Transcript Document

WIND AND PRESSURE (Chapter 5 Book)
• Wind is the movement of air, generally is horizontal
•Air tends to move from high to low pressure
H
L
When pressure in two locations is unequal: We have a pressure gradient
And a Pressure Gradient Force will produce a wind
A CONVECTIVE WIND SYSTEM (Page 130):
2
1
L
3
H
L
4
L
H
L
L
H
H
L
L
H
SEA AND LAND BREEZES
• An example of a convective wind system
THE CORIOLIS EFFECT AND WINDS
We know that: air moves from high to low pressure
H
F
L
F is the pressure
gradient force
GLOBAL SCALE:
Direction of wind is somewhat different due to Coriolis effect
WHAT IS THE CORIOLIS EFFECT?
It’s a result of the Earth’s rotation
It undergoes an apparent deflection of objects in movement:
TO THE RIGHT: Northern Hemisphere
TO LEFT: Southern Hemisphere
HOW IS THE CORIOLIS EFFECT IN WINDS?
Once air has been set in motion by the pressure gradient force,
there is an apparent deflection from its path, as seen by an observer
on the Earth.
HOW WOULD BE THE RESULTING PATH IN THE
SOUTHERN HEMISPHERE?
SURFACE WINDS ON AN IDEAL EARTH
IDEAL EARTH:
• No complicated pattern of land and water
• No seasonal changes
GLOBAL SURFACE WINDS
•Because air rises at Equator, a surface LOW is
generated
• At 30° latitude a surface HIGH is generated
(air descends as part of Hadley Cell)
•At 60° cold climate: surface LOW
ACTUAL SURFACE WINDS AND PRESSURE PATTERNS
(Figure 5.14 Book, very important for climate processes)
SUBTROPICAL HIGH PRESSURE BELTS
• Southern Hemisphere: a similar pattern with “ideal Earth”. There
are 4 centers (3 over the oceans and one over Australia)
• Northern Hemisphere: 2 centers of High pressure (Hawaiian High
and Azores High). They move northward during summer
•They have influence on North America during summer
ACTUAL SURFACE WINDS AND PRESSURE PATTERNS
THE ITCZ AND MONSOON CIRCULATION
•Sun is directly overhead: equator, tropics of Cancer and Capricorn,
depending on season
•Hadley cell circulation is driven by this heating.
The ITCZ changes with seasons
(South America, Africa, Asia)
Monsoon:
•Movement of ITCZ
•Change in pressure pattern
Monsoon (in Asia)
•Summer Monsoon: WET
Warm, humid air comes from
Indian Ocean and southwest
Pacific
•Winter Monsoon: DRY
Dry, continental air from
north
ACTUAL SURFACE WINDS AND PRESSURE PATTERNS
HIGHER LATITUDES :
•Southern and Northern hemisphere are different:
NH: two large continental masses
SH: large ocean, and a cold glacier land (Antartic)
•Northern Hemisphere:
Continents: surface HIGH pressure in winter (strong Siberian High)
surface LOW pressure in summer
•Southern Hemisphere:
Permanent ice sheet of Antartica
High)
permanent anticyclone (South Polar
OCEAN SURFACE CURRENTS
(Figure 5.22, relate with Figure 5.14)
Exchanging heat between low and high latitudes (important regulators of
air temperature)
WHAT IS UPWELLING?
It’s the rising of deeper colder water
•
Nutrient-rich water rises from deeper levels to replace
the surface water that has drifted away
•
Large fishing population in these areas
WHAT IS EL NIÑO?
It’s a disruption of the oceanatmosphere system in the Tropical
Pacific, having important
consequences for weather around
the world
NORMAL CONDITIONS
Trade winds blows toward
west across the Tropical
Pacific
warm water is piled up
in the west Pacific
WHAT IS EL NIÑO?
EL NIÑO CONDITIONS
Trade winds relax in central
and western Pacific
Depression of thermocline in
eastern Pacific
Reduce upwelling to cool
Rise in Sea Surface
Temperature in central and
East Pacific (coast of South
America)
global impacts
This phenomena takes place at intervals of ~3-8 years
(1982-83 and 1997-98 were the last extreme events)
WINDS ALOFT
How does air move at higher elevations?
The gradient force INCREASES with altitude
How does pressure changes with elevation?
stronger winds
800mb
colder
warmer
850mb H4
800mb
850mb
H
L
H3
900mb
H2
900mb
Low
Height
L
H
950mb
H
POLE (90°)
L
950mb
High
Height
H1
Sea level
30° latitude
Reasons why winds are stronger at upper levels (in mid-high latitudes)
1.
Difference of temperature (poles and 30° lat for example)
2. Pressure decreases with elevation :
SLOWER in WARMER air
FASTER in COLD air
Height (of pressure):
HIGH in WARMER air
LOW in COLDER air
Figure 5.17, Page 139
ROSSBY WAVES
•Undulations of upper air westerlies
Formation:
1. Waves arise in the Polar Front
2. Warm air pushes pole ward and a tongue of cold air
pushes southward (undulation development)
3. Waves are strongly developed. Cold air are “troughs”
of low pressure
4. Waves are pinched off, forming cyclones of cold air
ROSSBY WAVES
•They are important for poleward heat transport
•Reason for variable weather in midlatitudes
JET STREAMS
• Regions at high elevation with strong wind streams, wind speed
more than: 58 knots
•They take place where atmospheric pressure gradients are strong
•The greater the contrast in temperature,
the stronger the jet streams blow
•Jet streams tend to be weaker in summer than in winter
•Jet streams are less intense in the Southern Hemisphere due to
smaller land masses
Upper level weather map (200mb) for June 27th 2005
AIR MASSES
(Chapter 6)
•A large body of air with uniform temperature
and moisture characteristics.
• They acquire their characteristics in source regions
•Air masses move from one region to another (due to pressure gradient, upper
level winds, jet stream)
•When moving, the properties are influenced by the new environment
TYPICAL AIR MASSES
Air mass
Symbol
Source region
Properties
Maritime equatorial
mE
warm oceans in
equatorial region
warm, very moist
Maritime Tropical
mT
warm oceans in tropical
region
Warm, moist
Continental tropical
cT
Subtropical deserts
Warm, dry
Maritime polar
mP
Midlatitude oceans
Cool, moist(winter)
Continental polar
cP
Northern continental
interiors
Cold, dry (winter)
Continental artic
cA
Regions near north pole
Very cold and dry
Continental antartic
cAA
Regions near south pole
Very cold and dry
TYPICAL AIR MASSES
NORTH AMERICAN AIR MASSES
Strong influence on North American weather
1.
Continental Polar (cP)
Tongues of cold dry air; periodically extend
S,E producing cold temperatures and clear
skies
2. Continental Artic (cA)
When moving southward: severe cold wave
3. Maritime Polar (mP)
Unstable in winter: heavy precipitation over
coastal ranges
4. Maritime Tropical (mT)
Moves northward bringing moist unstable air:
thunderstorms
5. Continental Tropical (cT)
Does not move widely, influence
weather conditions over source region
FRONTS
Transition zone between two air masses of different characteristics
COLD FRONT
Transition zone where a cold air
mass invades a warmer air mass
Colder air mass remains in contact
with ground (because is denser)
It forces warm air mass to rise
If warm air is unstable:
thunderstorms
WARM FRONT
Transition zone where a warm air
moves into a region of colder air
Cold air remains in contact with
ground (denser)
It forces warm air mass to rise
If warm air is stable:
Steady precipitation
If warm air is unstable:
thunderstorms
OCCLUDED FRONT
When a cold front overtakes a warm front (cold fronts move at a
faster rate than warm fronts)
STATIONARY FRONT
A front that is not moving. Masses are not strong enough to
replace each other
TRAVELING CYCLONES AND ANTICYCLONES
CYCLONES
Air spirals inward and upward
condensation, precipitation
ANTICYCLONES
Air spirals outward and downward
condensation cannot occur
TYPES OF CYCLONES:
1.
2.
3.
Wave cyclones (middle and high latitudes, ~1000km or 600miles)
Tropical cyclones (hurricanes, typhoons)
Tornado
WAVE CYCLONES
The wave cyclone forms, intensify and dissolves along the polar front
FORMATION
Two anticyclones, one with warm air, and the other with cold polar air
are in contact in the polar front.
A. Early stage:
Wave formation in the polar front
Cold air southward, warm air northward
B. Open stage:
Wave is deepened and intensified
C. Occluded stage:
Cold front overtakes warm front (occluded front)
Precipitation is intensified
D. Dissolving stage:
The cold front is reestablished
HOW DOES A WAVE CYCLONE AFFECT WEATHER?
TROPICAL AND EQUATORIAL WEATHER SYSTEMS
• Weak upper level winds
•Air masses have similar characteristics (warm, moist)
•Air masses move slowly
•NO fronts and wave cyclones
•Intense convectional activity
TROPICAL CYCLONES: HURRICANES
•
•
Hurricanes are tropical cyclones with winds greater
than 74 mi/hr.
They circulate:
counterclockwise in Northern Hemisphere
clockwise in Southern Hemisphere
HOW ARE HURRICANES FORMED?
Thunderstorms
+
Ocean heat and moisture (ocean temps warmer than 26.5°C or 81°F)
+
High relative humidity in middle and upper troposphere
+
Low wind shear (change of wind speed with elevation,
storms grow vertically)
STAGES OF DEVELOPMENT
1. TROPICAL DEPRESSION
A low pressure is formed in the center of
the thunderstorm group (winds 23-39mph)
2. TROPICAL STORM
• Tropical depression intensifies (39-73
mph).
• It is assigned a name at this time.
•Strom is more circular in shape
3. HURRICANE
• Tropical storm becomes a hurricane when
wind speed reaches 74mph
• A pronounced rotation develops around the
central core
HOW IS THE RELATIONSHIP WITH EL NIÑO?
More tropical storms and hurricanes in Eastern Pacific
Decrease in Atlantic, Gulf of Mexico and Caribbean sea
THE GLOBAL CLIMATE
WEATHER
The condition of atmosphere at any particular time
and place. It’s always changing
Weather surface map, June 30th 2005
CLIMATE
The synthesis of weather, the average weather of
a region over a period of time
Annual average precipitation
CLIMATE CLASSIFICATION
1. LOW LATITUDE CLIMATES
• Influence of cT, mT, mE air masses
• Equatorial low pressure and subtropical high pressure belts, ITCZ
• Tropical cyclones
2. MIDLATITUDE CLIMATES
• Interaction of mT and cP air masses (polar front)
•Wave cyclones
3. HIGH LATITUDE CLIMATES
• Influence of cP and cA air masses (Northern Hemisphere), mP and
cAA air masses (Southern Hemisphere)
LOW LATITUDE CLIMATES
1.
2.
3.
4.
Wet Equatorial
Monsoon and trade-wind coast
Wet-dry tropical
Dry tropical
CLIMATE
PATTERNS
FACTORS
WET EQUATORIAL
Heavy precipitation, uniform
temperature all months
ITCZ, mE, mT air masses
MONSOON TRADE WIND
COAST
Heavy precipitation and warm
temperatures, with annual cycle
mE and mT brought by trade
winds; summer: ITCZ closer,
winter: high pressure. Monsoon
patterns (India)
WET-DRY TROPICAL
Very dry and warm in winter,
very wet and cold in summer
ITCZ proximity
DRY TROPICAL
Very low rainfall and high
temperatures
Subtropical high pressure cells
(adiabatic warming)
HIGH ELEVATION
Cold climate (higher: colder),
annual cycle
Orographic rainfall; ITCZ
influence
MIDLATITUDE CLIMATES
1.
2.
3.
4.
5.
6.
Dry subtropical
Moist subtropical
Mediterranean
Marine west-coast
Dry midlatitude
Moist continental
CLIMATE
PATTERNS
FACTORS
DRY SUBTROPICAL
Low precipitation, great annual
temperature cycle
Subtropical highs (25°-35°)
MOIST SUBTROPICAL
Abundant rainfall on eastern
side of continents all year
Summer: flow of warm/moist
air from oceans (mT)
Winter: wave cyclones
MEDITERRANEAN
Wet winter and very dry summer
Poleward movement of high
pressure cells during
summer(cT dominates)
MARINE WEST COAST
Precipitation all months, max in
winter. Small annual range temp
Westerlies bring mP air
masses. Orographic effect.
Summer: subtropical high
pressure poleward
DRY MIDLATITUDE
Very low precipitation
Rain shadow of mountains.
Summer: convection rainfall
MOIST CONTINENTAL
Precipitation all year, max in
summer. Strong annual cycle of
temp
Polar front (cP,mP,mT
interaction). Summer: mT,
winter: cP,cA