Atmospheric Moisture and Precipitation

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GEOG 1112 Weather and Climate
Violent Weather
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Midlatitude Cyclone

• Well-organized low pressure system that migrates
  across a region as it spins
• Develops along polar jet as it swings north and
  south in process called Cyclogenesis
• Both upper-level and surface conditions drive
  cyclogenesis
• Southerly swing of polar jet creates an
  upper-level trough supporting cyclogenesis

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Upper-levels and Cyclogenesis
Upper-level convergence pushes down on the
surface, creating high pressure Upper-level
divergence allows air to rise from surface,
creating low pressure
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Midlatitude Cyclone
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Classic Midlatitude Cyclone
L
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Midlatitude Cyclone Migration
Follows path of polar jet
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Two Types of Thunderstorms
1) Air mass thunderstorms (ordinary
thunderstorms) self-extinguishing localized
short lived phenomena limited vertical wind
shear.
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2) Supercell
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Supercell (Severe Thunderstorms) self-propagating
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Mesoscale Convective Complexes
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Squall Line Thunderstorms
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• Cloud-to-cloud lightning
• The most frequent type of lightning
• Occurs within a particular cloud or between
  clouds.
• Also called sheet lightning the sky is
  typically uniformly lit while the stroke is
  buried within the cloud.

• Cloud-to-ground lightning
• Begins when negative charges build in a cloud
  base.
• These negative charges are eventually discharge
  onto the positively charged ground.

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Thunder

• The rapid expansion of air associated with a
  lightning stroke causes thunder. The slower speed
  of sound, with reference to light, causes a lag
  between the stroke and the resulting thunder
• Rumbling thunder is typically caused by sound
  echoing off topographic features and buildings

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Tornadoes

• Small, intense cyclone
• Spawned by severe supercell thunderstorms
• Most destructive atmospheric phenomenon
• Mesocyclone within the supercell
• Gravity Waves
• Rated on Enhanced Fujita Scale EF0 (40-72 mph)
  E F6 (319-379 mph)
• Tornado rotation detected by Doppler radar

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Formation of a Tornado
Updrafts
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Tornadoes
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Tropical Cyclone

• Called hurricane (Atlantic NE Pacific), typhoon
  (NW Pacific) or cyclone (SW Pacific Indian)
• First Easterly Wave trough in tropical
  easterlies
• May become Tropical Depression rotation starts
• Next Tropical Storm sustained winds over 39 mph
• Finally, Hurricane sustained winds over 73 mph

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Tropical Cyclone Strength

• Storm Surge 1 cause of death and damage in
  tropical cyclones
• Debate recent increase in severe tropical
  cyclones Is it global warming or a natural
  cycle?

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Atlantic Hurricanes
Official season June 1-Nov 30 Peak
season Mid-Aug Mid-Oct
Hurricane tracks
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Conditions Necessary for Hurricane Formation

• Hurricanes form only over deep water layers with
  surface temperatures in excess of 27 oC (81 oF)
• Coriolis force is an important contributor, and
  as such, hurricanes do not form equatorward of 5o
  latitude
• Strong vertical shear must be absent

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Tropical Cyclone Structure
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Destruction by Hurricanes

• Winds and surge are typically most intense in
  the right front quadrant of the storm. Why?
  Combination of wind speeds and the speed of the
  storms movement.