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- Definition: Hurricanes have sustained winds of 120 km/hr (74 mph) or
greater.
- Size: Average diameters are approximately 600 km (350 mi). (one third
the size of mid-latitude cyclone)
- Duration: days to a week or more.
- Strength: Central pressure averages about 950 mb but may be as low as
870 mb.
- Power: The energy released by a single hurricane can exceed the annual
electricity consumption of the US and Canada.
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- Hurricanes form only over deep
(several tens of meters) water layers with surface temperatures in
excess of 27 oC.
- Poleward of about 25o,
water temperatures are usually below this threshold.
- Hurricanes are most frequent
in late summer and early autumn during high SST times.
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- Hurricanes obtain their energy
from latent heat release in the cloud formation process.
- Hurricanes occur where a deep
layer of warm waters exists and during the times of highest SSTs.
- For the N.H., August and September are the most active months.
- For the S.H., the hurricane season is January-March.
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- Tropical Disturbance: Clusters of small thunderstorms.
- Tropical Depression: When at least one closed isobar is present, the
disturbance is classified as a tropical depression.
- Tropical Storm: Further intensification, to wind speeds of 60 km/hr (37
mph), place the storm in the category of tropical storm.
- Hurricane: Hurricane status is gained when winds reach or exceed 120
km/hr (74 mph).
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- When a tropical disturbance reaches the stage of tropical storm, it will
be given a name.
- The name come from an A-W list created by World Meteorological
Organization (WMO).
- Six lists are created for the Atlantic Ocean, each list is used for one
hurricane season.
- The names of the hurricanes that cause devastating damages are removed
from the list forever.
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- The Saffir-Simpson scale.
- Five categories: larger numbers
indicate lower central pressure, greater winds, and stronger storm
surges.
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- A central eye surrounded by large cumulonimbus thunderstorms occupying
the adjacent eye wall.
- Weak uplift and low precipitation regions separate individual cloud
bands.
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- The eye is an area of descending air, relatively clear sky, and light
winds which is about 25 km (15 mi) in diameter on average.
- A shrinking eye indicates storm intensification.
- The eye wall is comprised of the strongest winds, the largest clouds,
and the heaviest precipitation with rainfall rates as high as 2500
mm/day (100 in.).
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- The horizontal pressure gradient with altitude decreases slowly.
- At about 400 mb, pressures within the storm are approximate to that
outside.
- Surface-400mb: Cyclonic circulation.
- 400mb-tropopause: anticyclonic circulation.
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- Winds and surge are typically most intense in the right front quadrant
of the storm where wind speeds combine with the speed of the storm’s
movement to create the area of highest potential impact.
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- Some tropical disturbances form in association with mid-latitude troughs
migrating toward lower latitudes, some form from ITCZ-related
convection, but most develop from easterly waves.
- Easterly waves, or undulations in the trade wind pattern, spawn
hurricanes in the Atlantic (typically 2–3000 km).
- Only about 10% tropical disturbances intensify into more organized,
rotating storms.
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- After making landfall, a tropical
storm may die out completely within a few days.
- Even as the storm weakens, it can
still bring in huge amount of water vapor and rainfall hundreds of
kilometers inland.
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- Heavy rainfalls
- Strong winds
- Tornadoes
- Storm Surges: A rise in water
level induced by the hurricane.
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- Most hurricanes also contain clusters of tornadoes.
- Most pf these tornadoes occur in the right front quarter of the
hurricane movement.
- It appears the slowing of the wind by friction at landfall contribute to
the formation of tornadoes.
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- Process 1: Hurricane winds drag
surface waters forward and pileup the waters near coasts.
- Process 2: Lower atmospheric
pressure raises sea level (for every 1 mb pressure decrease, sea level
raises 1 cm).
- Storm surges raise costal sea
level by a meter or two for most hurricanes, but can be as much as 7
meters.
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- Hurricane watch: if an
approaching hurricane is predicted to reach land in more than 24 hours.
- Hurricane Warning: if the time
frame is less, a warning is given.
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