Hurricanes

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The Earth's weather system represent complex
interactions between the oceans, the land, the
sun, and the atmosphere.
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• Weather is the condition of the atmosphere at a
  particular time and place. It refers to such
  conditions of the local atmosphere as
• temperature,
• atmospheric pressure,
• humidity (the amount of water contained in the
  atmosphere),
• precipitation (rain, snow, sleet, hail),
• wind velocity.

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• Vertical Structure of the Atmosphere
• General trends with increasing altitude
• Constituent gases tend to decrease in density
• Air pressure decreases
• Water vapor decreases dramatically
• Temperature fluctuates decreases in the
  troposphere where weather occurs.

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Composition of the Atmosphere

• The gaseous envelope that surrounds the planet.
• Because air pressure decreases with altitude, the
  amount of air per unit volume (density) also
  decreases with altitude.
• The relative proportions of the gases in the air
  are essentially constant regardless of altitude.
  Nitrogen, Oxygen, and argon make up 99.96 of the
  gases by volume.

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Greenhouse Effect

• Carbon dioxide, water vapor, methane, greenhouse
  gases and chlorofluorocarbons (CFC's) are some
  greenhouse gases of anthropogenic origin.
• .

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Air Pressure

• Air pressure decreases with altitude. At any
  given altitude, the air pressure is caused by the
  weight of air above. This means that the air
  near the ground is compressed by the weight of
  the air above it.
• As air pressure decreases, air density decreases.

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Water Vapor in the Atmosphere

• Water can exist in all 3 states at the normal
  range of earth temperature and pressure.
• Whenever matter changes from one state to
  another, energy is either absorbed or released.
• From gas to liquid - condenstation - heat energy
  is released
• From liquid to gas - evaporation - heat energy is
  absorbed

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Relative Humidity

• Air is saturated when evaporation condensation.
  Temperature dependent.
• Saturation vapor pressure of air at any given
  temperature cannot be exceeded.
• Relative humidity the ratio of the vapor
  pressure in a parcel of air to the saturation
  vapor pressure at the same temperature.

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Therefore, relative humidity can be changed by...

• Changing the water vapor content.
• Add water, increase relative humidity
• subtract water, decrease relative humidity
• Changing the temperature.
• Increase temperature, decrease relative humidity
• Decrease temperature, increase relative humidity.

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Adiabatic Processes

• Processes that occur without the addition or
  subtraction of heat from an external source.
• Compressional warming - when air is compressed,
  the temperature rises.
• Expansional Cooling - when air expands, the
  temperature decreases.
• Because Air pressure decreases with increasing
  altitude, rising air expands and sinking air is
  compressed.
• The adiabatic lapse rate - the way temperature
  changes with altitude in rising or falling air.

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Upward movement of air results from

• Density lifting - When warm, low-density air
  rises convectively and displaces cooler, denser
  air.
• Orographic lifting - When flowing air is forced
  upward over a mountain range.
• Convergence lifting - when flowing air masses
  converge and are forced upward.
• Frontal lifting - when two flowing air masses of
  different density meet.

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Warm Front
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Cold Front
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Atmospheric Stability

• Two assumptions
• Lifting processes force air upward.
• Rising air does not mix substantially with the
  surrounding atmosphere.
• A parcel of air forced to rise will expand and
  cool adiabatically.
• Lifting condensation level altitude at which
  the rising parcel reaches saturation temperature
  and cloud forms.

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Atmospheric Stability

• Atmospheric stability is a property of air that
  describes its tendency to remain in its original
  position (stable) or to rise (unstable) once the
  initial lifting force ceases.

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Atmospheric Stability

• Stable air - if an air parcel that is forced
  aloft cools faster than the surrounding
  environment.
• Cloud formation may occur when the saturation
  temperature is reached, but clouds would be
  layered without much vertical development - fair
  weather clouds.
• If the lifting forced ceased, the parcel would
  have the density to sink.

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Atmospheric Stability

• Unstable air - if an air parcel that is forced
  aloft cools slower than the surrounding
  environment.
• If the lifting force ceased, the parcel will
  continue to rise because it is warmer and more
  buoyant than its surroundings.
• If the air parcel rises to its condensation
  level, clouds with vertical development will form
  as the buoyant air rises on its own -
  thunderstorm clouds.

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Tornadoes
How a Tornado Works - Associated with strong
thunderstorms that develop when 3 main
atmospheric conditions occur simultaneously in
the central US 1) a northerly flow of warm,
moist air from the Gulf of Mexico 2) a cold,
dry air mass rapidly moving southward from Canada
or the Rocky Mountains 3) strong easterly jet
stream - These three air masses moving in
different directions produce shearing conditions
that are give thunderstorm clouds a "spin" -
Funnel clouds begin to form, they may (or may
not) touch down and develop into a tornado.
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Tornadoes.

• Warm moist air tropical air shoots upward as it
  meets colder, dryer polar air. As the warm moist
  air rises, it may meet varying wind directions at
  different altitudes due to a strong westerly jet
  stream. If these varying winds are staggered in
  just the right manner with sufficient speed, they
  will act on the upward rising air, spinning it
  like a top.

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Tornado Characteristics

• About 70 of all tornadoes on Earth occur in the
  central and southern US.
• One section of the nation is best at producing
  tornadoes. This area is called "Tornado Alley,"
  (shown on the map).- Northern Texas and Oklahoma
• Occur mostly in late spring - early summer when
  conditions are best for tornado formation but
  can occur anytime.
• Can move at speeds up to 60 mph and have max
  wind speeds of 300 mph.

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Tornado "magnitude" measured on the Fujita Scale
(F0-F5) based on Damage
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Wind

• Wind is a horizontal air movement arising from
  differences in air pressure.
• Wind results when air flows from a place of high
  pressure to one of low pressure.

Isobars - lines connecting places of equal air
pressure on a map.
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Wind

• The spacing of the isobars indicates the amount
  of pressure change over a given distance
  pressure gradient.
• Compare to the slope of a hill

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Wind Summary

• Horizontal pressure gradient is the driving
  force.
• Magnitude is determined from the spacing of the
  isobars.
• Direction is always from areas of high pressure
  to areas of low pressure (perpendicular to the
  isobars).

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Coriolis Effect

• Due to the rotation of the earth on its axis.
• Deflects all free moving objects to the right of
  their path in the Northern Hemisphere and to the
  left in the Southern Hemisphere.
• Strongest at the poles, nonexistent at the
  equator.

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The Coriolis Force and Wind

• Deflects to the Right in the Northern Hemisphere,
  Left in the Southern Hemisphere.
• Deflection increases with wind speed.
• Deflection is strongest at the poles and weakens
  equatorward.

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Convergent and Divergent Flow

• In the Northern Hemisphere
• Around a low pressure cell, an inward
  counterclockwise flow develops Centers of low
  pressure are called cyclones convergent flow
• Around a low pressure cell, an outward clockwise
  flow develops. Centers of high pressure are
  called anticyclones divergent flow.

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Low pressure center generally related to unstable
conditions and stormy weather
Vertical Flow
Net downward movement of air and fair weather
Net upward movement of air, often resulting in
cloud formation and precipitation.
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Around a surface high air is spiraling outward,
which leads to a downward flow of air at the
center of the high and convergence aloft.
Around a surface low air is spiraling inward,
which leads to an upward flow of air at the
center and divergence aloft.
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HURRICANES

• A hurricane is a massive tropical cyclone with
  rotary winds that exceed 74mph blowing
  counterclockwise around a relatively calm central
  area of very low pressure.
• Hurricanes in the Atlantic and East Pacific,
  typhoons in the west pacific, cyclones in the
  Indian Ocean and Southern Hemisphere.
• Form in late summer and early fall when ocean
  waters are warmest.

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Tropical systems are classified into four
categories according to its degree of
organization and maximum sustained wind
speed. Tropical disturbance, tropical wave
Unorganized mass of thunderstorms, very little,
if any, organized wind circulation. Tropical
depression Has evidence of closed wind
circulation around a center with sustained winds
from 20-34 knots (23-39 mph). Tropical storm
Maximum sustained winds are from 35-64 knots
(40-74 mph). The storm is named once it reaches
tropical storm strength. Hurricane or typhoon
Maximum sustained winds exceed 64 knots (74 mph).
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Hurricane Formation
The process by which a tropical cyclone forms and
subsequently strengthens into a hurricane depends
on three conditions 1. A pre-existing
disturbance with thunderstorms (typically
emerging from the coast of Africa) 2. Warm (at
least 80ºF) ocean temperatures to a depth of
about 150 feet 3. Light upper level winds that
do not change much in direction and speed
throughout the depth of the atmosphere (low wind
shear)
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Hurricane Formation

• Heat and energy for the storm are gathered by the
  disturbance through contact with warm ocean
  waters. The winds near the ocean surface spiral
  into the disturbance's low pressure area. The
  warm ocean waters add moisture and heat to the
  air which rises. As the moisture condenses into
  drops (thunderstorm bands), more heat is
  released, contributing additional energy to power
  the storm. If weather and ocean conditions
  continue to be favorable, the system can
  strengthen and the storm begins to take on the
  familiar spiral appearance due to the flow of the
  winds and the rotation of the earth (the stronger
  the winds, the greater the coriolis deflection).

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Warm, humid air spirals in from the sides and
rises, dumping vast amounts of rain and releasing
heat in the process. The heated, rising air
creates very low atmospheric pressures so low
that the pressure in the upper atmosphere above
the storm is actually higher than in the lower
atmosphere in the center of the storm. This
causes a downward flow of air in the center of
the storm, forming the eye of the hurricane.
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Hurricanes
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Once hurricanes form, they are pushed west with
the prevailing west-blowing winds in tropics. The
Coriolis force causes storms north of the equator
to travel in right-curving paths, and storms
south of the equator to travel in left-curving
paths.
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The Hurricanes End

• Strong upper level winds shear tear the hurricane
  apart.
• Moving over cooler water can lead to weakening.
• Moving over land shuts off the moisture source
  and reduces surface circulation due to friction.

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Weather Alerts

• A tropical-storm watch is issued when sustained
  winds from 39 to 73 mph (54.7 to 117.48 kph / 34
  to 63 kt) are possible in your area within 36
  hours.
• A tropical-storm warning indicates that these
  conditions are likely in your area within 24
  hours.
• A hurricane watch is issued when hurricane
  conditions (sustained winds greater than 74 mph /
  119 kph / 64 kt) are possible in your area within
  36 hours.
• A hurricane warning is issued when these
  conditions are likely in your area within 24
  hours.

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Hurricane Damage Storm surges are like a hill
of ocean water (sometimes as high as 20ft above
sea level) pushed up by a hurricane. Storm surges
are caused by two factors low atmospheric
pressure that pulls the ocean surface up, and the
spiraling, converging winds that push the ocean
water in toward the center of the storm.
Storm surges cause far more death and destruction
than the high winds in a hurricane.
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Ocean-front property is particularly susceptible
to damage from hurricane storm surges, when ocean
waves are very high and strong because of the
strength of the hurricane's winds.
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Storm Surge
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Flooding

• Hurricanes frequently produce huge amounts of
  rain, and flooding can be a significant problem,
  particularly for inland communities. A typical
  hurricane brings at least 6 to 12 inches of
  rainfall to the area it crosses.

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Many communities and small towns in eastern North
Carolina found themselves afloat following
Hurricane Floyd's massive rainfall and the
ensuing floods. Streams and waterways were
already swollen from Hurricane Dennis, which
struck eastern North Carolina (twice) just weeks
before Hurricane Floyd.
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Winds

• Hurricane winds not only damage structures, but
  the barrage of debris they carry is quite
  dangerous to anyone unfortunate enough (or unwise
  enough!) to be caught out in them.

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Hurricanes often cause severe structural damage.
This building, in the U.S. Virgin Islands, was
leveled.
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Hurricane prediction Hurricanes can be predicted
to some degree. Wind directions, sea surface
temperatures, and behavior of past storms can be
used to issue warnings about storm development,
possible landfall location, and storm size.
However the storms can unpredictably weaken,
strengthen, or change direction.
Weather satellites use different sensors to
gather different types of information about
hurricanes Visible - clouds, circulation
patterns Radar / Doppler radar - rain, wind
speeds, precipitation amounts Infrared -
temperature differences, cloud heights
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Visible Images
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Doppler Radar
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Infrared
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This photo is a composite of three days' views
(August 23, 24 and 25, 1992) of Hurricane Andrew
as it slowly moved across south Florida from east
to west.
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Hurricane Floyd was a Category 3 storm that
brought intense rains and record flooding to the
eastern United States and Canada. Nearly 90
percent of the fatalities associated with this
storm were drownings due to inland flooding.
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Hurricane Names

• When the the winds from these storms reach 39
  mph (34 kts), the cyclones are given names.
• Years ago, an international committee developed
  names for Atlantic cyclones. In 1979 a six year
  rotating list of Atlantic storm names was adopted
  alternating between male and female hurricane
  names.

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2006 Alberto Beryl Chris Debby Ernesto Florence Go
rdon Helene Isaac Joyce Kirk Leslie Michael Nadine
Oscar Patty Rafael Sandy Tony Valerie William
2008 Arthur Bertha Cristobal Dolly Edouard Fay Gus
tav Hanna Ike Josephine Kyle Lili Marco Nana Omar
Paloma Rene Sally Teddy Vicky Wilfred
2004 Alex Bonnie Charley Danielle Earl Frances Gas
ton Hermine Ivan Jeanne Karl Lisa Matthew Nicole O
tto Paula Richard Shary Tomas Virginie Walter
2005 Arlene Bret Cindy Dennis Emily Franklin Gert
Harvey Irene Jose Katrina Lee Maria Nate Ophelia P
hilippe Rita Stan Tammy Vince Wilma
2007 Allison Barry Chantal Dean Erin Felix Gabrie
lle Humberto Iris Jerry Karen Lorenzo Michelle Noe
l Olga Pablo Rebekah Sebastien Tanya Van Wendy
2009 Ana Bill Claudette Danny Erika Fabian Grace H
enri Isabel Juan Kate Larry Mindy Nicholas Odette
Peter Rose Sam Teresa Victor Wanda