131 terms

Earth Systems Geology: Ch. 4

Air pressure
the pressure exerted by the weight of air above a given point; expressed in millibars (mb) or inches of mercury (in. Hg)
-created by the motion, size, and number of the air molecules
Mercury barometer
-Measures air pressure
-A column of mercury in a glass tube under vacuum
-The mercury rises and falls with changes in air pressure
Aneroid barometer
-Measures air pressure
-A small, sealed chamber partially emptied of air. The chamber is connected to a mechanism that is sensitive to changes in the chamber. As air pressure varies, the mechanism moves needle on a dial
the horizontal motion of air across Earth's surface
-produced by the differences in air pressure btwn one location and another
-Refers to wind's updrafts and downdrafts
-Adds a vertical component to wind
What is used to measure wind?
1) Anemometer
2) Wind Vane
What are the driving forces within the atmosphere that determine speed and direction of winds?
1) Gravity
2) Pressure gradient force
3) Coriolis Force
4) Friction Force
How is a pressure gradient force created?
Its created bc of a pressure difference between high and low pressure areas
Winds tend to move from ______ to ______ pressure
high to low
lines of equal pressure plotted on a weather map
-The closer the isobars are together, the stronger the wind and intensity of the pressure gradient
How does the curvature of the Earth affect wind?
-Earth is a sphere and rotates on its axis, so the air moving across its surface appears to curve
In the Northern hemisphere, air curves to:
the right (clockwise)
In the Southern hemisphere, air curves to:
the left (counter clockwise)
Effects of wind friction near surface extend to:
about 500 m or 1650 ft
What variables affect friction?
1) Surface texture
2) Wind Speed
3) Time of day/year
4) Atmospheric conditions
Near the surface, friction disrupts equilibrium between:
pressure gradient and coriolis affect

-Bc friction decreases wind speed, it reduces coriolis force and makes wind move across isobars at an angle
What drives the global circulation of winds and ocean currents?
The imbalance btwn equatorial energy surpluses and polar energy deficits
Pressure and density _____ as altitude increases
Low density air means:
molecules are farther apart
Why is warmer air less dense than cooler air?
When air becomes heated, the temp rises, molecules become more active, the spacing between molecules increases and density is reduced. Air pressure is therefore also reduced
Why is moist air lighter?
Bc the molecular weight of water is less than that of the molecules that make up dry air
As water vapor in the air increases, density ______, so humid air exerts_______than dry air.
decreases; less air pressure
Air associated with low pressure
warm, humid air
Air associated with high pressure
cold, dry air
any instrument that measures air pressure
Today, atmospheric pressure is measured:
@ weather stations by electronic sensors that provide continuous measurements
Barometric pressure
The air pressure adjusted to a standard of normal sea level pressure
measures wind speed
Wind vane
determines winds direction
-usually measured 33ft above ground to reduce effects of local topography
How are winds named?
For the direction from which they originate
Westerly wind
-wind from west
-blows eastward
Southerly wind
-wind from south
-blows northward
Gravitational force exerts ________ on the atmosphere over all of earth
virtually uniform pressure
Pressure gradient force
-Drives air from areas of high barometric pressure to areas of low barometric pressure
-Exists bc earth's surface is unevenly heated
-The horizontal and vertical pressure gradient cause wind
Low barometric pressure
-less dense air
-warm air
High barometric pressure
-more dense air
-cool air
the rate of change in some property over a distance
Strongly subsiding and diverging air
-Associated w/ high pressure
-when air descends from upper atmosphere and diverges at surface
Converging and rising air
-Associated with low pressure
-when air at the surface converges from different directions and ascends to upper atmosphere
Closer isobars
indicates steep pressure gradient which causes faster air movement from high to low pressure area
Farther away isobars
indicates gradual pressure gradient and slower air flow
Coriolis force
makes wind travelling in a straight path appear to be deflected in relation to earth's rotating surface
-an effect of earth's rotation
Where is coriolis force weakest? strongest?
weakest at equator; strongest at poles
The strength of the coriolis force varies with _____
the speed of earth's rotation which varies with latitude
Deflection due to coriolis force occurs:
regardless of the direction youre going
Does deflection change the speed of the object?
The faster the objects speed, the:
greater the apparent deflection
The higher air rises....
the more speed it has so the coriolis force increases
Friction force
-In the boundary layer
-creates drag as wind moves across earth's surfaces
-its effects extend to about 500 m/1650 ft
Friction force _____ with height
Rougher surfaces produce:
more friction
Geostrophic winds
winds that flow around the pressure areas, parallel to the isobars, in the upper troposphere
Where winds spiral out from a high pressure area in a clockwise direction (northern hemisphere)
-high pressure system
-characterized by clear skies
-Winds spiral into low pressure area in counterclockwise direction (northern hemisphere)
-low pressure system
-characterized by cloudy and stormy weather
3 levels of atmospheric circulation
1) Primary circulation = general worldwide circulation patterns
2) Secondary circulation = migratory high and low pressure systems
3) Tertiary circulation = local winds and weather patterns
Meridional flows
winds that move principally north or south along meridians of longitutde
Zonal Flows
winds moving east or west along parallels of latitude
Equatorial low
-low primary pressure system at equator
-warm, low density, wet and rainy
-marked by the intertropical convergence zone (ITCZ)
-stimulated by thermal factors
Polar highs
high primary pressure systems at poles
-stimulated by thermal factors
-cold, high-density, dry
Subtropical highs
-primary pressure system
-formed by dynamic (mechanical) factors
-hot and dry high-pressure cells
Subpolar lows
-primary pressure system
-formed by dynamic (mechanical) factors
-cool and wet
The dynamic (mechanical) factors that cause the subtropical highs and subpolar lows are:
-The converging, rising air associated w/ low pressure and the subsiding, diverging air associated with high pressure
-They are mechanical bc they result from physical displacement of air
What happens in the Equatorial Low (Trough)?
-Energy surpluses from sun make light, low density air that ascends. Air is very moist and hot so it causes tons of rainfall throughout the zone. Tall, vertical cloud columns frequently reach tropopause and cause thunderstorms
Intertropical Convergence Zone (ITCZ)
-Location of the equatorial low
-Identified by bands of clouds along equator
-Shifts w/ the season
-Winds are calm bc of weak pressure gradient and ascent of air
Trade winds
-The surface winds converging at the equatorial low
-generated by the subtropical high pressure cells
-Northeast trade winds blow from the northern hemisphere and Southeast trade winds blow from the southern hemisphere
Most consistent winds on earth?
Trade winds
Hadley cells
circulation cells
the equatorial calms @ the ITCZ
What happens in the Subtropical highs?
-Broad high pressure zone of hot dry air bring clear, cloudless skies over Sahara and Arabian deserts.
How are subtropical highs formed?
These anitcyclones are formed as air above the subtropics is mechanically pushed downward and is then heated by compression on its descent to the surface
Bermuda High
-subtropical high pressure system in western Atlantic
Azores High
-The bermuda high that migrates to the eastern Atlantic during winter in northern hemisphere
Pacific High/Hawaiian High
The subtropical high pressure system that dominates the Pacific in July and goes south during winter
Calms of Cancer/Capricorn
-The subtropical belts where windless, hot, dry, desert air is caused by the subtropical highs
-Where most deserts occur
-Surface winds generated by the subtropical high pressure cells that flow towards higher latitudes
-Stronger in the winter
-Less consistent than trade winds bc of migratory pressure systems and topographic barriers
North Pacific Aleutian Low and North Atlantic Icelandic Low
-The subpolar lows dominant in winter that weaken/disappear in the summer
-Cyclonic low pressure systems that results from the warm westerlies meeting the cold polar air
-Causes cool rain like in Oregon, UK, etc.
Polar front
-The area of contrast btwn cold air from higher latitudes and warm air from lower latitudes
-Masses of air with different characteristics meet here
-Leads to condensation and precipitation as the warm air from the westerlies rises
What happens in Polar Highs?
-These cold, dry, anticyclonic high pressure systems descend and diverge and form the polar easterlies
Why are polar highs weak?
bc they receive little energy from the sun to put them in motion
Polar easterlies
the winds formed by the polar highs
-weak and variable
Antarctic High
The stronger, more persistent polar high formed over Antarctica
Constant isobaric surface
-A fixed pressure value used as a reference datum and plotted at its elevation above sea level
-This plotted value gives a surface of constant pressure (constant isobaric surface) so we can map pressures in the upper atmosphere
Rossby waves
-Great waving undulations within the westerly flow of the jet streams
-The large meanders in the jet stream
-Occur along the polar front and brings waves of cold air southward and waves of warm air northward
-Have major effect on weather
Jet streams
-Irregular concentrated bands of fast flowing, narrow wind currents occurring at several locations that influence surface weather systems
-Speeds exceed 190 mph
-strengthen during each hemispheres winter when they're closer to the equator
What causes variation of speed in the jet streams?
the pattern of high pressure ridges and low pressure troughs cause jet streams to meander and change speed
How to identify the variations in a constant isobaric surface
1) Ridges indicate high pressure anti-cyclones/convergence of the air/divergence of air at surface; the contours bend poleward
2) Troughs indicate low pressure cyclones/divergence of air/convergence of air at surface; contours bend equatorward
Polar jet stream
-Meanders along the polar front
-Moves cold air masses to the South, usually North America
-Influences storms
-Can migrate as far as Texas
Subtropical jet stream
-Meanders in the tropics and mid-latitudes
-Can occur over same place as polar jet stream and may even merge with it
Land and sea breezes
local winds produced along most coastlines
What causes sea breezes?
-Land warms faster than the ocean during the day so the warm air above land rises and triggers an onshore flow of cooler marine air
What causes warm nighttime breezes on the coast?
Land cools faster than the ocean so the cool air over the land sinks and flows toward low pressure areas over the water where the air is lifted, warmed, and then flows back over land
Mountain and valley breezes
-Local winds that result when mountain air cools rapidly at night and valley air gains heat rapidly during day
Santa Ana winds
-Result form a pressure gradient generated when high pressure builds over Great Basin of the U.S.
-Creates a strong, warms, dry winds that funnel downhill through mountains and flow over california
-Bring dust, dryness, and heat to CA coast that make dangerous wildfire conditions
Katabatic winds
-Drainage winds on a larger regional scale; stronger than local winds
-Developed in elevated areas where air at the surface cools and flows downslope
-Ex: the winds that blow off ice sheets
Different terrains produce:
different types of regional and local winds
-Seasonally shifting regional wind systems
-Involve an annual cycle of returning precipitation with the summer Sun'
-Occur over Southeast Asia, Indonesia, India, North Australia, and equatorial Africa
What causes monsoons?
-Unequal heating btwn land and ocean
-Shifting of ITCZ which brings moist air northward in the summer
North American monsoon
Brings heavy rainfall to Arizona, New Mexico, and west Mexico during late summer
Why do monsoons bring so much rain?
Warm summer air over landmasses creates low pressure and draws moist air from the oceans where the shifted ITCZ is
Driving force for ocean currents
1) the frictional drag of wind
2) Coriolis force
3) Water density differences
4) Configuration of continents and ocean floor
5) Moon and Earth interactions
Ocean currents are driven by
the atmospheric circulation of high pressure cells
-circulate clockwise in the northern hemisphere and counterclockwise in southern hemisphere
oceanic circulation systems
Equatorial ocean currents
-Driven by the trade winds which push the ocean currents westward
-These currents remain near the equator and aren't affects by coriolis force
Western intensification
The phenomenon of the westward moving equatorial ocean currents piling up against the eastern shores of continents
-The water then goes where it can, spilling into other currents
Effects of western intensification
-Ocean currents along the eastern shorelines of continents, like the Gulf Stream and Kuroshio, are much stronger than currents on the western shorelines
Upwelling current
Where surface water is swept away from a coast bc of surface divergence caused by coriolis force or by offshore winds and cool water from great depths rises to replace it
Downwelling current
Where excess water gravitates downward
-Creates the deep currents that flow vertically and along ocean floor redistributing heat energy and salinity
Thermohaline circulation (THC)
-Deep currents produced by differences in water density due to temperature and salinity
THC currents travel _____ than wind-driven surface but carries _____ volumes of water
slower; larger
Ocean freshening
-When ocean surface water freezes in the polar regions and the salt is squeezed out of it, causing it to be salt-free when it melts again
-Process is being accelerated due to climate change
How could increased rates of ocean freshening damage the THC?
-A large input of freshwater into North Atlantic could reduce the density of seawater enough that downwelling wouldn't occur, so THC would halt
Warm currents flow near ______ while cold, salty currents flow______
surface; deeper
El Nino-Southern Oscillation (ENSO)
-A shifting of sea-surface temps, air pressure, and winds across the equatorial Pacific region
Best waters to fish in?
where upwelling of cold, nutrient rich water occurs
El Nino: ENSO's warm phase
-For unexplained reasons, pressure patterns and surface ocean temps in Pacific shift from their usual locations
How does El Nino happen?
1) Because of higher pressure than normal in the western Pacific, trade winds start moving eastward instead of their normal westward movement
2) Warm surface water is no longer pulled westward, thermocline lowers in eastern Pacific, and upwelling stops which deprives marine life of nutrients
3) No upwelling of cold water creates a surface pool of warm water (El Nino) in eastern Pacific near South American coast
transition layer btwn warm surface water, and colder, deeper water
How often do es El Nino occur? How long does it last each time?
Usually every 3 to 7 yrs

Can last months or years
La Nina: ENSO's Cool Phase
-When surface waters in central and eastern Pacific cool below normal
-Weaker and less consistent than El Nino;
Effects of El Nino
1) Hurts South American fisheries
2) Linked to unusually intense weather and short-term climate effects like droughts (in South Africa, southern India, Australia, Philippines), hurricanes (in the Pacific), heavy rain (in the U.S. and Central/South America)
3) Weakens Atlantic hurricane season
Effects of La Nina
1) Brings wetter conditions and floods to Indonesia, South Pacific, and Brazil
2) Strengthens the Atlantic hurricane season
Pacific Decadal Oscillation (PDO)
-Shifting patterns of SSTs, air pressure, and winds in the Pacific between east coast of Asia and west coast of North America
-Lasts 20-30 yrs
-Strongest in the northern Pacific ocean
PDO cool phase (negative phase)
-Higher than normal temps dominate the northern and tropical regions of the western Pacific and lower temps occur in east near US
PDO warm phase (positive phase)
-Lower than normal temps in northern and western Pacific and higher temps in the east
Effects of the PDO
1) Negatively effects fisheries on US Pacific coast during positive phase
2) Associated with poleward expansion of subtropical deserts and shifting of storm tracks to higher latitudes
North Atlantic Oscillation (NAO)
-North-south fluctuation of atmospheric variability btwn Icelandic Low and Azores High in the Atlantic where pressure differences alternate from a weak to strong pressure gradient
-Flips unpredictably; can change from week to week
NAO positive phase
-Strong pressure gradient is formed by lower than normal Icelandic low pressure system and higher than normal Azores high pressure system
-Stronger westerly winds and jet streams
-Causes mild winters in US and warm, wet, stormy weather in Europe
NAO negative phase
-Weaker pressure gradient btwn Azores and Icelandic systems because
-weaker westerly winds and jet streams
-Causes moisture in Mediterranean, cold dry winters in Europe, and cold snowy winters in US
Arctic Oscillation (AO)
Variable fluctuations btwn middle and high latitude air mass conditions over the Northern Hemisphere
-Associated with NAO
AO positive phase
-Lower pressures than normal over North Pole and higher pressures at lower latitudes
-Causes strong westerly winds and jet streams
AO negative phase
-Higher than normal pressure over North Pole and lower pressures over central Atlantic
-Causes weaker wind flow