Atmospheric Pressure and Wind Systems

Atmospheric Pressure and Wind Systems
Global Distribution of Air Pressure: - Global Surface (Horizontal) Pressure Belts Nature of Winds - Causes of Wind - Cyclones and Anticyclones

Atmospheric Pressure and Wind Systems
General Circulation of the Atmosphere - Global Surface Wind Systems - Regional Wind Systems - Local Wind Systems

Atmospheric Pressure It is the force or weight of an air column exerted on the surface It’s measured using mercury Barometer Torricelli (1643) measured air pressure with a mercury barometer

Atmospheric Pressure Standard barometric pressure: - at sea-level: inches or mb or 14.7Ibs/in2 or 1kg/cm2 - high pressure is any value higher than mb ( mb) - low pressure is any value lower than mb ( mb) Note: 1cm = 13.3mb or 1in = 34mb

Mercury Barometer Invented By Torricelli

Atmospheric Pressure Air pressure varies vertically and horizontally Heating or warming surface temperatures cause air pressure to decrease due to: - air expansion, and - increased vibration/collision of air molecules

Atmospheric Pressure Cooling or cold surface temperatures cause air pressure to increase due to: - air contraction or crowding of air molecules - reduced vibration/collision of air molecules

Atmospheric Pressure In general, cold surfaces in winter develop thermally induced areas of high pressure while, warm surfaces in summer develop thermally induced low pressure Strongly rising air often produces low pressure at the surface (a dynamic low)

Atmospheric Pressure Strongly descending air often produces high pressure at the surface (a dynamic high)

Global Belts of Low & High Pressures
The global belts of low and high pressures include: - Equatorial trough of low pressure - Subtropical high pressure - Sub-polar low pressure - Polar high pressure

Global Belts of Low and High Pressures

Equatorial Trough of Low Pressure - centered at the equator - occurs between latitudes 5oN & S - It’s thermally induced Sub-Tropical Pressure Belt - centered at latitudes 30oN & S - occurs between lat. 25o and 35oN & S - dynamically induced - zone of air subsidence - zone of major deserts

Location of Major Deserts at Sub-Tropical Pressure Belts

Sub-Polar Low Pressure - centered at lat. 60o N & S - dynamically induced due to strong lifting of warm air as it meets cold air from the pole

Polar High Pressure - centered at the poles - forma a circular pressure cap over the polar region - thermally induced

The global pressure belts represent average pressure conditions Belts shift several degrees of latitudes annually following the overhead sun Belts are better defined in the southern hemisphere than in the north because of large homogenous water body

Poorly defined belts in the N.H. because of remarkable land and water contrasts In winter, the sub-polar low in N.H. is not continuous, hence: - over warmer oceans, the Aleutian low and the Icelandic low persist - but, over colder land surface, the Siberian & Canadian high pressures form instead

Average Atmospheric Pressure (Winter)
Canadian High Icelandic Low Siberian High

In summer, the subtropical high pressure belt is not continuous in N.H., hence: - over colder oceans, the Hawaiian and Bermuda high pressure persist - But, over warmer land surface, Asian low pressure develops in this belt of high pressure

Average Atmospheric Pressure (Summer)
Hawaiian High Bermuda High Asian Low

The Nature of Atmospheric Pressure
Mapping pressure with isobars Pressure measured with a barometer Typical units are millibars or inches of mercury Contour pressure values reduced to sea level Shows highs and lows, ridges and troughs

Nature of Winds Some Definitions: Wind: Air in horizontal motion Updraft: Small-scale air in upward vertical motion Ascent: Large-scale air in upward vertical motion Downdraft: Small-scale air in downward vertical motion Subsidence:Large-scale air in downward vertical motion

The Nature of Wind Origination of wind
Uneven heating of Earth’s surface creates temperature and pressure gradients Direction of wind results from pressure gradient Winds blow from high pressure to low pressure

The Nature of Wind Wind speed
Tight pressure gradients (isobars close together) indicate faster wind speeds Wind speeds are gentle on average

Vertical Variations in Pressure and Wind
Atmospheric pressure decreases rapidly with height Atmospheric surface pressure centers lean with height Winds aloft are much faster than at the surface Jet streams

Causes of Wind The principal causes of wind are: - solar energy - pressure gradient force - coriolis force - frictional force

Causes of Wind Solar Energy: - differences in the distribution of solar energy determines low and high pressure belts - air moves from high to low pressure areas

Causes of Wind Pressure Gradient Force: - spatial variation of pressure produce pressure gradient force - the force causes air to move from high to low pressure area across the isobars - determines wind direction and causes wind to blow at right angle or perpendicular to the isobars

Pressure Gradient Force

Pressure Gradient Force Only

Isobars: Pressure Gradient

Causes of Wind - determines wind speed such that steeper pressure gradient force produces stronger wind speed

Causes of Wind Coriolis Force: - produced by earth rotation - it turns wind to the right of its direction in the N.H. and to the left in the S.H Coriolis effect increases in strength poleward - It only affects wind direction, not wind speed, though faster winds turn more

Coriolis Effect

Geostrophic Wind

Coriolis Effect and Geostrophic Winds

Causes of Wind - prevents the wind from flowing down the pressure gradient - when pressure gradient force is equal to coriolis force, GEOSTROPHIC WIND develop and moves parallel to the isobars

Causes of Wind Frictional Resistance: - caused by surface roughness or molecular friction within the air stream - reduces wind speed by 60% on land and produces wind turbulence (eddying and swirling motions) - Does not affect upper levels

Frictional Resistance

Causes of Wind - wind speed is faster over water bodies because of smoother surface (only 40% reduction in speed) - interferes with coriolis force by causing a less than 90o deflection - produces o change in wind direction

Causes of Wind results in wind blowing at some intermediate angle between pressure gradient and coriolis forces

The Influence of Pressure Gradient Force (PGF), Coriolis Effect (CE), and Frictional Resistance (FR) on Wind Direction 900mb 950mb 1000mb 1050mb CE

Effects of Friction, Coriolis Effect, and Pressure Gradient Force on Wind Direction

Cyclones Cyclones describe wind flow pattern around a low pressure center air converge at the low pressure center and rises to the upper level clouds can easily form in the N.H., airflow is a counterclockwise in-spiral into the low pressure center

S.H. Cyclone (surface) Clockwise Flow
N.H. Cyclone (surface) Counterclockwise Flow

Converging & Rising Air in a Cyclone
Cyclones Converging & Rising Air in a Cyclone Descending & Diverging Air in a Anticyclone

Cyclones in the Southern Hemisphere, airflow is a clockwise in-spiral into the low pressure center commonly associated with bad weather

Anticyclones air flow pattern around a high pressure center air divergence at the high pressure center leading to air subsidence from the upper level Northern Hemisphere: air flow is a clockwise out-spiral from the high pressure center

N.H. Anticyclone (surface) Clockwise Flow
S.H. Anticyclone (surface) Counterclockwise Flow

Converging & Rising Air in a Cyclone
Cyclones Converging & Rising Air in a Cyclone Descending & Diverging Air in a Anticyclone

Anticyclones Southern Hemisphere: air flow is counterclockwise Commonly associated with fine weather condition

Global Surface Wind Systems
Main prevailing surface winds are: - Trade winds - Westerly - Polar Easterlies there are four zones of variable winds and calms over the four existing pressure belts in each hemisphere Monsoon winds

Trade Winds - originates from the equator ward side of the subtropical high pressure belt - the wind is deflected to the west in its movement to the equator to blow as an easterly wind - in N.H., it is the North-East (NE) Trade wind

High Pressure Belt as Source of Surface Winds

Hadley Cell: Air Flow Between Equatorial Low and Sub-Tropical High Pressure Belts

Trade Winds Belt

- in S.H., it’s South-East Trade wind - trade winds are persistent with a steady direction - originates as warm dry winds and prevails between the equator & lat. 30º - could cause heavy precipitation over tropical oceans

- The trade winds become monsoon wind in SE Asia and Africa once they cross the equator - Could cause dry and dusty winds when it blows over continents, especially over desert areas - Causes Harmattan wind in West Africa in December through March

Westerlies - originates on the poleward side of the Subtropical High Pressure Belt - wind prevails between lat. 30º and 60º - disrupted by landmasses in N.H. - they are strong and persistent winds in Southern Hemisphere

The Westerlies Belt

- wind velocities increase poleward in the south and sailors describe these increases by terms like: roaring forties furious fifties screaming sixties - mid-latitude depressions are associated with this wind

Polar Easterlies - Global Surface Wind Systems - wind moves from east to west - it is cold and dry

Polar Easterlies Belt

Intertropical Convergence Zone (ITCZ) - it is where the 2trade winds meet at or close to the equator - zone of calm and variable winds (Doldrums) - zone of weak horizontal airflow

- ITCZ shifts north or south following the overhead sun - zone of instability and rising air (updraft) during thunderstorms

Local Wind Systems The main types of local winds described are: - Land and Sea Breeze - Chinook/Foehn Winds - Drainage Winds or Katabatic Winds - Mountain and Valley Winds

Local Wind Systems: Land & Sea Breeze
Land and Sea Breeze - commonly experienced along tropical coastlines any time of the year and in summers in mid-latitudes - involves sea breeze on land during the day and land breeze over the ocean surface at night

- caused by differential heating of land and water to form a small-scale convectional circulation - low pressure develops over warm land in the day causing low pressure to develop over land - high pressure develops over relatively colder ocean surface during the day

Land and Sea Breeze

- hence, sea breeze develops and blows - at night, low pressure develops over relatively warmer ocean surfaces and high pressure over relatively colder land surface - hence land breeze develops & blows offshore over the oceans at night

Local Wind Systems: Chinook/Foehn Winds
It is a local downslope wind It’s called Chinook (snow-eater) in the Rockies and foehn wind in the Alps It begins as moisture laden wind that is forced to rise the windward side of the slope

It causes heavy precipitation on the wind ward side & a relatively dry wind is pulled over the leeward side of the mountain the descending air is compressed and warmed up adiabatically the wind arrives the base of the mountain on the leeward side as a warming, drying wind

Chinook Wind

It’s capable of melting snow very rapidly and makes it possible to keep the animals longer in the field in winter A similar wind is called Santa Anas in California Santa Anas is noted for its high speed, high temperature and extreme dryness

Santa Anas provides ideal condition for wildfires in summers and fall in California to spread rapidly Chinook wind causes extreme dryness or the rain shadow effect on the leeward side

Chinook Wind and Rainshadow Effect

Local Wind Systems: Drainage or Katabatic Wind
Common in cold uplands, high plateaus or high interior valleys of high latitudes, examples: Greenland and Antarctica it involves the spill over of cold & dense air across low mountain divides (or through passes) downslope to lowland regions under the force of gravity

Hence, it is called Gravity-Flow Winds It is called the Mistral wind along the Rhone Valley in southern France Mistral wind originates in the Alps and channeled through the Rhone valley at high velocity to the Mediterranean Sea

It is also called Taku winds in southeastern Alaska

Local Wind Systems: Mountain and Valley Winds
It’s a daily cycle of airflow between the valley side slopes and the valley bottoms Valley side slopes are heated more rapidly during the day than the valley bottom Hence, low pressure develops on the valley side slopes and high pressure at the valley bottom receiving less heat during the day

Mountain and Valley Breeze

Hence, valley breeze invades the slope at daylight when pressure is low Valley breeze are prominent during summer An opposite process, the mountain breeze, operates at night

Valley side slopes lose heat very rapidly and become chilled at night Hence, high pressure develops on the slopes & causing chilled and dense mountain side air to slip downslope as mountain breeze at night Mountain breeze is more prominent in winter

Regional Wind Systems: Monsoonal Wind Systems
Monsoon winds are seasonal wind shift of up to 180o Monsoonal winds blow onshore in summer Monsoonal winds blow offshore in winter Well developed in the trade wind belts due to shifts in positions of ITCZ an unequal heating of land and water

Monsoonal Wind Systems
Best developed along the West African coast, India, and China. Minor systems are recognized in Northern Australia It brings heavy monsoonal rain to these regions in summer and dry dusty winds in winter

Monsoonal System in Africa

Minor Monsoonal System

Monsoonal System in India

Monsoonal System in China

El Nino and La Nina Warming of waters in the eastern equatorial Pacific Associated with changes in weather patterns worldwide Typically occurs on time scales of 3 to 7 years for about 18 months

El Nino and La Nina In normal years, the coasts of Ecuador and Peru are washed by cold Peruvian current But in some years when the Equatorial currents are weak, warm ocean currents flow southward to replace the cold Peruvian current This happens close to the end of the year and the natives named it El Nino (the child) after the child Jesus because of the Christmas season

El Nino: Walker Circulation Patterns

El Nino and La Nina Locally, El Nino causes: - abnormal weather patterns with abnormally high amount of rain inland - hence, abnormal high crop yields and devastating floods in Ecuador and Peru observed - but the fishing industry is usually devastated because the warm waters blocks the upwelling of nutrient rich cold waters

El Nino and La Nina The effects of El Nino are felt across the globe: - Causes severe drought in Australia, Indonesia, the Philippines and Africa Sahel El Nino brought severe storms accompanied by unusual beach erosion, landslide and floods to California - Heavy rains and flood in Texas and the Gulf states and less hurricane events

El Nino and La Nina The effects of El Nino are felt across the globe: - Suppression of Atlantic hurricanes - allows a pool of warm water over the Pacific to develop which in turn displaces the paths of both the polar and subtropical jet streams - hence, subtropical jet brought heavy winter precipitation to the Gulf coast and the polar jet brought milder winter far north

El Nino and La Nina The effects of El Nino are felt across the globe: - or warmer than normal winter in northern United States and Canada persists

El Nino and La Nina During an El Nino year, high pressure develops in the western pacific near Australia and low pressure in east pacific When El Nino comes to an end, the pressure situation reverses such that east pacific has high pressure and the west low pressure This phenomenon is referred to as El Nino Southern Oscillation (ENSO)

El Nino and La Nina What was once regarded as the normal condition with high pressure and cold current in eastern pacific is now referred to as La Nina Researchers restrict La Nina to periods when surface temperatures are colder than average

El Nino and La Nina La Nina has its distinct weather patterns: - colder than normal winter of the Pacific Northwest and Northern Great Plains - Warming experienced in the rest of the United States - Great hurricane activity producing more than 20 times more damage than El Nino years

Review Questions for Topic 5

1) The force exerted by gas molecules on some area of Earth’s surface or any other body is called what? Density Wind Atmospheric pressure Friction Geotropism Figure 5-1

1) The force exerted by gas molecules on some area of Earth’s surface or any other body is called what? Density Wind Atmospheric pressure Friction Geotropism Level of Difficulty: 1 Text Reference: The Nature of Atmospheric Pressure Geography Standard: 7 Blooms Taxonomy: Knowledge Figure 5-1 Explanation: Gas molecules, when in contact with a surface, will exert a force on that surface. This force corresponds to atmospheric pressure.

2) Lines drawn on maps joining areas of equal atmospheric pressure are called what?
Wavelengths Isotherms Contours lines Isohyets Isobars Figure 5-4

2) Lines drawn on maps joining areas of equal atmospheric pressure are called what?
Wavelengths Isotherms Contours lines Isohyets Isobars Level of Difficulty: 1 Text Reference: The Nature of Atmospheric Pressure Geography Standard: 7 Blooms Taxonomy: Knowledge Figure 5-4 Explanation: Lines of constant pressure, by definition, are called isobars.

3) Due to Coriolis force, freely moving objects in the Northern Hemisphere appear to be deflected to
the left. the right. the ocean. the east. the west.

3) Due to Coriolis force, freely moving objects in the Northern Hemisphere appear to be deflected to
the left. the right. the ocean. the east. the west. Level of Difficulty: 2 Text Reference: The Nature of Wind Geography Standard: 7 Blooms Taxonomy: Knowledge Figure 5-6 Explanation: In the Northern Hemisphere, winds are deflected to the right by Coriolis. In the Southern Hemisphere, winds are deflected to the left

4) The air that descends and spirals out of the subtropical high pressure belt is the source of
polar easterlies. trade winds and the westerlies. Chinooks. land and sea breeze. the monsoons.

4) The air that descends and spirals out of the subtropical high pressure belt is the source of
polar easterlies. trade winds and the westerlies. Chinooks. land and sea breeze. the monsoons. Level of Difficulty: 2 Text Reference: The General Circulation Geography Standard: 7 Blooms Taxonomy: Skills Figure 5-14 Explanation: As seen in Figure 5-14, the subtropical high is the source location for both the mid-latitude westerlies and the trade winds.

5) A sea breeze is experienced
in the night. in winter. during the day. at dawn only. when air over land is too heavy to be lifted by convective currents.

5) A sea breeze is experienced
in the night. in winter. during the day. at dawn only. when air over land is too heavy to be lifted by convective currents. Figure 5-34a Level of Difficulty: 2 Text Reference: Localized Wind Systems Geography Standard: 7 Blooms Taxonomy: Skills Explanation: During the day, the land heads faster than the water, creating a thermal low on land and a thermal high over water. The winds will blow from high to low pressure, creating a sea breeze.

6) The semi-permanent area of high pressure over the poles of Earth is an example of
a subtropical high. a sea breeze. a dynamic high. a thermal high. a midlatitude anticyclone. Figure 5-14

6) The semi-permanent area of high pressure over the poles of Earth is an example of
a subtropical high. a sea breeze. a dynamic high. a thermal high. a midlatitude anticyclone. Level of Difficulty: 3 Text Reference: The General Circulation Geography Standard: 7 Blooms Taxonomy: Skills Figure 5-14 Explanation: The area of high pressure over the poles is an example of a thermal high. When the air gets extremely cold and dense over these regions, it becomes very heavy, thus exerting higher pressure on the surface as a result of its temperature.

7) An El Niño is observed as
a cooling of equatorial Pacific waters. high pressure over western South America. a warming of eastern equatorial Pacific waters. low pressure over Australia. rainy conditions for Australia. Figure 5-37

7) An El Niño is observed as
a cooling of equatorial Pacific waters. high pressure over western South America. a warming of eastern equatorial Pacific waters. low pressure over Australia. rainy conditions for Australia. Figure 5-37 Level of Difficulty: 3 Text Reference: El Niño-Southern Oscillation Geography Standard: 7 Blooms Taxonomy: Skills Explanation: During an El Niño event, water over the eastern equatorial Pacific (off the west coast of South America) becomes abnormally warm, resulting in a switch of the Walker Circulation.

8) Waves in the jet stream pattern are called
Kelvin waves. electromagnetic waves. shallow water waves. Coriolis waves. Rossby waves. Figure 5-24

8) Waves in the jet stream pattern are called
Kelvin waves. electromagnetic waves. shallow water waves. Coriolis waves. Rossby waves. Figure 5-24 Level of Difficulty: 2 Text Reference: The General Circulation Geography Standard: 7 Blooms Taxonomy: Knowledge Explanation: The wave patterns which give the midlatitudes a majority of its weather that are embedded in the jet stream pattern are called Rossby waves.

9) Which type of wind is an example of a katabatic wind?
Foehn Gale Bora Chinook Santa Ana

9) Which type of wind is an example of a katabatic wind?
Foehn Gale Bora Chinook Santa Ana Level of Difficulty: 2 Text Reference: Localized Wind Systems Geography Standard: 7 Blooms Taxonomy: Knowledge Explanation: A bora wind is a strong cold wind that affects the lee slopes of a mountain range. It is katabatic because it is a cold wind.

10) Air which has decreased in density and temperature will
be warmer. have a higher air pressure. have a lower air pressure. sink. compress. Figure 5-3

10) Air which has decreased in density and temperature will
be warmer. have a higher air pressure. have a lower air pressure. sink. compress. Level of Difficulty: 2 Text Reference: The Nature of Atmospheric Pressure Geography Standard: 7 Blooms Taxonomy: Skills Figure 5-3 Explanation: Air temperature and density are directly related to pressure. If air temperature and density decrease, the pressure must also decrease as a result of the temperature and density decrease.

The General Circulation of the Atmosphere
Atmosphere is in constant motion Major semipermanent conditions of wind and pressure—general circulation Principal mechanism for longitudinal and latitudinal heat transfer Second only to insolation as a determination for global climate

Simple example: A non-rotating Earth Strong solar heating at equator Little heating at poles Thermal low pressure forms over equator Thermal high forms over poles Ascending air over equator Descending air over poles Winds blow equatorward at surface, poleward aloft Figure 5-12

Observed general circulation Addition of Earth’s rotation increases complexity of circulation One semipermanent convective cell near the equator Three latitudinal wind belts per hemisphere Hadley cells Figure 5-14

Seasonal differences in the general circulation Figure 5-15

Components of the general circulation Subtropical highs Persistent zones of high pressure near 30° latitude in both hemispheres Result from descending air in Hadley cells Subsidence is common over these regions Regions of world’s major deserts No wind, horse latitudes Figure 5-16

Trade winds Diverge from subtropical highs Exist between 25°N and 25°S latitude Easterly winds: southeasterly in Southern Hemisphere, northeasterly in Northern Hemisphere Most reliable of winds “Winds of commerce” Figure 5-17

Trade winds (cont.) Heavily laden with moisture Do not produce rain unless forced to rise If they rise, they produce tremendous precipitation and storm conditions Figure 5-20

Intertropical Convergence Zone (ITCZ) Region of convergence of the trade winds Constant rising motion and storminess in this region Position seasonally shifts (more over land than water) Doldrums Figure 5-21

Westerlies Form on poleward sides of subtropical highs Wind system of the midlatitudes Two cores of high winds – jet streams Rossby waves Figure 5-22 Figure 5-24

Polar highs Thermal highs that develop over poles due to extensive cold conditions Winds are anticyclonic; strong subsidence Arctic desert Polar easterlies Regions north of 60°N and south of 60°S Winds blow easterly Cold and dry

Polar front Low pressure area between polar high and westerlies Air mass conflict between warm westerlies and cold polar easterlies Rising motion and precipitation Polar jet stream position typically coincident with the polar front Figure 5-25

The seven components of the general circulation Figure 5-26

Vertical wind patterns of the general circulation Most dramatic differences in surface and aloft winds is in tropics Antitrade winds Figure 5-28

Modifications of the General Circulation
Seasonal modifications Seven general circulation components shift seasonally Components shift northward during Northern Hemisphere summer Components shift southward during Southern Hemisphere summer Figure 5-29

Monsoons Seasonal wind shift of up to 180° Winds onshore during summer Winds offshore during winter Develop due to shifts in positions of ITCZ and unequal heating of land and water Figure 5-30

Major monsoon systems Figure 5-32

Minor monsoon systems Figure 5-33

Localized Wind Systems
Sea breezes Water heats more slowly than land during the day Thermal low over land, thermal high over sea Wind blows from sea to land Land breezes At night, land cools faster Thermal high over land, thermal low over sea Wind blows from land to sea Figure 5-34

Valley breeze Mountain top during the day heats faster than valley, creating a thermal low at mountain top Upslope winds out of valley Mountain breeze Mountain top cools faster at night, creating thermal high at mountain top Winds blow from mountain to valley, downslope Figure 5-35

Katabatic winds Cold winds that originate from cold upland areas, bora winds Winds descend quickly down mountain, can be destructive Foehn/Chinook winds High pressure on windward side of mountain, low pressure on leeward side Warm downslope winds Santa Ana winds Figure 5-36

Atmospheric Pressure and Wind Systems

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