Atmospheric Circulation

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Atmospheric Circulation

• F.6 Geography

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1. Atmospheric circulation (????)

• horizontal and vertical flow,
• driving forces of air movement,
• influences on surface wind system

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2. Major wind systems

• patterns and characteristics of the trades,
  westerlies, polar winds and monsoons

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3. Air masses

• nature and types
• influences on weather and climate

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4. Atmospheric disturbances

• typhoons and man responses
• changes of wind speed and direction

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EXPLANATORY NOTES

• 1. Atmospheric circulation is the mechanism
  through which the energy surpluses and deficits
  are balanced, and the balance involves air
  movement of different scales.

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• 2. Students are expected to
• understand the flows and driving forces of
  atmospheric circulation
• relate atmospheric circulation to surface wind
  systems,
• understand the nature and characteristics of
  air masses and
• note how people response to atmospheric
  disturbances, e.g. typhoons

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3. Temporal and spatial scales for atmospheric
motions
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I. INTRODUCTION ??

• The atmosphere acts as heat engine in which the
  difference in the temperature between the poles
  and the equator provides the energy supply to
  drive the planetary atmospheric circulation.
• Large-scale air circulation transport heat, both
  sensible heat and latent heat present in water
  vapor.

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• Because of the global radiation imbalance
• -- a surplus in low latitudes and
• a deficit in high latitudes
• Atmospheric circulation must transport heat
  across the latitudes from the regions of surplus
  to the region of deficit.

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• The variable heating of different parts of the
  atmosphere sets up variations in pressure, which
  in turns sets the air in motion.
• Wind is air in motion and it dominantly
  horizontal.

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(II) ATMOSPHERIC CIRCULATION (GENERAL
CIRCULATION) (????)  

• HORIZONTAL AND VERTICAL AIR FLOW (???????)
• THE IMPORTANCE OF AIR MOVEMENT (????????)
• 1. Thermal redistribution
• 2. Transfer of water vapour

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1. HORIZONTAL MOVEMENT WINDS(?)

• Horizontal movement, or wind, is by far the
  faster and consists of air movements parallel to
  the surface.
• Horizontal movements or wind is an important
  climatic factor for a number of reasons.

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(1) Thermal Re-distribution (?????)

• Wind balanced warm and cold bodies of air,
  thereby modifying the thermal characteristics of
  places related to their radiation regime.
• Such modification may have a considerable effect
  on the air temperature of a place

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• Air movement is important to weather and climate,
  and human significance.
• For convenience, air motion may be resolved into
  two components horizontal and vertical

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• A change in wind direction may cause changes
  in temperature.
• For example, HK in winter was affected by NW
  monsoon winds.

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(2) Moisture Transfer (????)

• Wind action transports water vapor. In
  particular, moisture is brought from areas where
  it is abundant, such as over the oceans, to areas
  where it is often deficient, such as over the
  interiors of continents.

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• e.g. Onshore winds in E and SE China, including
  HK, in summer.
• Example Figure below illustrates the
  significance of a seasonal reversal of wind
  direction in rainfall amount for Hong Kong. It
  shows the effects of monsoon winds on rainfall.

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(3) Environmental hazards (????)

• Air in rapid motion is, a severe environmental
  hazard. On average, more lives are lost each year
  as a result of tropical storms than from the
  combined effects of fire, lightning, floods,
  tidal waves and earthquakes.

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2. VERTICAL MOVEMENT

• Vertical motions, on the other hand, involve
  sinking and rising masses of air perpendicular to
  the surface and are usually 100-1000 times slower
  than their horizontal counterparts.
• Vertical movements of air, although normally less
  rapid than their horizontal counterparts, are
  very important, since they strongly influence
  whether the climate and weather will be cloudy
  and rainy or clear and dry.

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• Areas where air is sinking are relatively
  cloud-free and dry, e.g. TD
• whereas in areas characterized by rising air
  motion the opposite weather types tend to
  prevail. e.g TRF.

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3. RELATIONSHIP BETWEEN AIR MOTION (BOTH
HORIZONTAL VERTICAL) AND THE GLOBAL ENERGY
BUDGET.

• Air in motion, however, has an even more
  fundamental function to fulfilled at a global
  scale -- the transfer of heat.
• It will be recalled from energy budget that
• THE UNEQUAL HEATING OF THE EARTH SURFACE BY
  THE SUN PRODUCES A LATITUDINAL CONTRAST IN ENERGY
  BUDGETS between about 40 N and 35 S, where the
  amount of incoming radiation exceeds that lost by
  the cooling of the earth-atmosphere system,
  whereas towards the poles the reverse applies.

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• Obviously, if such a situation persisted ,it
  would cause the low latitudes to be very much
  hotter than they are at present, and the high
  latitudes to be very much more cold.
• Atmospheric movement implies the existence of a
  mechanisms whereby heat is moved from the surplus
  areas to the deficit areas to compensate for the
  shortfall in the energy budget of the latter.

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B. DRIVING FORCES THAT CONTROL AIR MOVEMENT

• SPATIAL VARIATION OF TEMPERATURE (??????)
• The energy required to drive the gigantic
  circulation of the earth surface is provided by
  the temperature contrasts between cold polar
  region and warm tropical air region. Why there is
  unequal heating on the earth surface ?

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2. AIR PRESSURE (????)

• Although not readily noticeable, air exerts a
  pressure on every surface exposed to it.
• That pressure can be considered as resulting from
  the weight of overlying air pressing down on a
  given area.

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AIR MOTION IS A RESPONSE TO A FORCE OR FORCES OF
SOME KIND

• ATMOSPHERIC MOTION IS CONTROLLED BY THE
  INTERPLAY BETWEEN 5 FORCES
• 1. THE PRESSURE-GRADIENT FORCE
• 2. THE CORIOLIS FORCE
• 3. FRICTION

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• Air motion is initiated by a pressure gradient
  between places, with initial movement occurring
  from high to low pressure locations. The air is
  pushed from areas of high pressure to areas of
  low pressure. The air ought to move at right
  angles to the isobars.
• (Spatial variations of pressure are depicted on
  maps by means of isobars, which are lines
  connecting places having the same barometric
  pressure).

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1. THE PRESSURE GRADIENT FORCE (????)

• -- INFLUENCE THE DIRECTION AND SPEED OF WIND

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• The gradual change of pressure between different
  areas is known as pressure gradient.
• Where a pressure gradient exists, air molecules
  tend to drift in the same direction as that
  gradient. This tendency for mass movement of air
  is referred to as the pressure gradient force.
• The magnitude of the pressure gradient force is
  directly proportional to the steepness of the
  gradient.

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• A simple relationship between pressure-gradient
  and wind speed exists the steeper the
  pressure-gradient, the faster the wind speed.

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• Falling pressure (low pressure) generally
  generate the onset of poor unstable weather, and
  a rising barometer(high pressure) suggests a
  trend towards sunny stable weather conditions.
• A pressure gradient exists both vertically and
  horizontally.  

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(a) Vertical Pressure Gradient

• Pressure decreases vertically. As we move upwards
  through the atmosphere, the weight of overlying
  air diminishes.
• Obviously, the layers closest to the surface will
  have the greatest weight overlying them and thus
  the pressure will be greatest. Therefore, rapid
  decrease in air pressure occurs with increasing
  height.

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(b) Horizontal Pressure Gradient

• Pressure varies laterally because of the
  temperature differences resulting from
  differences in the intensity of solar heating of
  the atmosphere.
• Where solar radiation is intense, the air warms
  up, expands and its density decreases. As a
  result, air pressure falls.
• Where cooling occurs, the air contracts, its
  density increase and air pressure becomes
  greater.

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2. Coriolis force

• As the earth will rotates , the wind blowing in
  Northern Hemisphere will deflected to its right.
• In Southern Hemisphere, it will deflected to its
  left.
• The force exerted greatest in pole, but lowest in
  equator

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3. Friction

• All types of obstacles produce frictional drag
  where the wind blowing through.
• Frictional drag acts in a direction opposite to
  the path of motion and can cause deceleration
• It also reduces the magnitude of the Coriolis
  force which is dependent on wind speed

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• It will disturbed the combined Coriolis force and
  frictional force and cause the wind to blow
  obliquely across the isobar

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(c) Pressure in the Upper Atmosphere

• But the pattern of air pressure close to the
  surface is reverse in the upper atmosphere.
• This is because as cold air contracts, the upward
  decline in pressure is rapid and at any constant
  height above a zone of cool air the pressure is
  relatively low. (High pressure at lower
  atmosphere, but low atmosphere at the upper)
• Conversely, warm air expands and rises, so that
  the vertical pressure gradient is less steep.
  Above areas of warm air(low pressure), therefore,
  the pressure tend to be relatively high (high
  pressure).

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Figure 4.8 Upper Westerlies
wind
Pressure decreases
Pressure gradient force
Coriolis force
Cold
Warm
North Pole
Equator
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• With increasing altitude, wind tend to be
  prevailing westerly
• It blow at high speed , (125km hr)
• Band of rapid air movement in the upper called
  jet streams
• At this height, the frictional effect of the
  ground surface upon winds is very weak
• Air flows nearly approximates to the geostrophic
  winds
• The lower air density at high altitude also
  allows air to flow more easily

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• The upper air westerlies occur as wave-like
  forms, called Rossby Waves
• This is due to the effects of land and sea
  difference on the surface and relief differences
  along the same latitude
• Three to six Rossby waves encircle the globe in
  amplitudes covering 15to 20 of latitude