Project Atmosphere American Meteorological Society

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Project AtmosphereAmerican Meteorological Society

• Weather series The Coriolis Effect
• Debra L. Zolynsky

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Project AtmosphereAmerican Meteorological Society
American Meteorological Society Homepage
Supported by the National Science Foundation and
the National Oceanic and Atmospheric
Administration
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The Coriolis Effect
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Forces Governing Wind

• Forces acting on air parcels, which either
  initiate or modify motion, are the consequence
  of
• 1. Air pressure gradients
• 2. The centripetal force
• 3. The Coriolis effect
• 4. Friction
• 5. Gravity

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1. Air Pressure Gradients
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1. Air Pressure Gradient Force

• This force exists whenever air pressure varies
  from one place to another.
• Can arise from contrasts in air temperature,
• Differences in water vapor concentration, or
• Diverging and converging winds.

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1. Air Pressure Gradients
Air pressure gradients are always measured in the
direction of greatest change, that is,
perpendicular to isobars.
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1. Air Pressure Gradient Force

• The force that causes the air to move as the
  consequence of an air pressure gradient is known
  as the pressure gradient force and it always acts
  directly across isobars and towards low pressure.

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Air (just as water) flows from areas of high
pressure to areas of low pressure.
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2. Centripetal Force

• Newtons first law of motion an object in
  straight-line,unaccelerated motion remains that
  way unless acted upon by an unbalanced force.
• When driving around a curve at high speed, you
  feel a force pushing you outward. What you are
  feeling is the tendency for your body to continue
  in a straight path as the vehicle follows a
  curved path.

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2. Centripetal Force

• A centripetal force operates whenever air parcels
  follow a curved path.
• The centripetal force arises from an imbalance in
  other forces operating in the atmosphere.

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3. Coriolis Effect

• The turning of moving objects paths as seen from
  our vantage point on Earth is the Coriolis
  effect.
• The effect of the Earths rotation on
  horizontally moving objects is greatest at the
  poles and zero at the equator.
• It always acts perpendicular to the direction of
  motion left in the southern hemisphere, and
  right in the northern.

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3. Coriolis Effect

• More later

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4. Friction

• Friction is the resistance that an object or
  medium encounters as it moves in contact with
  another object or medium.
• The friction of fluid flow (either liquid or gas)
  is called viscosity.
• Eddies are visible swirls of fluid.
• The rougher the surface of the Earth, the greater
  the eddy viscosity.
• Turbulence is fluid flow characterized by eddy
  motion.
• Turbulence is experienced as gusts of wind.

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5. Gravity

• Gravity always acts directly downward.
• For this reason, gravity does not modify
  horizontal wind.
• Gravity influences air that is ascending or
  descending, such as in convective currents.
• Gravity is responsible for the downhill drainage
  of cold, dense air.

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Joining Forces

• Forces interact in the atmosphere to control the
  vertical and horizontal flow of air, that is, the
  wind. These interactions result in.
• 1. Hydrostatic equilibrium.
• 2. The geostrophic wind.
• 3. The gradient wind, and.
• 4. Surface winds (horizontal winds in the
  atmosphere.

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Explanation

• The air parcel moves from high pressure to low
  pressure
• directly across isobars. As the air parcel
  accelerates, the
• Coriolis force strengthens and causes the air
  parcel to turn gradually
• to the right always remaining at right angles to
  the parcels
• direction of motion. Eventually, forces attain a
  balance, known as
• geostrophic equilibrium, so that the geostrophic
  wind
• blows at a constant speed in a straight
• path parallel to isobars with the lowest
• air pressure to the left of the direction of air
  motion.

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Questions?