AOSC Lesson 11

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AOSCLesson 11
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Centrifugal Force
Fig. 6.11
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The Mechanism for Geostrophic Flow
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GEOSTROPHIC FLOW

• EVENTUALLY A BALANCE IS REACHED BETWEEN THE
  CORIOLIS FORCE AND THE PRESSURE GRADIENT FORCE
• IF THERE IS NO FRICTION THEN THIS OCCURS WHEN THE
  PARCEL OF AIR IS PARALLEL TO THE ISOBARS.
• AT THIS POINT THERE IS NO NET FORCE ON THE AIR
  PARCEL - NO ACCELERATION - IT NOW MOVES WITH
  CONSTANT VELOCITY.
• UNDER THESE IDEALIZED CONDITIONS THE AIRFLOW IS
  SAID TO BE IN GEOSTROPHIC BALANCE.
• WIND GENERATED IS CALLED THE GEOSTROPHIC WIND
• NORMALLY ONLY APPLIES TO WINDS ALOFT.
• SURFACE WINDS ARE SUBJECT TO FRICTION.

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Fig. 6-15, p. 172
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GRADIENT WINDS

• WINDS AROUND CENTERS OF HIGH OR LOW PRESSURE
  FOLLOW CURVED PATHS IN ORDER TO STAY PARALLEL
  WITH THE ISOBARS.
• THESE WINDS ARE CALLED GRADIENT WINDS.
• LOW PRESSURE CENTERS ARE CALLED CYCLONES -
  ROTATION IS COUNTER-CLOCKWISE - SAME AS THE EARTH
  
• CENTERS OF HIGH PRESSURE ARE CALLED
  ANTI-CYCLONES.
• IN SOUTHERN HEMISPHERE THE DIRECTIONS OF FLOW ARE
  REVERSED

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Fig. 6-12, p. 168
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Fig. 6-18, p. 175
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Fig. 6-24, p. 181
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SURFACE WINDS

• FRICTION AFFECTS WINDS ONLY CLOSE TO THE EARTH'S
  SURFACE.
• NOW WE MUST BALANCE THREE FORCES - CORIOLIS,
  PRESSURE GRADIENT AND FRICTION.
• NET EFFECT IS TO INDUCE A NET INFLOW AROUND A
  CYCLONE, AN EFFECT KNOWN AS CONVERGENCE.
• AROUND AN ANTICYCLONE WE GET A NET OUTFLOW,
  DIVERGENCE

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Aneroid Barometer
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Aneroid Barometer

• On an aneroid barometer fair corresponds to high
  surface pressure.
• Air is subsiding and is subject to adiabatic
  heating. This lowers the relative hunidity. Hence
  possibility of clouds forming is low.
• Rain corresponds to low pressure.
• Air is rising and is subject to adiabatic
  cooling. The raises the relative humidity.
  Possibility of clouds is high.

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Effect of temperature on pressure

• Initially the pressure above each city is the
  same. But if we heat the air above one of them
  the column of air will expand. If we cool the air
  above the other it will contract.
• If we now look at the pressure at the top pf the
  cool air it will be lower than that in the heated
  column. This is because pressure is defined as
  the weight of air above a given altitude.
• The weight of air above the cold column is less
  than that at the same altitude in the not column
• This will cause air to move from the hot column
  to the cold column.
• Hence the pressure will increase in the cold
  column at the ground, and the pressure will
  decrease in the hot column.

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Fig. 6-22, p. 179
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Sea (Land) Breeze Effect
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SEA BREEZES

• ARE THE RESULT OF DIFFEERENTIAL HEATING OF THE
  OCEAN AND THE LAND
• DURING THE DAY THE LAND HEATS UP QUICKLY WHILE
  THE OCEAN HEATS UP SLOWLY
• HIGH TEMPERATURE OVER THE LAND, LOWER TEMPERATURE
  OVER THE OCEAN
• AT THE SURFACE - HIGH PRESSURE OVER THE OCEAN,
  LOW PRESSURE OVER THE LAND - CAUSES WIND AT THE
  SURFACE TO FLOW FROM THE OCEAN TO THE LAND (SEA
  BREEZE)
• AT NIGHT THE LAND COOLS RAPIDLY TO A TEMPERAURE
  BELOW THAT OF THE OCEAN. WIND REVERSES - FLOWS
  FROM THE LAND TO THE OCEAN AT THE SURFACE (LAND
  BREEZE)

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Stepped Art
Fig. 7-6, p. 175
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Fig. 6.26
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Fig. 6.26
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Fig. 6.26
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Fig. 6.26