Physical Geography Exam 2

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Air Pressure
the measured weight of air as it exerts pressure on Earth’s surface
decreases with increasing altitude
Air density is greatest near the Earth’s surface
Air pressure is influenced by air temperature:
Warm air results in lower air pressure
Cooler air results in higher air pressure
Measuring Air Pressure:
Mapping Air Pressure:
Air pressure changes with altitude
Average air pressure at sea level = 1013.25 mb
High-pressure system
A circulating body of air that exerts relatively high pressure as air sinks toward the surface
Air flow diverges
Low-pressure system
A circulating body of air where relatively less pressure is created as air rises away from the surface
Air flow converges
Pressure systems
create large-scale circulatory systems that are interconnected by airflow
the process by which air flows horizontally from high-pressure to low-pressure
Map of Atmospheric Pressure:
Isobars indicate the geographic patterns of pressure systems
Red arrows illustrate the path of airflow relative to pressure systems
Wind Direction
Winds are named for the direction in which they originate
causes motion in the atmosphere
Pressure gradient
The greater the difference in pressure, the steeper the gradient
The steeper the gradient, the faster the airflow
Coriolis Force
Due to Earth’s rotation
Deflects objects traveling in the atmosphere
Earth’s eastward rotation below
Northern Hemisphere, deflection is to the right
Southern Hemisphere, deflection is to the left
Frictional Forces
Occurs at ground level
Strongest at surface, diminishing at about 1500 m (5000 ft)
Causes wind to slow down and move in irregular ways
Convection loops
spiraling descending and rising air are linked horizontally by advection
Hadley Cell
the tropical convection loop
Air at tropics is warmed by year-round direct sunlight
Intertropical Convergence Zone (ITCZ)
Warming creates a zone of low pressure at Equator as air rises into the atmosphere
Winds converge into ITCZ by advection
Subtropical High Pressure System (STH)
Air rising from ITCZ eventually sinks at subtropics creating zones of high pressure
Dry and warm winds diverge from STH
Ferrel Cell
the circulatory loop that mixes cool polar air with warm tropical air
Polar Front
the line of contact between contrasting air masses at about 60 N/S
Polar Jet Stream
formed by high-altitude winds that are formed with the temperature/pressure gradient
Rossby Waves
develop as undulations in the Polar Front and moderate significant temperature difference on either side
Polar Cell
the circulatory loop in the polar regions
Polar High-Pressure System
Air flowing northward from midlatitudes sinks, producing a weak high-pressure system
Consists of masses of rotating, descending dry air that flows toward the Polar Front
Circulatory Loops and Wind Patterns
Trade Winds ITCZ
STH Westerlies and Trade Winds
Westerlies and Polar Easterlies Polar Front
Polar High Polar Easterlies
Monsoonal Winds
Seasonal shift of the ITCZ and prevailing wind direction in the subtropics
Asian monsoons
Winter: ITCZ in south
Cold air, high pressure
Summer: ITCZ in north
Warm air, low pressure
Sea Breeze
Breeze blows from high- pressure sea to low- pressure land
Land Breeze
Breeze blows from high- pressure land to low- pressure sea
Valley Breeze
Breeze blows upslope as mountain slopes heat up
Mountain Breeze
Breeze blows downslope as mountain slopes cool off
Katabatic Winds
Extremely cold, dense air flows downslope under force of gravity
Flow at great speeds
Chinook Wind
Occurs when a steep pressure gradient develops in mountainous regions
high pressure on windward side
low pressure on leeward side
Currents and Gyres
Surface currents are driven by winds as energy transfers by friction
form as continents block the movement of water
Oceanic Conveyor Belt
Slow vertical mix of water between layers of the ocean
Downwelling currents
caused by high-density water that is cooler and saltier
Upwelling currents
caused by low-density water that warms in tropical regions
El Niño
Reversal of “normal” flow of currents and winds in tropical Pacific
Occurs every 3-8 years
Affects climate
Changes ocean surface temperature
Changes patterns of precipitation
Wind farms
Collection of turbines used to harness wind power
Conversion to clean usable energy
Hydrogen Bonding
Attraction between the hydrogen atoms of water molecules
Explains water’s physical states
Hydrologic Cycle
Movement of water between various storage locations
Amount of water is finite
Total amount evaporated equals the total precipitated globally
Yet, local and regional imbalances occur
refers to the concentration of water vapor in the air
Maximum Humidity
Maximum amount of water vapor that a body of air can hold
Subject to air temperature
Warm air can hold more water vapor than cold air
the point where the air cannot hold any more water vapor at its current temperature
Specific Humidity
How much water vapor is actually in the air
Relative Humidity
Ratio of specific humidity to maximum humidity
How close the air is to saturation, at its current temperature
Lower latitudes
Specific humidity is low
Relative humidity is high
Higher latitudes
Specific humidity is high
Relative humidity is low
Diurnal cycle
Maximum humidity increases with warming
Specific humidity is constant
Relative humidity gradually decreases
Dew-Point Temperature
Temperature at which a mass of air is saturated
Related to changes in relative or specific humidity
results in evaporation directly from leaf pores in plants into the atmosphere
the combination of evaporation and transpiration
Evapotranspiration rates depend on
Net radiation which increases heating
Air temperature which influences maximum humidity
Relative humidity and moisture capacity of air
Dry Adiabatic Lapse Rate
Applies to unsaturated air
Dry air cools or warms at 10C/1000 m or 5.5F/1000 ft
Wet Adiabatic Lapse Rate
Applies to air that reaches the level of condensation, or the altitude of saturation
Rate varies with moisture content and temperature
Average rate is about 10C/1000 m or 5.5F/1000 ft
visible masses of suspended, minute water droplets or ice crystals
Two necessary conditions for cloud formation
Air must be saturated
Either by cooling below the dew point or by adding water vapor to the air

There must be a substantial quantity of small airborne particles for water vapor to collect
Such particles are known as condensation nuclei

Clouds are classified based on form and altitude


Radiation fog
develops at night when air cools to the dew point and is held below a temperature inversion, or an overlying body of warmer air
Advection fog
develops when warm air flows over a cooler surface, cooling it to the dew point
Sea fog
develops when cool marine air comes in direct contact with colder ocean water
Windward side
Air cools at DAR to dew point
Forms clouds that cool at WAR and precipitation follows
Leeward side
Air descends downslope, warming at DAR
Creates rain shadow of dry conditions
Orographic Uplift
Airflow interrupted by a mountain range
Convectional Uplift
Unequal heating of Earth’s surfaces
Stable Air
little convection and no precipitation
Unstable Air
strong convection bubbles lift and create precipitation
Air Mass
a large body of the lower atmosphere with uniform conditions of temperature and moisture
air mass source region
any large body of land or water where air derives its characteristics
Boundaries between differing air masses
When one air mass advances in a front,
frontal uplift causes clouds and/or precipitation
Warm Front
Warm air advances
Warm air slowly lifted
Cold Front
Cold air advances
Warm air rapidly lifted
Midlatitude Cyclones: Interactions at the Polar Front
A well-organized low-pressure system that migrates across a region while it spins
Midlatitude Cyclones: Upper Air Flow
500-mb upper air-pressure surface
Occurs at a specific but varying altitude over any given place on Earth
Vertically divides atmosphere in two, from surface to top
Explains pressure changes associated with temperature change
High-pressure ridges
form when height of pressure surface is higher and anticyclones occur
Low-pressure troughs
form when height of pressure surface is lower and cyclones occur
is the process that forms midlatitude cyclones
Conditions in upper atmosphere and surface are significant
Upper-level convergence sends air to the surface, creating high pressure
Upper-level divergence allows air to rise, creating low pressure
Evolution of Thunderstorms
Cumulus Stage
Begins with convection or advancing cold front into mT air
Rapid rising air forms cumulus clouds
Developing Stage
Condensation releases latent heat
Mature Stage
Very unstable air with development of strong updrafts
Intense precipitation brings cold air down to create downdrafts
Dissipation Stage
Collisions among ice crystals and rain droplets cause difference in electrical charge within clouds
Ground has positive (+) charge
Most lightning within clouds from positive (+) to negative (-)
In strong storms, leader (-) from cloud meets streamer (+) from ground, creating “spark” as cloud-to-ground lightning
Small, intense cyclone formed in supercell thunderstorms
are large rotating updrafts
Form at high altitudes with strong updrafts and wind shear
A horizontal vortex of air gets pulled vertically in updrafts
Tropical Cyclones
Develop in homogeneous air masses at low latitudes
Fueled by abundant water vapor and latent heat
Early Formation:
Easterly Wave slow-moving trough migrates along tropical easterlies
Upper air converges on windward side and diverges on leeward side, causing rapid uplift
Tropical storms in the Atlantic or eastern Pacific with very high winds
Rare combination of environmental variables:
Warm ocean surface
High evaporation
Favorable upper air winds
High pressure aloft
Anatomy of a hurricane:
Around the eye, air flows inward and upward
In the eye, air flows toward surface and warms
Atlantic Hurricane Tracks
Originate in West Africa driven by trade winds
Intensify over warm tropical Atlantic
Driven northeast by westerlies
the state of the atmosphere at a specific place and time on Earth’s surface
the long-term average values of weather elements, such as temperature and precipitation
Köppen Climate Classification
Most widely used classification system
Stems from the recognized relationship between major vegetation regions and regional climate characteristics
System’s description of world climates is based on
Average monthly temperature
Average monthly precipitation
Total annual precipitation
Tropical (A) Climates
Surrounds the Equator from 25 N/S
Consistently warm average temperatures
Subcategories based on precipitation only
Tropical rainforest (Af)
Tropical monsoon (Am)
Tropical savanna (Aw)
Arid and Semi-Arid (B) Climates
Poleward of A climates
Subtropical high creates precipitation patterns
Subcategories based on precipitation and temperature:
Hot low-latitude desert (BWh)
Cold midlatitude desert (BWk)
Hot low-latitude steppe (BSh)
Cold midlatitude steppe (BSk)
Mesothermal (C) Climates
20 to 60 N/S
Distinct warm seasons and cold seasons
Humid Subtropical Hot-Summer (Cfa, Cwa)
Mediterranean Dry-Summer (Csa, Csb)
Marine West Coast (Cfb, Cfc)
Microthermal (D) Climates
35 to 60 N/S
Longer cold seasons and limited warm seasons
Humid Continental Hot-Summer (Dfa, Dwa)
Humid Continental Mild-Summer (Dfb, Dwb)
Subartctic (Dfc, Dwc, Dwd)
Polar (E) Climates
Nonmountainous areas poleward of 70 N/S
Long, cold winters with little precipitation
Tundra (ET)
Ice cap (EF)
Categories: Physical Geography