Download Wind – horizontal motion of air across the Earth`s surface

GEOG 140
Intro Physical Geography
Lecture Notes
Atmospheric and Oceanic Circulations
Wind – horizontal motion of air across the Earth’s surface
-Caused by air pressure (density) differences between one location and another
-two properties
-speed
-direction
Global Winds-several forces determine both speed and direction of winds
-gravitational force
-almost uniform
-compresses the atmosphere near the ground worldwide
-density decreases as altitude increases
-pressure gradient force
-air moves from areas of high pressure to areas of low pressure
-greater the differences in pressure, the higher the wind speed
-Coriolis force
-deflective force caused by Earth’s rotation
Pressure Gradient Force
-high and low pressure systems mainly the result of unequal heating of the
Earth’s surface
-pressure gradient illustrated on weather maps by isobars (areas of equal
pressure)
-tighter the lines (isobars) the greater the pressure gradient
-winds move at right angles to the isobars
-high pressure areas – descending, diverging airflows
-low pressure areas-ascending, converging airflows
Coriolis Force
-appears to deflect wind from a straight path
-Northern Hemisphere-to the right
-Southern Hemisphere-to the left
-increases from the Equator (zero) to the poles (maximum)
-pressure gradient + coriolis force =geostrophic wind
-wind flows parallel to isobars (balance between pressure gradient and
deflection of Coriolis force
-high and low pressure areas develop rotary motion
-Northern Hemisphere high (anticyclone) –clockwise
-Northern Hemisphere low (cyclone) - counterclockwise
-wind flowing between highs and lows
Friction Force
-effect of surface friction extends to a height of about 500 meters (1650 ft)
-varies with
-surface texture
-wind speed
-time of day and year
-atmospheric conditions
-near surface-disrupts the balance between pressure gradient and Coriolis
force
-close to surface –friction decreases wind speed which reduces
Coriolis force
-wind crosses isobars at an angle
Global Patterns of Pressure
-Four pressure areas located in Northern Hemisphere and four located in Southern
Hemisphere
-two (in each hemisphere) result of temperature factors
-equatorial low-pressure trough (warm/wet)
-polar high-pressure cells (cold/dry)
-two (in each hemisphere) result of dynamic factors
-subtropical high-pressure cells (hot/dry)
-subpolar low-pressure cells (cool/wet)
-Equatorial low-pressure trough
-narrow band of low pressure circling the Earth
-constant high sun and consistent daylength
-warmer creates lighter, less dense, ascending air
-converging moist air (full of latent heat)
-vertical clouds with heavy rain
-line of cloud cover in equatorial regions
-Intertropical Convergence Zone (ITCZ)
-migrates in the tropics in response to the subsolar point
-air converges from both sides of the equator
-northeast and southeast trades
-hadley cell circulation
-at the equator/ITCZ lack of winds-duldrums
-Subtropical high pressure cells –hot dry desert air
-20o-35o latitude in both semesters
-broad zone of hot dry air
-absence of clouds on satellite images
-decending air of Hadley cell circulation
-decending air compresses and heats
-hotter air has high moisture capacity
-air is actually dry because the majority of the moisture falls over
equator
-Subpolar –low pressure cells -cool and moist air
-two cells at around 60o N latitude in winter
-Aleutian low
-Icelandic low
-weaken/disappear in summer with strengthening of high in subtropics
-contact zone –polar front
-discontinuous belt surrounds Antarctica
-Polar high-pressure cells- frigid dry deserts
-weak polar high pressure cells (Anatarctic high more pronounced)
-anticyclonic winds move away from the poles
-decending and diverging
-clockwise –N. Hemisphere
-counterclockwise-S. Hemisphere
-polar easterlies –weak, variable winds
Upper Atmospheric Circulation
-Rosby waves
-Jet Streams
Local Winds
-Land-sea breezes
-Mountain-valley breezes
-Kadiabatic winds
Monsoonal Winds
Oceanic Currents
-surface currents
-ocean currents driven by the circulation around subtropical high pressure
cells
-Northern Hemisphere-winds and ocean currents move clockwise
-Southern Hemisphere-winds and ocean currents move counterclockwise
-along the equator
-tradewinds force water westward
-coriolis force keeps flow constrained in a narrow band (coriolis
force zero at the equator)
-as water reaches western edge of ocean it “piles up” and spills
north and southward in strong currents
-Gulf Stream_Atlantic
-Kuroshio-Pacific
-deep currents
-surface water swept away from coasts replaced by upwelling current
-cold, nutrient rich water rises from great depths
-Pacific Coasts of N. and S. America
-tropical/mid latitude west Coast of Africa
-surface water piles up and sinks
-water gravitates downward in a downwelling current
-important mixing currents run along ocean floor
-transport heat energy