Transcript The Nature of Storms

Chapter 13
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How thunderstorms form:
 1. Moisture: There must be an abundant source
of moisture in the lower levels of the atmosphere.
 2. Lifting: Some mechanism must lift the air so
that the moisture can condense and release latent
heat.
 3. Instability: The portion of the atmosphere
through which the cloud grows must be unstable.
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Thunderstorms are often classified according
to the mechanism that caused the air to rise:
 If the air rose because of unequal heating of
Earth’s surface within one air mass, the
thunderstorm is called an air-mass thunderstorm.
▪ 1. mountain thunderstorms occur when an air mass
rises as a result of orographic lifting.
▪ 2. sea-breeze thunderstorms are caused by extreme
temperature differences between the air over land and
the air over water.
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The second main classification of
thunderstorms is frontal thunderstorms,
which are produced by advancing cold fronts
and warm fronts.
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A thunderstorm usually has 3 stages:
 1. cumulus stage – warm air starts to rise nearly
vertically upward (updrafts), condensing and
creating a cumulus cloud. The cloud will continue
to grow as long as warm air below it continues to
rise.
2. mature stage - Raindrops start to fall through
the cloud when the rising air can no longer hold them
up. Cool air sinks rapidly to the ground along with the
precipitation creating
downdrafts. Updrafts and
downdrafts form a convection
cell that produces gusty surface
winds.
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3. dissipation stage – after 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.
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A line of thunderstorms can last for hours or
even days as they continually regenerate
themselves with new, warm air that is
introduced into the updrafts.
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Severe thunderstorms can produce some of the
most violent weather conditions on Earth.
 They can develop into supercells, which are
characterized by intense, rotating updrafts.
 Supercells often produce:
▪ damaging wind
▪ large hail
▪ tornadoes
• These can last for
several hours and have
updrafts as strong as 240
km/hr.
• Only about 10% of the
~100,000 thunderstorms
per year in the U.S. are
considered severe.
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Lightning is electricity caused by the rapid
rush of air in a cumulonimbus cloud.
 1. Separation of the positive and negative charges
of the cloud into two levels:
▪ the + charge at the top
▪ the - charge at the bottom
 2. A flow of negative charge rushes toward the
Earth. This is known as a stepped leader.
▪ The + charges of the Earth are attracted to this stepped
leader, so a flow of + charge moves into the air.
 3. When the stepped leader and the + charge
from the earth meet, a strong electric current
carries + charge up into the cloud.
▪ This illuminated electric current is known as the return
stroke
▪  LIGHTNING
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Return stroke channel is 100 million V of
electricity!
A lightning bolt heats the surrounding air to
~ 30,000°C
Thunder is produced as this superheated air
rapidly expands & contracts.
Lightning accounts for ~7,500 forest fires
each year the U.S.
Lightning strikes in the U.S. cause an average
of 300 injuries & 93 deaths per year.
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Violent downdrafts that are concentrated in a
local area are called downbursts.
 Wind speeds exceeding 160 km/hr
▪ Macrobursts = cause of path of destruction up to 5km
wide, have wind speeds of more than 200 km/hr and can
last up to 30 minutes.
▪ Microbursts = affect areas of less than 3km but can have
winds exceeding 250 km/hr & last less than 10 minutes.
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Each year $1 billion in damage is caused by hail.
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Hail is caused when raindrops are lifted up into
the atmosphere by updrafts during a
thunderstorm and then supercooled by
temperatures below freezing, turning them into
ice balls.
Hail
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If winds are weak, weather systems & storms
move slowly causing them do dump their rain
over a limited location.
 If the rain falls faster than the ground can absorb
it, or faster than the streams & rivers can
transport it out of the area, flooding can occur.
▪ *Fact: floods are the main cause of thunderstormrelated deaths each year.
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A tornado is a violent, whirling column of air
in contact with the ground.
 *before it reaches the ground it’s called a funnel
cloud.
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1. Winds of different speeds from different directions
(wind shear) creates a horizontal rotation in the lower
atmosphere parallel to the ground.
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2. Strong updrafts tilt the rotating the column upward
into the thunderstorm creating a funnel cloud.
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3. A tornado forms within the rotating winds.
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Tornadoes are classified according to the
Fujita tornado intensity scale: ranks
tornadoes according to their
 (1) path of destruction
 (2) wind speed
 (3) duration
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Most tornadoes form in the spring during the
late afternoon & evening, when temperature
contrasts between polar air & tropical air is the
greatest.
 Occur most frequently in the Central U.S., where
continental polar air collides with maritime tropical air
moving north from the Gulf of Mexico.
▪ These occur in a region known as “Tornado Alley” – extends
from northern Texas through Oklahoma, Kansas & Missouri
▪ *Fact: more than 700 tornadoes touch down each year in the
U.S.
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In the U.S. an average of 80 deaths and 1,500
injuries result from tornadoes annually.
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The National Weather Service issues tornado
warnings & watches before a tornado actually
strikes.
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During a severe thunderstorm, the presence of dark,
greenish skies, a towering wall of clouds, large
hailstones, and a loud, roaring noise are signs of an
approaching or developing tornado.
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Large, rotating, low-pressure storms called
tropical cyclones form during summer and
fall in the topics.
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Warm air at tropics rises and condenses into
clouds and rain.
Rising air creates an area of L pressure @
ocean surface.
The Coriolis effect causes the moving air to
turn counterclockwise in the N. hemisphere.
  this produces the cyclonic rotation of a tropical
cyclone.
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Tropical cyclones require 2 conditions to
form:
 1. abundant supply of warm ocean water
 2. some sort of disturbance to lift warm air & keep
it rising.
▪ They occur in all tropical oceans except the S. Atlantic &
the Pacific Ocean west of the S. American Coast.
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Western Pacific Ocean = “Typhoons”
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Indian Ocean = “Cyclones”
Atlantic Ocean, Caribbean Sea, Gulf of Mexico,
& west coast of Mexico = “Hurricanes”
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 They occur most frequently in late summer and early fall,
when the Earth’s oceans contain their greatest amount of
stored heat energy.
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Tropical cyclones move according to the wind
currents that steer them.
 In the tropics, tropical cyclones are caught up in
the circulation of high-pressure systems.
▪ They move to the west then turn poleward when they
reach the edges of the H pressure system.
▪ Then they are guided by the prevailing Westerlies.
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1. Tropical disturbance  area of thunderstorms continues
for 24 hours or more.
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2. Tropical Depression  when the winds increase to
(23 - 39 mph), organized circulation of wind in the center of
the thunderstorms is detected.
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3. Tropical Storm  forms when the maximum sustained
winds have intensified to between (39-73 mph). It becomes
better organized and begins to look like a hurricane with a
circular shape.
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4. Hurricane  if air pressure continues to fall & winds
reach sustained winds of 74 mph.
Katrina
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Development of calm center = eye
Strongest winds concentrated in a band
immediately surrounding the eye = eyewall
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Saffir-Simpson hurricane scale classifies
hurricanes according to:
 1.wind speed
 2. air pressure in the center
 3. potential for property damage.
▪ Ranges from Category 1  Category 5
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A hurricane will last until it can no longer
produce enough energy to sustain itself.
 When the storm moves over land and no longer
has access to the warm ocean surface.
 Or the storm moves over colder waters.
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Strong winds moving onshore in coastal
areas are partly responsible for another major
hurricane threat: storm surges.
 Occurs when hurricane force-winds drive a mound
of ocean water toward coastal areas, where it
washes over the land.
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The National Hurricane Center – issues
regular advisories that indicate a storm’s
position, strength and movement.
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Droughts are extended periods of well-below
normal rainfall.
 The result of shifts in global wind patterns that
allow H pressure systems to persist for weeks or
months over continental areas.
▪ Sinking air will resist any lifting moisture and
condensation cannot occur.
Folsom Lake, Northern CA – Drought
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Heat waves are extended periods of abovenormal temperatures.
 Formed by the same H pressure that cause
droughts.
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Air sinks and warms by compression
Blocks cooler air masses from moving into the area
Difficult for condensation to occur = little cloud cover
Jet stream is farther north and weak so the system does
not move very much
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Cold wave is an extended period of belownormal temperatures.
 Caused by systems of continental polar or arctic
origin.
▪ During winter, little sunlight available & snow
covered surface radiates heat back to space.
▪  development of large pools of extremely cold
air
▪ Cold air sinks, pressure increases creating H pressure
system.
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Wind chill factor is a phenomenon measured by the
wind-chill index, which estimates the heat loss from
human skin caused by a combination of wind and cold
air.
Study, Study, Study!!!
Test: Wednesday,
April 8th