Announcements

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Announcements
Homework 4 (last one!) - Due next Tuesday, 22
November.

• Final Exam Review Session
• Review session Wed. 7 Dec, 600 - 700 pm, room
  TBD.
• Will be cumulative.
• I will put together a set of questions from
  throughout the semester for you to study from.

Thanks to those who handed in questions this
week! You now have a mini-study guide available
online my lecture questions. See Comments
column of Syllabus page
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Key Concepts
Fronts, Mid-latitude Cyclones
4 types of air masses (cP, cT, mP, mT)
4 types of fronts (warm, cold, stationary,
occluded), and their symbols
Understand why different air masses move over or
under one another and what general types of
clouds form depending on how the air masses meet
4-5 steps to mid-latitude cyclogenesis
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Key Concepts
Thunderstorms
3 stages of thunderstorm development (cumulus,
mature, dissipating)
Gust front - edge of downdraft below the cloud
near the surface
Updraft/downdraft - rapid vertical motion within
a thunderstorm
Conditions favoring an ordinary thunderstorm
Conditions favoring a severe thunderstorm
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Fronts
4 types of fronts
Cold front - cold, dense air moving in a into a
region of warm, lighter air
Warm front - warm, light air moving in a into a
region of cold, denser air
Stationary front - cold, dense air stationary
against a region of warm, lighter air
Occluded front - cold front meeting a warm front,
can result in either cold occlusion or warm
occlusion.
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Fronts
Cold Front
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Fronts
Warm Front
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Fronts
Stationary Front
17 Nov 06 UTC (11 PM LT)
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Fronts
Occluded Front
Cold air occlusion - Warm air from warm front
rides up over steep cold front air as cold front
catches up with the warm front
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Fronts
Occluded Front
Warm air occlusion - Cold air in front of warm
front rides under entire cold front air mass as
it catches the warm front.
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Mid-latitude Cyclogenesis
Steps to cyclogenesis
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Mid-latitude Cyclogenesis
Steps to cyclogenesis
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Thunderstorms
Three stages of development
1. Cumulus (growth) stage - Warm, humid air rises
and forms a single or cluster of cumulus clouds
2. Mature stage - Cloud grows above freezing
level, precipitation, most intense stage
3. Dissipation stage - Upward motion in cloud is
diminished, cloud dissipates
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Thunderstorms
Cumulus (growth) stage - Warm, humid air rises
and forms a single or cluster of cumulus clouds,
typically no rainfall yet
updraft
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Thunderstorms
Mature stage - most intense stage
Precipitation forms and falls, warming air in
storm
Gust front - leading edge of downdraft air near
the surface below cloud
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Thunderstorms
Dissipation stage
Updrafts are reduced in intensity
Downdrafts dominate the storm, dissipating it
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Thunderstorms
Conditions favoring storm development
Ordinary Thunderstorms - develop in weak to no
vertical shear environments
Severe Thunderstorms - develop in strong vertical
shear environments
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Thunderstorms
Ordinary Thunderstorm
Updraft and downdraft can mix
updraft
downdraft
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Thunderstorms
Severe Thunderstorm
Wind shear separates the downdraft flow from the
updraft flow prolonging the life of the updraft
downdraft
tilted updraft
Storm motion
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Thunderstorms
Severe Thunderstorm Characteristics
Large hail
Strong surface wind gusts
May produce a tornado
Longer duration than an ordinary storm
Multicellular or supercell structure
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Lightning
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Seven Cloud-to-ground flashes
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Three Upward Flashes from communications towers







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Electrification
You cannot get cloud-to-ground lightning until
there is a big voltage difference between the
cloud and ground
So, how does this occur??
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Noninductive Mechanism
For those that are curious
Focus on this one!
Lower (warmer) region
Upper (colder) region
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Inductive Charging Mechanism
Electric field must already exist for this
mechanism to work
Electric field
Net negative charge
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A cloud is charged to 100 million Volts, relative
to ground!
Lightning Flash
Lets start to dissect a cloud-to-ground
lightning flash
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1. Stepped leader starts at base of cloud,
invisible to human eye, increases electric
potential between tip of leader and ground
Lightning Flash
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2. As leader moves towards the ground, positive
charge builds up at surface
Lightning Flash
Objects that point up above the surface (tree,
tower, lightning rod) are more likely to provide
a good positive charge build-up
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3. Return stroke - positive upward charge meets
leader, positive charge heads towards cloud base,
very bright!
Lightning Flash
Generally one channel formed
Strokes are typically a few to 100s of
microseconds
3-4 strokes in one flash, we cannot see this,
looks like one bright flash, flash lasts on the
order of a second or less
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Lightning Flash
Streak Camera photo of a cloud-to-ground flash
(1 flash) with 12 return strokes
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Lightning Safety
A lightning strike can ruin your whole day
Dont stand under a tall pole or a single tree
(or in a small grove)
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Speed of Lightning
How Far Away is Lightning?
Speed-of-sound 1100 ft/sec (330 m/sec) gt one
mile in 5 seconds!! (1 km in 3 seconds)!!