
• Development of a sea breeze and a land breeze.
At the surface, a sea breeze blows from the water onto the land...
• 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.
Sea Breeze Front
• Leading edge of the sea breeze
• Produces a rapid drop in temperature just
behind it (as much as 9°F)
• Cumulus clouds often form along this front
• Main cause of Florida’s abundant rainfall
Seasonally Changing Winds
• Monsoon Wind System – changes directions
seasonally – blows from one direction in
summer and the opposite direction in the winter.
• Especially well-developed in eastern and
• During winter, air over the continent becomes
much colder than air over ocean. High pressure
sets up over Siberia and air flows from land to
Changing annual wind flow patterns
associated with the winter Asian
monsoon. Clear skies and winds blow
from land to sea
Changing annual wind flow
patterns associated with the
summer Asian monsoon. Warm
humid air blows up from equator
bringing rainy weather.
Mountain and Valley Breezes
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.) Mountain breezes are also called
gravity or nocturnal drainage winds.
As mountain slopes warm during the day, air rises and often condenses
into cumuliform clouds, such as these.
• Any downslope wind
• Usually reserved for downslope winds that are
much stronger than mountain breezes.
• Katabatic (or fall) winds can rush down slopes
at hurricane speeds, but most are not that
intense and many are on the order of 10 kts or
– Bora – a cold gusty northeasterly wind with speeds
sometimes in excess of 100 kts. (Northern Adriatic)
– Mistral – cold (less violent wind) that descends the
western mountains and into the Rhone Valley of
Chinook (Foehn) Winds
• Warm dry wind that descends the eastern slope of the
• Region of influence extends from NE New Mexico into
Canada. (Similar winds occur along the leeward slope of
• Snow Eater
• Occur when strong westerly winds aloft flow over a N-S
trending mountain range producing low pressure on the
eastern side of the mountains. This trough of low
pressure forces air downslope. As the air descends it is
compressed and warms. (Compressional heating)
See pg 177 of book
Conditions that may enhance a chinook.
A chinook wall cloud forming over the Colorado Rockies (viewed from the plains)
Santa Ana Winds
• Warm dry wind that blows from the east or
northeast into Southern California
• Air descends from the elevated desert plateau, it
funnels through mountain canyons in the San
Gabriel and San Bernardino Mountains, finally
spreading over the Los Angeles Basin and San
• It lifts dust and sand, dries out vegetation, sets
stage for serious brush fires, especially in Fall.
(1961 Bel Air Fire burned for 3 days, destroyed
484 homes and caused $25 million in damage.
Santa Ana Winds
• Surface weather map
showing Santa Ana
conditions in January.
for this particular day are
given in °F. Observe that
the downslope winds
blowing into Southern
temperatures into the
upper 80's, while
readings were much
• Dust devils (whirlwinds)
A haboob approaching Phoenix, Arizona. The dust cloud is rising to a
height of about 450 m (1475 ft) above the valley floor. Haboob forms
as cold downdrafts along leading edge of a thunderstorm lift dust or
sand into huge dark cloud that may cover over a hundred kilometers
• The formation of a dust
devil. On a hot, dry day,
the atmosphere next to
the ground becomes
unstable. As the heated
air rises, wind blowing
past an obstruction twists
the rising air, forming a
rotating column, or dust
devil. Air from the sides
rushes into the rising
column, lifting sand, dust,
leaves, or any other loose
material from the surface.
• A dust devil forming
on a clear, hot
summer day just
south of Phoenix,
• Dust devils are not
General Circulation of the
• Represents the average air flow around
• Actual winds at any one place may vary
considerably from average
• Model for how heat is transported from the
equator to the poles
• Underlying cause is the unequal heating of
Global Winds - Single Cell Model
• The general circulation of
air on a non-rotating earth
uniformly covered with
water and with the sun
directly above the
equator. (Vertical air
motions are highly
exaggerated in the
• Does not actually exist on
earth (Coriolis produces
zonal winds at all
• Diagram shows the
names that apply to
the different regions
of the world and their
Three Cell Model
• The idealized wind and
distribution over a
• Tropical regions still
receive an excess of heat
and the poles a deficit
• Doldrums – equatorial
waters, air is warm,
gradients are weak,
• Horse latitudes ?
Three Cell Model
• Trade winds – blow from
northeast in N. Hemi and
southeast in S. Hemi
• Intertropical Convergence
Zone – Surface region of
convergence between trades
• Subtropical Highs –
semipermanent high in the
subtropical high pressure belt
centered near 30° latitude.
Bermuda high, Pacific Ridge.
• Westerlies – prevailing
Three Cell Model
• Polar Front – A semi
front that separates tropical air
masses from polar air masses
• Subpolar low – Belt of low
pressure between 50° and 70°
latitude. In N. Hemi this belt
consists of the Aleutian low
and the Icelandic low.
• Polar easterlies – a shallow
body of easterly winds located
at the high latitudes poleward
of the subpolar low.
Three Cell Model (wind regimes)
• Diagram shows the
names of surface
winds and pressure
systems over a
uniformly watercovered rotating
Semi-permanent pressure systems
• Highs and Lows that move only slightly
during the course of the year
• Bermuda High (Atlantic)
• Pacific High or Pacific Ridge
• Greenland-Icelandic Low or Icelandic Low
• Aleutian Low
Average sea-level pressure distribution and surface wind-flow patterns for
January. The heavy dashed line represents the position of the ITCZ.
Average sea-level pressure distribution and surface wind-flow patterns for July.
The heavy dashed line represents the position of the ITCZ.
A winter weather map depicting the main features of the general circulation
over North America. Notice that the Canadian high, polar front, and
subpolar lows have all moved southward into the United States, and that
the prevailing westerlies exist south of the polar front. The arrows on the
map illustrate wind direction.
Idealized View of Major Pressure Systems
• Major pressure
systems and idealized
air motions (heavy
blue arrows) and
of the general
shaded light blue
General Circulation and
• The position of the major features of the
general circulation and their latitudinal
displacement strongly influence the
climate of many areas.
• In general we expect abundant rainfall
where air rises (tropics and ITCZ) very
little where it sinks (vicinty of subtropical
highs and polar regions).
During the summer, the Pacific high moves northward. Sinking air along its
eastern margin produces a strong subsidence inversion, which causes
relatively dry weather to prevail. Along the western margin of the Bermuda high,
southerly winds bring in humid air, which rises, condenses, and produces
Average annual precipitation for Los Angeles, California, and Atlanta, Georgia.
Note: they are on different sides of semi-permanent highs
Westerly winds and the Jet Stream
• Relatively strong winds concentrated
within a narrow band in the atmosphere.
• Nearly continuous
• Jet Core often exceeds 100 kts
• Found at the tropopause at elevations
between 33,000 and 46,000 ft, but may
occur at higher and lower altitudes.
A jet stream is a swiftly flowing current of air that moves in a wavy west-toeast direction. In the Northern Hemisphere, it forms along a boundary
where colder air lies to the north and warmer air to the south. In the
Southern Hemisphere, it forms where colder air lies to the south and
warmer air to the north.
• Subtropical jet stream – the jet stream typically found
between 20° and 30° latitude and at altitudes between
12 and 14 km.
• Polar front jet stream (Polar jet) – the jet stream that is
associated with the polar front in middle and high
latitudes. Usually located between 9 and 12 km.
• Low level jets – peak winds less than 60 kts that form
above the Central Plains during the summer- contributes
to thunderstorm activity
• Tropical easterly jet – summertime – over tropics
• Stratospheric polar jet – forms in the polar winter near
the top of the stratosphere
Average position of the polar jet stream and the subtropical jet stream, with
respect to a model of the general circulation in winter. Both jet streams are
flowing into the page, away from the viewer, which would be from west to east.
Position of the polar jet stream and the subtropical jet stream at the 300-mb
level during the morning of March 10, 1998. Solid gray lines are lines of equal
wind speed (isotachs) in knots. Heavy lines show the position of the jet
streams. Heavy blue lines show where the jet stream directs cold air
southward, while heavy red arrows show where the jet stream directs warm air
Global Wind Patterns and the Oceans
• Winds blowing over the oceans cause the
surface water to drift along.
• The general atmospheric circulation
influences the ocean currents.
• Ocean currents move with the prevailing
• Gulf stream, California current, etc.
Average position and extent of the major surface ocean currents. Cold
currents are shown in blue; warm currents are shown in red. Names of the
ocean currents are given in Table 7.2.
The Gulf Stream (dark red band) and its eddies are revealed in this satellite
mosaic of sea surface temperatures of the western North Atlantic during June,
1984. Bright red shows the warmest water (about 27°C or 80°F), followed by
orange and yellow. Green, blue, and purple represent the coldest water.
Why is the air cool off the west
• Average sea surface
along the west coast
of the United States
• As winds blow parallel to the west coast of North
America, surface water is transported to the right
(out to sea). Cold water moves up from below
(upwells) to replace the surface water.
Under ordinary conditions, higher pressure over the southeastern Pacific and
lower pressure near Indonesia produce easterly trade winds along the equator.
These winds promote upwelling and cooler ocean water in the eastern Pacific,
while warmer water prevails in the western Pacific. When the trades are
exceptionally strong, water along the equator in the eastern Pacific becomes
quite cool. This cool event is called La Niña.
During El Niño conditions, atmospheric pressure decreases over the eastern
Pacific and rises over the western Pacific. This change in pressure causes the
trades to weaken or reverse direction. This situation enhances the countercurrent
that carries warm water from the west over a vast region of the eastern tropical
Pacific. The thermocline, which separates the warm water of the upper ocean from
the cold water below, changes as the ocean conditions change from non-El Niño
to El Niño.
Average sea surface temperature departures from normal as measured by
(a) During El Niño conditions, upwelling is greatly diminished and warmer than
normal water (deep red color), extends from the coast of South America
westward, across the Pacific.
During La Niña conditions, strong trade winds promote upwelling, and
cooler than normal water (dark blue color) extends over the eastern and
Typical winter weather patterns across North America during an El Niño warm event.
Typical winter weather patterns across North America during a La Niña cold event.
Regions of climatic abnormalities associated with El Niño-Southern Oscillation conditions. A strong ENSO
event may trigger a response in nearly all indicated areas, whereas a weak event will likely play a role in
only some areas. Note that the months in black type indicate months during the same years the major
warming began; months in red type indicate the following year. (After NOAA Climatic Prediction Center.)