Download Santa Ana Winds

Santa Ana Winds
•  Surface weather map
showing typical Santa
Ana conditions. High Desert
Elevation
~1500-2000 ft
Santa Ana Winds
~1500 meters
0 meters
Santa Ana Winds
~875 mb
~1500 meters
~875 mb
Horizontal Pressure Gradient
Starts air moving.
0 meters
~1000 mb
Santa Ana Winds
~875 mb
~1500 meters
~875 mb
Horizontal Pressure Gradient
Starts air moving.
As air moves toward coast, it
will descend
0 meters
~1000 mb
Clicker Question
Set Frequency to "BB"
As air descends, it will
(A) cool at the dry adiabatic lapse rate
(B) warm at the dry adiabatic lapse rate
(C) cool at the moist adiabatic lapse rate
(D) warm at the moist adiabatic lapse rate
Clicker Question
Set Frequency to "BB"
As air descends, it will
(A) cool at the dry adiabatic lapse rate
(B) warm at the dry adiabatic lapse rate
(C) cool at the moist adiabatic lapse rate
(D) warm at the moist adiabatic lapse rate
Santa Ana Winds
~875 mb
~1500 meters
~875 mb
Horizontal Pressure Gradient
Starts air moving.
As air moves toward coast, it
will descend
0 meters
~1000 mb
Sinking air compresses and warms
at rate of 10 C / km
Santa Ana Winds
Prevailing
Wind
Direction
Thermal Circulations:
• Circulation set up by change in air temperature
warm air
cold air
Psurf=1000mb
Psurf=1000mb
Clicker Question
Set Frequency to "BB"
warm air
cold air
PA
PB
Psurf=1000mb
Psurf=1000mb
2 km
Two locations both with equal surface pressure = 1000mb, but one has a column of cold air and the other a column of warm air. What can you say about the pressures PA and PB at a height of 2 km?
(A) PA > PB
(B) PA < PB
(C) PA = PB
Clicker Question
Set Frequency to "BB"
warm air
cold air
PA
PB
Psurf=1000mb
Psurf=1000mb
2 km
Two locations both with equal surface pressure = 1000mb, but one has a column of cold air and the other a column of warm air. What can you say about the pressures PA and PB at a height of 2 km?
(A) PA > PB
(B) PA < PB
(C) PA = PB
P(z)
PSURF
Pressure ∝ atmospheric mass above = (total mass) – (mass below)
Start with two locations with same temperature
atmospheric
columns of air
Psurf=1000mb
Psurf=1000mb
Now warm one and cool the other
- warm column expands (air is less dense)
- cold column contracts (air is more dense)
warm air
atmospheric
columns of air
cold air
Psurf=1000mb
Psurf=1000mb
Now warm one and cool the other
- warm column expands (air is less dense)
- cold column contracts (air is more dense)
warm air
atmospheric
columns of air
cold air
PA
PB
Psurf=1000mb
Psurf=1000mb
At level Z, about 50% of mass above on cold side
At level Z, about 67% of mass above on warm side
Since Pressure ∝ mass above:
at level Z, PB > PA
height = Z
Set Frequency to "BB"
Clicker Question
cold air
warm air
PA
PB
Psurf=1000mb
PGF
2 km
Psurf=1000mb
Two locations both with equal surface pressure = 1000mb, but one has a column of cold air and the other a column of warm air. What can you say about the pressures PA and PB?
- Warm air less dense
(B) PA < PB
- Vertically, pressure decreases slower in warm column - Temperature difference creates PGF aloft
Thermal Circulations:
• Circulation set up by change in air temperature
• Sea-Breeze is a thermal circulation
- diurnal time scale
• Monsoons
- seasonal time scale (months)
SEA BREEZE CIRCULATION
Begin with isobars parallel to surface
- no horizontal pressure gradients
(at start, air same temperature over land and water)
SEA BREEZE CIRCULATION
Begin with isobars parallel to surface
- no horizontal pressure gradients
(at start, air same temperature over land and water)
Sun’s energy
heats surface
Specific heat of land lower than specific heat of water:
- land warms faster than water
SEA BREEZE CIRCULATION
Begin with isobars parallel to surface
- no horizontal pressure gradients
(at start, air same temperature over land and water)
Sun’s energy
heats surface
KEY FACTOR!!!
Specific heat of land lower than specific heat of water:
- land warms faster than water
SEA BREEZE CIRCULATION
Land warms and
heats air above
- conduction
- convection
Air above land warms: first through conduction, then convection.
- at first, convective currents do not travel very high, however
as land continues to warm, convection brings warm air higher.
SEA BREEZE CIRCULATION
Warmer air column
less dense
Isobars over land
“tilt”
Warmer air over land less dense than colder air over water:
--> isobars lift up over the land
--> one must climb higher to pass through the same mass of air
SEA BREEZE CIRCULATION
Tilted isobars cause horizontal
pressure gradient
- Air flows offshore aloft
Tilted isobars result in horizontal pressure gradient aloft
--> still no horizontal pressure gradient at surface
Aloft, air flows from HIGH pressure to LOW pressure.
(Assume no Coriolis effect here)
SEA BREEZE CIRCULATION
Surface pressure over
land decreases as air
aloft moves away
Surface pressure over
ocean increases as
air aloft moves over
As air moves from HIGH to LOW aloft, less air mass over land
--> surface pressure over land decreases
As air moves toward LOW, the mass of air over water increases
--> surface pressure over water increases
SEA BREEZE CIRCULATION
Pressure gradient near surface
results in onshore flow
Now have horizontal pressure gradient near surface
Near surface, air flows from HIGH to LOW
--> near surface ONSHORE flow: from water towards the land
--> aloft, flow is OFFSHORE: from land towards the water
SEA BREEZE CIRCULATION
Air added at surface rises to
replace air removed aloft
Air added aloft sinks to replace
air removed at surface
- Over the land, air rises from the surface to replace the air leaving aloft.
- Convection, initiated by the warm land surface augments this rising air.
- Over the ocean, air sinks to replace the surface air that moves inland.
- Over the land, the air is rising. If the rising air contains enough water
vapor, condensation will take place and clouds will form over the land.
- NOTE: The HIGHS and LOWS labeled are relative to pressures at a
given HORIZONTAL level.
Annual Thunderstorm Frequency
WINTER
Changing annual wind flow patterns
associated with the winter Asian
monsoon. Clear skies and winds blow
from land to sea
SUMMER
Changing annual wind flow
patterns associated with the
summer Asian monsoon. Warm
humid air blows up from equator
bringing rainy weather.
North American Monsoon
(Arizona Monsoon)
Early Summer
Late Summer