Exam 2 review - Lecture notes 2 PDF

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Professor name: Vicki Drake
Santa Monica College...

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PHYSICAL GEOGRAPHY - EXAM 2 STUDY GUIDE

Atmospheric Pressure What is atmospheric pressure? ! •

Measure of the “force” of the air above a point of observation!

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Measured as a force/area!

What are typical measurements of air pressure? ! •

Baseline for measuring air pressure is mean sea level!

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Using force/area as measure, mean sea level air pressure: 14.7 lbs/in2!

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Millimetres or inches of mercury or the “bar”!

Constant pressure chart helps pilot determine they are flying at correct altitude (using altimeter)! Can you diagram Northern and Southern Hemisphere air flow around Highs and Lows? !

Cyclonic air flow •

PGF starts the air moving toward the centre of the low!

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Coriolis force deflects the air from its intended path and friction slows the air down!

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Northern hemisphere: counterclockwise and into the centre of a low!

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Southern Hemisphere: clockwise and into the centre of a low!

Anticyclonic air flow •

PGF pulls the air out from the centre of the high!

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Coriolis force deflects the air from its intended path, friction slows the wind down!

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Northern hemisphere: clockwise and out of the centre of a high!

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Southern Hemisphere: counterclockwise and out of the centre of a high!

Horizontal air movement due to a difference in surface air pressure •

Air movement based on 2 of newton’s “laws of motion”!

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An object in motion or at rest will tend to stay in motion or at rest until a force is exert on it!

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To overcome inertia, the force needed is equal = mass X acceleration produced by force!

Global Circulation Thermal Air pressure: •

unequal heating of land and water and energy transfer via conduction and convection!

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Thermal low air pressure develops over warm to hot surfaces via conduction and convection!

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conduction: a warm to hot surface transfers heat energy to the air, which becomes warmer, more buoyant and less dense than surrounding air!

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convection: warm air rises!

Thermal high pressure develops over cool to cold surfaces! •

conduction: cool air is more dense, less buoyant than surrounding air!

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convection: cool air sinks!

Dynamic Air pressure: •

high altitude winds (near the top of the troposphere) and earth’s rotation!

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Dynamic highs have a core of warm descending air! •

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Warm air is pushed back down to the surface, warming considerably as it descends!

Dynamic lows have a core of cool rising air! •

Low cool air is pulled up!

Pressure Gradient Force: controls both wind direction and wind velocity! •

Net force produced when differences in horizontal air pressure exist!

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Always directed from high pressure to low pressure and moves at right angles across isobars!

Coriolis Force: controls wind directly only (deflects air flow from original direction)! •

Apparent force due to the rotation of earth on its axis!

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Almost 0 at equator and strongest near poles!

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Higher the velocity of moving object, strong Coriolis affects the object!

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Coriolis affects only wind direction!

Friction: controls wind velocity only (acts to slow wind down close to surface)! •

Effect of friction is observed closest to earth’s surface - “boundary layers”!

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Friction slows down wind speed!

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Friction layer varies in height across the earth, but for the most part lies within about a kilometre of the surface!

Geostrophic Wind Development •

Theoretical horizontal wind blows in a straight path at a constant speed, parallel to isobars!

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Exists at altitudes above 1000 metres (above the boundary/friction layer)!

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Develops when the PGF and CF are in dynamic balance!

Pacific Ocean semi-permanent air pressure systems •

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Pacific high! •

Located at approx. 30N off the coast of California!

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Seasonally shifting!

Aleutian low! •

Located at approx. 60N in gulf of Alaska!

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Seasonally shifting!

Atlantic Ocean semi-permanent air pressure systems •

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Bermuda-Azores high! •

Located approx. 30N!

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Shifts seasonally!

Icelandic low! •

Located approx. 60N near Iceland!

El Nino southern oscillation events •

Southern oscillation refers to the reversal of air pressure systems during an El Nino!

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Upwelling along Peruvian coast ceases!

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Trade winds weaken !

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Weak equatorial eastward current develops !

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Moisture and temperature patterns alter as air pressure systems ‘flip-flop’ !

Pacific Ocean Currents! •

California current: south flowing cold current!

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Equatorial current: westerly flowing warm current!

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Kuroshio current: northerly flowing warm current!

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North Pacific drift: easterly flowing somewhat warm current!

Atlantic Ocean Currents •

Gulf stream: northerly flowing warm current!

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Labrador current: southerly flowing cold current!

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North Atlantic drift: easterly flowing somewhat warm current!

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Canary current: southerly flowing cool current!

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Equatorial current: series of westerly flowing warm current!

Jet streams •

Swift river of air found in the upper troposphere!

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Two are found in each hemisphere!

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Polar jet stream! •

Moves west to east at an altitude of about 10 kilometres!

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Air flow is intensified by the strong temperature and pressure gradient that develops when cold air from the poles meets warm air from the tropics! •

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forms because of this temperature gradient!

Subtropical jet stream! •

Located approx. 13 kilometres above the subtropical high pressure zone!

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Reason for its formation is similar to polar jet stream: Differences in temperature and humidity between subtropical and tropical air at 30 N/S!

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It is weaker. Slower wind speeds are a result of weaker latitudinal temperature and pressure gradient!

Sea Breeze/Land breeze •

Due to temperature differences between coastal and inland regions!

Monsoons •

means “changing wind directions with a change in season”. !

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Monsoons are associated with heavy precipitation. !

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India has two monsoons: Winter and Summer ! •

Winter: landmass very cold, high pressure out from centre!

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Summer: landmass very hot, low pressure

Santa Ana Winds •

Santa Ana Winds are warm, dry, high velocity winds that flow into southern California. !

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The winds usually begin in late summer-early fall and extend to December; typically ending with the arrival of southern California’s winter rainy season. !

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The source region for the winds is northeast of southern California near Utah. !

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In the late summer-early summer, a large high pressure system (The Great Basin High) develops near the area of Utah, western United States. !

Humidity Hydrologic cycle: continuous movement of water on, above and below the surface of the Earth! Absolute Humidity ! •

Measures the mass of water vapor in a given volume of air !

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Registers a change in water vapor, even if no water vapor has been added or lost !

Specific Humidity ! •

Measures the mass of water vapor in an air “parcel” in a given mass of air!

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More effective measure of true water vapor amounts: not based on changing volume!

Relative Humidity ! •

A ratio between the actual amount of water vapor present (Specific Humidity/Absolute Humidity) at a given temperature and the maximum amount of water vapor air can ‘hold’ as a gas at any given temperature !

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Not a true measure of water vapor in an air parcel !

Dew Point Temperature ! •

Water vapor in air condenses into droplets at the ‘Dew Point Temperature’ !

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If air temperature and Dew Point temperature are far apart, relative humidity is low!

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If air temperature and Dew Point temperature are close, relative humidity is high!

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If they’re equal, relative humidity equals 100%!

What is vapor pressure? the pressure of a vapor in contact with its liquid or solid form ! Actual vapor pressure (AVP)? Measurement of amount of water vapor in a volume of air! Saturated vapor Pressure (SVP)? When reaches SVP, water in air will condense! A$rain shadow desert$is a dry area on the leeward side of a mountainous area! Why is it warmer and dryer on the lee side of a mountain than on the windward side? As the wind blows across a$mountain$range, air rises and cools and clouds can form on the$windward side. When the air sinks on the$leeward side$of the mountain$range, it is usually much drier and$warmer than$it was to begin with.!

% Lapse Rates: Changes in air temperature with changes in elevation ! Dry Adiabatic Lapse Rate! • Constant rate of cooling (or heating) for an unsaturated parcel of air! • Cooling rate for rising unsaturated air! • Heating rate for sinking unsaturated air!

Wet Adiabatic Lapse Rate! • Not a constant rate of cooling: a variable rate for rising unsaturated parcel of air! • Always a lower rate of cooling than the DALR! • No rate for heating! Environmental Lapse Rate ! Stability: A measure of the resistance of an air parcel to being displaced vertically – either up or down in the atmosphere! Stable Air resists any vertical displacement. ! • If the air parcel is forced to rise, it will return to original elevation once the force is removed ! • If DALR > ELR = Stable Air! • If WALR> ELR = Stable Air ! Unstable Air does not resist vertical displacement! If the air parcel is forced to rise, air will continue to rise on its own ! • If DALR < ELR = unstable Air! • If WALR < ELR = unstable Air! Convective Instability • In large thunderstorms, air rises quickly creating a disparity in temperature and air density between the base and top of cloud ! • Cloud bases, at lower altitudes, tend to be slightly warmer and more ‘wet’ (droplets) ! • Cloud tops, at higher altitudes, tend to be colder and ‘drier’ (ice crystals) ! Cloud development What are the four Mechanisms for uplifting air to start the ‘cooling’ process? • Orographic uplift! • Convective uplift! • Frontal uplift ! • Convergence uplift!

What are Condensation nuclei • Microscopic particulates in the atmosphere! • soot, volcanic ash, salt crystals, sand, clay, ice crystals! • CN are needed to increase condensation of water vapor! • Without CN in the atmosphere, relative humidities of >100% would be required for condensation! Collision and coalescence • Larger cloud droplets pass through the cloud, riding updrafts and downdrafts and colliding with other smaller cloud droplets. ! • At the right speed and angle of incidence, two cloud droplets collide and join forces to become a larger droplet. ! • This process continues until the cloud droplet is large enough to fall from the cloud ! Factors that control the size of rain droplets: • Initial liquid water content of the cloud ! • Initial size of the cloud droplet! • Larger cloud droplets are favored for growth ! • “Thickness” of cloud (cloud height) and time! • The longer a cloud droplet stays inside the cloud, the bigger it can grow ! • Taller cumulus-type clouds produce much larger rain droplets than the thinner stratus-type clouds ! • Strength and number of updrafts and downdrafts within the cloud ! • The more ‘elevator’ rides the cloud droplet takes from the base to the top of the cloud increases the final size of the droplet ! Four major classes of clouds • stratus: sheetlike clouds! • cumulus: puffy clouds! • cirrus: wispy clouds! • nimbus: violent rain clouds!

Clouds with vertical development • Cloud base at elevations ranging between 1000 m to 13,000 m (3,000 - 40,000 ft) ! • White to light gray due to ice crystals near tops and water droplets near base ! • Cumulus: puffy clouds that can develop into larger thunderstorms ! • Cumulus Castellanus – first indicator of thunderstorms! • Cumulus humulis – “fair weather” cumulus! • Cumulus congestus – smaller thunderstorm cloud ! • Cumulonimbus: towering thunderstorm! Relationship between cloud development and stability

• Stable Air resists vertical displacement, and produces more stratiform-type clouds ! • Unstable Air does not resist vertical displacement, and produces more cumuliform- type clouds ! Precipitation types

• RAIN ! • Water droplets averaging 2 mm in diameter ! • Virga – rain falling from base of cloud that evaporates before touching ground! • SNOW ! • Ice Crystal growth; unique ! • Fall Streak – snow falling from high altitude clouds that is sublimated back into atmosphere before touching ground !

• SLEET ! • Ice crystal falling from cloud base; passes through warmer atmosphere and melts; refreezes in atmosphere before coming into contact with surface !

• FREEZING RAIN ! • Ice crystal falling from cloud base; passes through warmer atmosphere and melts; does not re-freeze until coming into contact with an object at the surface (tree, building)!

• HAIL ! • Layered ice pellet (ball) formed in powerful Thunderstorms. !

Air Masses and Fronts cP (continental polar) ! •

cP – air mass is cold with low humidity!

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Source regions include parts of Alaska, Canada, Siberia and Northern Russia !

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cP air masses mix with air over warmer Great Lakes in winter to produce “Lake-Effect Snows” in Buffalo, New York !

mP (maritime polar) ! •

mP air mass has low to medium temperatures, with high specific humidity !

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Source regions include: Gulf of Alaska, northern Pacific and northern Atlantic oceans !

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Modified cP air moving into Gulf of Alaska (off Alaska) becomes mP air – cool, wet, and unstable air that can bring winter storms to western USA !

cT (continental tropical) ! •

cT air mass has very high temperatures and very low humidity – it is unstable at low elevations !

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Source regions include northern Mexico and southwestern USA (Sonoran Desert region) !

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Long lasting cT air masses in southwestern USA responsible for prolonged drought conditions. !

mT (maritime tropical)! •

mT air mass has medium to high temperatures and very high humidity !

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Source regions include: Gulf of Mexico, subtropical Pacific Ocean, and subtropical Atlantic Ocean. %

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mT source region east of Rockies is Gulf of Mexico; usually confined to southern states, but may move into Central Plains in winter producing milder temperatures, fog and heavy precipitation events !

cA (continental arctic) •

cA air mass is very cold with very low humidity !

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Source regions include: northern Alaska, northern Canada and northern Russia !

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cA air seldom reaches continental USA, however, when it does, extremely low (sub-freezing) winter temperatures in parts of North Dakota, Minnesota, and Montana can result !

mE (maritime equatorial)/eT (equatorial tropical) •

mE or eT air mass is very warm with very high humidity !

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Source regions are the equatorial waters of the Pacific and Atlantic Oceans !

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Can play a role in the ‘birthing grounds’ regions for hurricanes !

Cold front •

Leading edge of a cool or cold air mass, replacing warmer air !

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Lowest temperatures and pressure readings found as cold front passes !

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Cold dense air wedges beneath warmer air, forcing air to rise !

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If rising air is unstable with high humidity, cumuliform clouds will form!

Warm front •

Leading edge of a warm or hot air mass, replacing cooler air !

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Warmest temperatures usually just as front passes over !

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Cloud types include stratiform with some cirrocumulus!

Stationary front The stationary front marks the boundary between warm and cold air masses! May represent dry weather with no precipitation or widespread cloudiness with light precipitation! Occluded front Occluded fronts develop when either a cold front overtakes a warm front; or a warm front overtakes a cold front ! Cold occlusions result in violent thunderstorms/tornadoes! Warm occlusion result in “sandwiching” of warm air between 2 cold air masses! Mid-latitude cyclones •

Mid-latitude or frontal cyclones are large traveling atmospheric cyclonic storms up to 2000 kilometers in diameter with centers of low atmospheric pressure. !

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Mid-latitude cyclones form along the polar front as the result of the dynamic interaction of warm tropical and cold polar air masses !

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Normally, individual frontal cyclones exist for about 3 to 10 days moving in a generally west to east direction. !

1. Cold polar air meets warm tropical air along polar front: stationary front develops! 2. Rossby wave develops as warm air moved pole-ward and cold air moves equator ward! 3. Cyclonic rotation of air about low air pressure begins, surface convergence cause uplift! 4. Cold front overtakes warm front: cold occlusion develops! 5. Full development of occluded front! 6. Weakening of pressure gradient and system dissipation; back to stationary front!

Thunderstorms, Tornadoes and Hurricanes Air Mass

Severe

Short lived afternoon summer thunderstorms

Long lived thunderstorms

Form away from fronts

Form in strong vertical, wind shears along cold fronts

Usually not associated with tornado activities Associated with high winds, gust fronts, microburst, hail and tornadoes Experience all 3 lifecycle stages in 1 hour! • Updrafts cut off by downdrafts! • Most dangerous stage: mature

Severe Thunderstorms last for hours – Longest stages are I and II. ! • Tilted updrafts and downdrafts. % Updrafts and downdrafts do not interfere with each other ! • Most dangerous stage: Mature and dissipating %

Characteristics of Severe thunderstorms: Microbursts are localized downdrafts of air that hit the ground and spread out horizontally ! •

Windspeeds of 75 m/sec (~168 mph) !

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May evolve Into gust fronts !

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Responsible for knocking down trees and associated with damage usually attributed to tornadoes. Several airline...