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):

H LL

H LL H LL

H HL

1 2

3 4

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 LF

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 rotationIt undergoes an apparent deflection of objects in movement:TO THE RIGHT: Northern HemisphereTO 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

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(Figure 5.14 Book, very important for climate processes)

THE ITCZ AND MONSOON CIRCULATION

ACTUAL SURFACE WINDS AND PRESSURE PATTERNS

•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)

•Movement of ITCZ •Change in pressure pattern

Monsoon (in Asia)

Monsoon:

•Summer Monsoon: WET

Warm, humid air comes from Indian Ocean and southwest Pacific

•Winter Monsoon: DRY

Dry, continental air fromnorth

HIGHER LATITUDES :

ACTUAL SURFACE WINDS AND PRESSURE PATTERNS

•Southern and Northern hemisphere are different: NH: two large continental massesSH: 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 permanent anticyclone (South Polar High)

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 ocean-atmosphere 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

EL NIÑO CONDITIONS

WHAT IS EL NIÑO?

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 stronger winds

How does pressure changes with elevation?

H L Sea level

H2

950mb

H3

H4

900mb

850mb

800mb

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 airFASTER in COLD air

950mb

900mb

850mb

800mb

High

HeightLow

Height

Height (of pressure):HIGH in WARMER airLOW in COLDER air

warmercolder

HLH1

L H

Figure 5.17, Page 139

POLE (90°) 30° latitude

ROSSBY WAVES

1. Waves arise in the Polar Front

•Undulations of upper air westerlies

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

Formation:

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 riseIf 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 riseIf warm air is stable:Steady precipitationIf 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

Air spirals inward and upward

condensation, precipitation

Air spirals outward and downward

condensation cannot occur

CYCLONES ANTICYCLONES

TYPES OF CYCLONES:

1. Wave cyclones (middle and high latitudes, ~1000km or 600miles)

2. Tropical cyclones (hurricanes, typhoons)3. Tornado

WAVE CYCLONES

The wave cyclone forms, intensify and dissolves along the polar front

Two anticyclones, one with warm air, and the other with cold polar air are in contact in the polar front.

FORMATION

A. Early stage:Wave formation in the polar frontCold air southward, warm air

northwardB. 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 CYCLONES: HURRICANES

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

• 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 whenwind 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 PacificDecrease in Atlantic, Gulf of Mexico and Caribbean sea

THE GLOBAL CLIMATE

WEATHER

CLIMATE

The condition of atmosphere at any particular time and place. It’s always changing

The synthesis of weather, the average weather of a region over a period of time

Weather surface map, June 30th 2005

Annual average precipitation

CLIMATE CLASSIFICATION

1. LOW LATITUDE CLIMATES

2. MIDLATITUDE CLIMATES

3. HIGH LATITUDE CLIMATES

• Influence of cT, mT, mE air masses• Equatorial low pressure and subtropical high pressure belts, ITCZ• Tropical cyclones

• Interaction of mT and cP air masses (polar front)•Wave cyclones

• Influence of cP and cA air masses (Northern Hemisphere), mP and cAA air masses (Southern Hemisphere)

LOW LATITUDE CLIMATES

1. Wet Equatorial2. Monsoon and trade-wind coast3. Wet-dry tropical4. 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. Dry subtropical 2. Moist subtropical3. Mediterranean4. Marine west-coast5. Dry midlatitude6. 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