Thunderstorms

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Thunderstorms

• Chp 13.1

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Types of thunderstorms

• Orographic thunderstorms

• air that is forced up by a mountain or hillside
• As air strikes the windward side, it is uplifted
  and cooled.
• Windward slopes of mountains tend to be the
  rainy sides
• the leeward side is dry.
• Dry climates found in the "rain shadow" of tall
  mountain systems

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• the result of localized convection in an unstable
  air mass.
• a relatively weak thunderstorm that usually does
  not last much longer than an hour, moves slowly
  and does not produce severe weather
• a garden variety thunderstorm.
• Air mass thunderstorms tend to occur within a
  maritime tropical air mass
• The main threats from an air mass thunderstorm
  are lightning and brief heavy rain.
• air mass thunderstorms tend to occur in the
  afternoon hours because of uplifting produced
  from heat of the sun.

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• occur along the boundaries of weather fronts
  (e.g. cold front).
• An intense line of thunderstorms that forms right
  along or just ahead of a cold front is called a
  squall line.
• Severe thunderstorms, supercells, and/or
  tornadoes can form

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Stages of Development
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Cumulus stage

• Starts with a cumulus cloud. 
• Heating by the sun
• Air starts to rise vertically upward
• The cloud will continue to grow as long as warm
  air below it continues to rise.

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Mature Stage

• When the cumulus cloud becomes very large, the
  water in it become large and heavy.
• Raindrops start to fall through the cloud
• cool dry air starts to enter the cloud.
• Because cool air is heavier than warm air, it
  starts to descend in the cloud (known as a
  downdraft).
• The downdraft pulls the heavy water downward,
  making rain.
• This cloud has become a cumulonimbus cloud
  because it has an updraft, a downdraft, and rain.
  
• Lightning and thunder start to occur, as well as
  heavy rain.
• The cumulonimbus is now a thunderstorm cell.

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Dissipating Stage

• After about 30 minutes, the thunderstorm begins
  to dissipate.
• This occurs when the downdrafts in the cloud
  begins to dominate over the updraft.
• Since warm moist air can no longer rise, cloud
  droplets can no longer form.
• The storm dies out with light rain as the cloud
  disappears from bottom to top.
• The whole process takes about one hour for an
  ordinary thunderstorm

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Assignment

• P. 333 1 - 5

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Severe Weather

• Section 13.2

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Supercells

• responsible for producing tornadoes, large hail
  and other dangerous winds
• warm, humid air in the lower atmosphere, colder
  air aloft and something to give the air near the
  ground an upward shove.
• needs strong winds coming from different
  directions at different altitudes high above the
  ground
• can last for hours,
• Some supercells produce little rain, others,
  downpours that can cause flash floods.
• Supercells are most common on the Great Plains,
  but also occur in other parts of the USA.
• supercells can be detected using the National
  Weather Service's Doppler weather radar.
• allow forecasters to peek inside a supercell and
  look for a Tornado Vortex Signature, or TVS.

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Lightning

• Lightning strikes about 100 times every second.
• Lightning is an electric current.
• many small bits of ice (frozen raindrops) bump
  into each other as they move around in the air.
• All of those collisions create an electric
  charge. After a while, the whole cloud fills up
  with electrical charges.
• The positive charges or protons form at the top
  of the cloud and the negative charges or
  electrons form at the bottom of the cloud.
• Since opposites attract, that causes a positive
  charge to build up on the ground beneath the
  cloud.
• The grounds electrical charge concentrates around
  anything that sticks up, such as mountains,
  people, or single trees.
• The charge coming up from these points eventually
  connects with a charge reaching down from the
  clouds and - zap - lightning strikes!

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• http//www.youtube.com/watch?vgKPwkau0Dh0
• http//www.youtube.com/watch?v_X_7YRVGvtA

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• 54,000 degrees Fahrenheit. That is six times
  hotter than the surface of the sun
• Thunder is caused by lightning.
• When a lightning bolt travels from the cloud to
  the ground it actually opens up a little hole in
  the air, called a channel.
• Once then light is gone the air collapses back in
  and creates a sound wave that we hear as thunder.
  
• The reason we see lightning before we hear
  thunder is because light travels faster than
  sound!

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• Next time you see a storm, count the number of
  seconds between when you see the lightning and
  hear the thunder.
• Take the number of seconds and divide by 5 and
  that will tell you how far away the storm is in
  miles.
• For example If you counted 10 seconds between
  the lightning and the thunder, the lightning is 2
  miles away!

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Lightning Safety

• IF YOURE OUTDOORS
• Keep an eye at the sky. Look for darkening skies,
  flashes of lightning, or increasing winds.
• Lightning often proceeds rain, so dont wait for
  the rain to begin.
• If you hear the sound of thunder, go to a safe
  place immediately.
• The best place to go is a sturdy building or a
  car, but make sure the windows in the car are
  shut.
• Avoid sheds, picnic areas, baseball dugouts and
  bleachers. If there is no shelter around you,
  stay away from trees.
• Crouch down in the open area, keeping twice as
  far away from a tree as far as it is tall.
• Put your feet together and place your hands over
  your ears to minimize hearing damage from
  thunder.
• If youre with a group of people stay about 15
  feet from each other.
• Stay out of water, because its a great conductor
  of electricity. Swimming, wading, snorkeling and
  scuba diving are not safe.
• Also, dont stand in puddles and avoid metal.
• Stay away from clotheslines, fences, and drop
  your backpacks because they often have metal on
  them.
• If youre playing an outdoor activity, wait at
  least 30 minutes after the last observed
  lightning strike or thunder..

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• IF YOURE INDOORS
• Avoid water. Its a great conductor of
  electricity, so do not take a shower, wash your
  hands, wash dishes or do laundry.
• Do not use a corded telephone. Lightning may
  strike exterior phone lines.
• Do not use electric equipment like computers and
  appliances during a storm.
• Stay away from windows and doors and stay off
  porches

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Severe Storms

• Formation of hail based on amount of updraft

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Tornadoes

• A violent, whirling column of air in contact with
  the ground.
• Extreme pressure gradient between the center and
  outer portions cause the rotation and winds.
• Classification
• Fujita scale
• Developed in 1971 by T. Theodore Fujita of the
  University of Chicago

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Tornado Alley
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Killer tornadoes
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Hurricanes

• Tropical cyclone
• Large, rotating, low-pressure storms
• Thrive on the energy from warm tropical oceans
• Originate along the ITCZ
• Require abundant supply of very warm ocean water
• Some sort of disturbance to lift the warm air and
  keep it rising

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Stages

• Tropical waves
• Tropical depressions
• Hurricane
• Minimum winds of 74 mph sustained
• Eye calm center of the storm
• Eyewall strongest winds of the hurricane

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Saffir-Simpson hurricane scale
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Hurricane Names