Geography 103
Weather
Lecture 11: California Weather (Quiz 3,
final)
1. Synoptic weather systems
(1). The
North Pacific Subtropical Anticyclone (Hawaiian High).
(2). The
(3). The
(4). The extratropical cyclones
A. High-latitude
type (northern type)
B.
Middle-latitude type (western type)
C. Low-latitude
type (southwestern type)
(5). Tropical storms (hurricanes): rare.
2. Normal summer weather patterns
(1). The
North Pacific Subtropical High (Hawaiian High).
Strong, expansion, and at higher latitude (40 oN).
A. Following the Northward migration of the sun
B. Thermal effect or differential heating between land and ocean (relatively
cool ocean
water compared to the nearby continent).
(2). Heat Low over southwest
USA.
A. Extends over the central valley of California (California Heat Low).
B. Shallow (5000 ft or 850 mb).
C. Summer monsoon:
(A). Orographic and convective showers.
(B). Over the mountain and desert areas.
(C). Warm moist air )unstable) from the Gulf of Mexico
or the
D. Intense and persistent trade wind inversion.
E. Advection sea fog.
F. Infrequent tropical storms (hurricanes)
1918, 1932, 1939, 1976-78.
G. Temperature rises with increasing distance from the coast.
3. Normal winter weather patterns
(1). The
North Pacific subtropical High
Weak, shrinking,
and at lower latitude (20 oN).
(2). The Aleutian Low
intensifies.
Thermal effective or differential heating between land and ocean (relatively warm ocean
compared
to the nearby continent).
(3). Radiation fog over the
central valley of California.
(4). Temperature decreases
from the coast toward inland.
(5). Precipitation:
A. Precipitation season
October to May of the following year.
B. Wettest month
Following the southward migration of the jet stream.
(A). Northern California: December.
(B). Central California: January.
(C). Southern California: February
C. Mean annual rainfall at LA civic Center: 14 inches.
4. Major winter storm tracks
(1). High-latitude type
(Northern type)
A. Originates north of 45 oN off the
B. Moves southward to southern California then shifts toward east.
C. Brings light and short-duration showers to southern
(A). The amount of precipitation is dependent on the
nearness of the center of the
storm to southern
(B). Cloudy or less than 1 inch precipitation/storm.
D. Tends to have Santa Ana wind following the passage of the storm.
(2). Middle-latitude type
(West type)
A. Originates between 35 and 45 oN in the
central Pacific (north of Hawaii).
B. Travels eastward approximately along 40 oN
to north and central California.
C. Bring median amount of precipitation to southern California (1-2 inches).
D. Storm center: near San Francisco.
E. The situation is terminated when the Alaskan low dissipates.
(3). Low-latitude type
(Southwest type)
A. Originates north of Hawaii but south of 35 oN
in the central Pacific.
B. Travels toward central and southern California.
C. Brings heavy precipitation to southern California (more than 2 inches)
(A). Pineapple express ( a series of wave cyclones).
(B). Associated with a subtropical jet.
D. The situation is terminated when the blocking dissipates or moves out of the
Gulf of Alaska.
5. The North Pacific Subtropical High
(Hawaiian High)
(1). Causes
A. Dynamic effect
Upper-level subsidence results from the meridional
circulation (Horse latitude).
B. Thermal effect (differential heating between land and ocean)
(A). Winter: Low over ocean and High over land
(relatively cold).
(B). Summer: Low over land and High over ocean
(relatively cold).
(2). Winter
A. The North Pacific High weakens and shrinks due to the thermal effect
(relatively warm ocean compared to the nearby land).
B. The position displaces toward the equator following the migration of the sun
toward
the Southern Hemisphere.
(3). Summer
A. The North Pacific Subtropical High intensifies and expands due to the
thermal effect
(relatively cold ocean compared to the nearby land).
B. The position shifts toward higher latitudes following the migration of the
sun northward.
(4). Dynamic effect always
creates the subtropical High. Only the thermal effect is responsible
for the weakening and intensification of the High.
6. The marine layer
(1). Definition: A cool and
moist air layer in a few thousand deep below an inversion layer.
(2). Causes:
A. The trade wind inversion created by the compressional
heating of subsiding air associated
with the North Pacific Subtropical High (The upper air originates from the
equator, the
Hadley circulation).
B. Cold ocean current off the California coast
(A). Water comes from north.
(B). Upwelling: Surface ocean water moves away from the coast due to Ekman’s spiral
(Surface ocean water moves to the right of the north wind due to the effect of Coriolis
force).
(3). Spatial distribution in
southern California
A. Increasing depth from the coast toward inland.
B. Ridge: North of the Palos Verdes.
C. Trough:
(4). Diurnal Variation
A. Minimum thickness: Late afternoon near sunset.
B. Maximum thickness:
(A). Coast: Early morning (9 am).
(B). Inland: Noon.
7. The trade wind inversion
(1). Causes
A. The compressional heating of the subsiding air associated with
the North Pacific
Subtropical High above the marine layer.
B.
(2). Seasonal variation
A. Winter
(A). Frequent ground surface inversion layers.
(B). Weaker and at lower elevation in summer.
B. Summer
(A). More frequent inversion but very few ground surface inversion layers.
(B). The strongest inversion during the year.
C. Spring (March-April)
Very weak and least frequencies of inversion layers.
(3). Geographical
variation
A. Root region
(A). The eastern part of the North Pacific Subtropical High (
(B). Intense and persistent inversion.
(C). Surface divergence (north winds).
(D). Cold ocean current off the coast.
(E). Arid (dry) climate.
B. Downstream region
(A). The western portion of the North Pacific Subtropical
High (East and
(B). Weak or absence of inversion.
(C). Surface convergence (South winds)
(D). Warm ocean current (solar heating when surface ocean water
travels thousands of
miles from the east to the west in the tropic).
(E). Humid climate.
C. Locations mean inversion base mean magnitude
Los Angeles 1500 ft 12 oC
Honolulu 6000 ft 6 oC
Manila 18000 ft 2 o C
8. Winds
(1). Land and sea breezes
A. Dominate daily
wind regimes (70% of the time).
B. Daytime:
(A). Sea breezes
Winds blow from sea to land or onshore winds.
(B). A weak surface low over land and a weak surface high over sea.
Land warms more rapidly than the adjacent sea due to lower specific heat
(about 0.2 cal/goC).
C. Night
(A). Land breezes
Winds blow from land to sea or offshore winds.
(B). A weak surface low over sea and a weak surface high over land.
D. 3 sea breeze convergence zone
(A).
(B). Palos Verdes
(C).
(2). The Santa Ana wind
A. Definition
A hot , dry, Foehn-like wind, generally from the NE
or E, especially in the pass
and river valley of Santa Ana, California, where it is further modified as a
mountain
gap wind.
B. Synoptic weather patterns
(A). The
a. The High weakens with elevation and becomes a low at the 500-mb
level.
b.
1035 mb (10 mb higher than
the normal) over the
in homepage)
(B). The front
The Great Basin High builds up following the passage of a cold front Northern
type)
(C). A surface low to the south of
(D). An upper-ridge (500-mb) just off the west coast
of the
C. Effects
(A). Lower or destroy the inversion layer.
(B). Strong winds
a. High pressure gradient force.
b. Canyon effect
Cajon pass, banning pass, Santa Clara (Antelope Valley) pass, and Newhall pass.
c. Wind speed greater than 12 knots.
10% of the Santa Ana wind speed exceeds 50 knots.
(C). Relative humidity: 10-20%.
(D). Visibility
a. Strong Santa Ana wind: May cause low visibility because of blowing dusts.
b. Weak Santa Ana wind:
(a). Clear sky when the Santa Ana wind reaches the ground surface.
(b). Heavy fog or smog when the Santa Ana wind blows above the marine layer.
(E). Air temperature
The warmest temperature in winter (90 oF or higher)
D. Santa Ana condition
(A). Weak wind or calm associated with the Great Basin High.
(B). May cause high temperature in winter and summer.
Northridge temperature exceeds 100 oF in
summer.
E. Seasonal variation
(A). Late autumn and winter.
(B). December and January: Most frequent.
(C). Approximately 16 Santa Ana winds per year.
F. Diurnal variation
Stronger at night.
(A). Night: Santa Ana wind is accelerated by the land breeze since both winds
are
offshore.
(B). Daytime:
wind).
9. Catalina Eddy
(1). Definition
Sub-synoptic (mesoscale) cyclonic circulation in the bight (small gulf)
of southern
(2). Causes
A. Orographical induced cyclonic vortex in the lee of
the east-west oriented Mountain Ynez
in Santa Barbara.
B. Approach of an upper trough.
(3). Effects
A. Deepening of the marine layer (higher ceiling)
Stratus formation.
B. Winds: SE winds along the coast (San Diego to Point Conception).
C. Pollution is advected (horizontal movement or
wind) offshore.
10. Stratus and fog
(1). Stratus
season
A. April to October.
B. July and August: Most frequent.
C. March and November: Minimum frequency.
(2). Formation
A. Advection
Stratus and fog are formed over ocean and advected
inland by strong sea breezes.
B. Deepening of marine layer
Marine layer rich in condensation nuclei.
(3). Synoptic
patterns
A. The North Pacific Subtropical High.
(A). Radiation fog: Clear night and light wind.
(B). The inversion base determines the marine
layer’s depth.
Stratus forms when marine layer becomes deep, allowing rising air parcel to
reach
dew point temperature below the inversion base.
B. An approaching low and front:
Thickening the marine layer and intensifying the onshore flow.
(4). Diurnal
variation
A. Most frequent stratus and fog at night and early morning.
B. Stratus retreats seaward leaving an open hole off southern
(A). Burnoff:: evaporation
of clouds and fog due to solar heating during the day.
(B). Upper return flow (offshore wind) of surface sea breezes carries clouds
seaward.
C. Ceiling: 1000 to 2000 ft.
D. Thickness of stratus: 500 to 1000 ft.