Atmospheric Circulation

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Atmospheric Circulation

• The fairly constant low pressure area at the
  equator, called the doldrums, is associated
  with sultry air and variable breezes.
• Scientist call this area the intertropical
  convergence zone or ITCZ.

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Atmospheric Circulation

• The term intertropical convergence zone is used
  to reflect the influence of wind convergence on
  conditions near the equator.
• Strong heating in the ITZC causes warm moist to
  rise, expand and lose moisture as rain.
• These are perfect conditions for the success of
  tropical rain forests.

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Atmospheric Circulation

• The ITZC does not coincide with the geographical
  equator at 0-degrees of latitude, but it lies at
  the meteorological equator or thermal equator
  which changes with the seasons.

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Monsoons

• A monsoon is a pattern of wind circulation that
  changes with seasons. The word is derived from
  an Arabic word which means seasons.
• Monsoons are linked to the different specific
  heats of land and water.
• In spring the land heats more rapidly than the
  ocean.

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Monsoons

• Air above the land becomes warmer and rises.
  Relatively cool air from over the ocean moves in
  to replace the rising air over land.
• Continental heating causes this humid air to
  rise, condense, and form clouds and rain.

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Monsoons

• In autumn the land cools more rapidly than the
  adjacent ocean. Air cools and sinks over the
  land, and dry surface winds move seaward.
• The intensity and location of monsoon activity
  depends on the position of the ITCZ.

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Mini-Monsoons

• Land breezes and sea breezes are small, daily
  mini-monsoons.
• Morning sunlight falls on both the land and the
  adjacent ocean evenly.
• The temperature of the water doesnt rise as much
  as the temperature of the land. The inland rocks
  transfer heat to the air and causes it to rise.

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Mini-Monsoons

• This creates a low pressure area over the land
  and causes cooler (higher pressure) air from over
  the ocean to move in and replace the air rising
  over the land.
• This is called a sea breeze.
• The situation reverses after sunset and the
  result is a land breeze.

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Storms

• Storms are regional atmospheric disturbances
  characterized by strong winds and frequently
  accompanied by precipitation.
• The two great storms on Earth are tropical
  cyclones and extratropical cyclones.

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Storms

• Most of us are quite familiar with tropical
  cyclones as the Atlantic hurricanes which occur
  in summer in the Northern Hemisphere.
• Extratropical cyclones are winter disturbances
  which residents of the Eastern Seaboard are
  familiar with. These mainly occur in the Ferrel
  cells of each hemisphere

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Storms

• Both kinds of cyclones are huge masses of low
  pressure air in which winds converge and ascend.
• The most powerful storm on Earth is the
  hurricane.
• Hurricanes occur in all tropical oceans except
  the equatorial South Atlantic.

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Hurricanes

• From above, tropical cyclones appear as circular
  spirals. They may be 1,000 kilometers (620
  miles) in diameter and 15 kilometers (9.3 miles)
  high.
• The center of the storm is call the eye and may
  be 13 - 16 kilometers (8-10 miles) in diameter.

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Hurricanes

• Hurricanes form from warm, humid air masses
  between 10 and 20 degrees latitude in both
  hemispheres.
• They do not form closer to the equator because
  the Coriolis effect is too weak to initiate the
  rotary motion.
• A tropical cyclone usually develops from a small
  tropical depression.

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Hurricanes

• When air containing a tropical disturbance is
  heated over tropical water with a temperature of
  about 26-degrees C (79-degrees F), or greater
  circular winds begin to blow in the vicinity of
  the wave, and some of the warm, humid air is
  forced upward.
• Condensation takes place and the storm takes
  shape.

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Hurricanes

• If conditions are ideal the storm will reach
  hurricane status, with wind speeds in excess of
  119 kilometers (74 miles) per hour in two to
  three days.
• The centers of most tropical cyclones move
  westward and poleward at 5-40 kilometers ( 3-25
  miles) per hour.

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Hurricanes

• Tropical cyclones are an ideal machine for
  cashing in on water vapors latent heat of
  evaporation.
• Three aspects of a tropical cyclone can cause
  property damage and loss of life.
• They are wind, rain and the storm surge.
• The most dangerous of these is the storm surge.

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Hurricanes

• The storm surge is a wall of seawater forced on
  shore by the low pressure of the hurricane.
• The water height increases when waves and strong
  hurricane winds ram the water mass ashore.

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Hurricanes

• You may have noticed that tropical cyclones turn
  counterclockwise in the Northern Hemisphere.
• This does not mean that the Coriolis effect does
  not apply to them.
• This apparent anomaly is caused by the Coriolis
  deflection of winds approaching the center of a
  low pressure area from a great distance.

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In the diagram below note the rightward
deflection of the approaching air. The edge spin
given by this approach causes the storm to spin
counterclockwise.
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Tropical Storm speed

• A tropical disturbance is a rotary circulation
  with little to no development on the ocean
  surface, but better developed in the upper
  atmosphere.
• A tropical depression is a rotary circulation on
  the ocean surface and sustained wind speed of
  less than 39 miles per hour.
• A tropical storm is a low-pressure area with a
  distinct rotary circulation and sustained wind
  speeds of 39 to73 mph.

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Categories of Storms
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Hurricanes

• Tropical cyclones are natures escape valves,
  flinging solar energy poleward from the tropics.