GEOG 1112: Weather and Climate

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GEOG 1112 Weather and Climate

• Atmospheric Pressure, Wind, Global Circulation

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Air Pressure

• Vertical and horizontal pressure differences
  that lead to atmospheric motions.

• Weight of air exerting pressure on surface of
  Earth
• Air pressure decreases with altitude
• Warm air is lighter than cool air, so it tends to
  rise cool air sinks

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Air Pressure Altitude
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Vertical Changes in Pressure

• Pattern of pressure a non-linear decrease with
  height

• Why?

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Atmospheric Pressure Systems
High-pressure system -also called
anticyclone -circulating body of
air -descending air -clockwise circulation
in northern hemisphere Low-pressure
system -also called cyclone -circulating body
of air -rising air -counterclockwise circ. in
northern hemisphere
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Atmospheric Pressure Map
Isobars lines of equal air pressure on a map
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Direction of Air Flow

• Unequal heating of land surfaces
• Pressure gradient force air flows from high to
  low
• Coriolis force deflection or change in
  direction caused by Earths rotation
• Frictional forces places a drag on that air flow

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Isobars and Pressure Gradient

• The closeness of isobars decides the gradient
  between high and low pressure
• The closer the isobar, the higher the winds and
  vice versa
• Winds at the surface always blow at right angles
  to isobars

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Pressure Gradient
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Pressure Gradient Force
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Coriolis Force

• Due to Earths rotation
• Pulls wind to right in northern hemisphere
• Pulls wind to left in southern hemisphere
• Strongest at poles
• None at equator

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Geostrophic Wind

• Horizontal wind in the upper atmosphere that
  moves parallel to isobars. Results from a balance
  between pressure gradient force and Coriolis
  force.

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Frictional Forces

• Near surface, friction (F) works against pressure
  gradient force (PGF), so resulting wind direction
  is between pressure gradient force and coriolis
  force (CF)

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Three Forces Combined
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Rossby Waves
Figure 6.17
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Jet Stream
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The Polar Front and Jet Streams

• Strong boundaries often occur between warm and
  cold air. In the mid-latitudes, the polar front
  marks this thermal discontinuity at the surface.

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Global Pressure Atmospheric Circulation
H

• Unequal heating of tropics and poles
• Global circulation on non-rotating, uniform Earth

L
L
H
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Global Circulation Model
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Seasonal Changes in Circulation

• ITCZ migrates with subsolar point
• Trade winds, STHs, westerlies, all follow the
  ITCZ north south with seasons

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Seasonal Changes in Circulation

• Monsoon
• Seasonal shift of prevailing wind dir. due to
  land/water contrasts
• Happens throughout subtropical regions
• Most significant in South SE Asia

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South Asian Monsoon

• Summer Monsoon
• Warm air over Asia
• Rising air/Low pressure
• Warm, moist wind rain
• ITCZ swings north

• Winter Monsoon
• Cold air over Asia
• Sinking air/High pressure
• Cool, dry NE winds
• ITCZ far south

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Local Wind Systems

• Sea Breeze
• Sun-heated air over land rises Low pressure
• Cooler air over water sinks High pressure

• Land Breeze
• Night air over land cools fast High pressure
• Air over water stays warmer Low pressure

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Local Wind Systems Mountain-Valley Breeze
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Subtropical High-pressure Cells
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Global Oceanic Circulation
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Oceanic Circulation

• Thermohaline Circulation warm water in gulf
  stream cools and evaporates as it heads north
  making it heavier downwelling, then a long trip
  to the Pacific where upwelling occurs

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ENSO
Normal
El Niño