Transcript 20.3 Severe Storms

Starter
1. In which atmospheric layer of Earth do thunderstorms occur?
a.
b.
c.
d.
Thermosphere
Mesosphere
Stratosphere
Troposphere
2. Which gas makes up the largest component of Earth’s atmosphere?
a.
b.
c.
d.
Argon
Oxygen
Nitrogen
Carbon Dioxide
3. Which is a major source of air pollution created by humans?
a.
b.
c.
d.
Hydroelectric plants
Industrial factories
Reforestation
Nuclear power
4. Which statement describes the general movement of air masses?
a.
b.
c.
d.
They
They
They
They
move
move
move
move
from
from
from
from
areas
areas
areas
areas
of
of
of
of
high pressure to areas of low pressure.
high altitude to areas of low altitude.
low humidity to areas of high humidity.
low temperature to areas of high temperature.
Objectives
 Explain the formation of a
thunderstorm
 Describe the conditions needed for a
tornado to form
 Identify the conditions that must exist
for a hurricane to form
20.3 Severe Storms
 A thunderstorm is a storm that generates
lightning and thunder. Thunderstorms
frequently produce gusty winds, heavy rain,
and hail.
20.3 Severe Storms
 Occurrence of Thunderstorms
• At any given time, there are an estimated 2000
thunderstorms in progress on Earth. The
greatest number occur in the tropics where
warmth, plentiful moisture, and instability are
common atmospheric conditions.
 Development of Thunderstorms
• Thunderstorms form when warm, humid air rises
in an unstable environment.
Stages in the Development
of a Thunderstorm
During the cumulus stage, warm moist air is supplied to the
cloud. Heavy precipitation falls during the mature stage. The
cloud begins to evaporate during the dissipating stage.
20.3 Severe Storms
 Tornadoes are violent windstorms that take
the form of a rotating column of air called a
vortex. The vortex extends downward from
a cumulonimbus cloud.
 Occurrence and Development of Tornadoes
• Most tornadoes form in association with severe
thunderstorms.
• A mesocyclone is a vertical cylinder of rotating air
that develops in the updraft of a thunderstorm.
Formation of a Mesocyclone
A mesocyclone can occur before the formation of a tornado.
First, stronger winds aloft cause lower winds to roll. Updrafts
tilt the rolling air so that it becomes nearly vertical. When the
rotating air is completely vertical, the mesocyclone is made.
20.3 Severe Storms
 Tornado Intensity
• Because tornado winds cannot be measured
directly, a rating on the Fujita scale is determined
by assessing the worst damage produced by the
storm.
 Tornado Safety
• Tornado watches alert people to the possibility of
tornadoes in a specified area for a particular time.
• A tornado warning is issued when a tornado has
actually been sighted in an area or is indicated by
weather radar.
Fujita Tornado Intensity Scale
<72
72-112
113-157
158-206
207-260
>260
Fujita Scale of Tornado Intensity
SCALE WIND SPEED
POSSIBLE DAMAGE
Enhanced,
Operational
Fujita Scale
F0
40-72 mph
Light damage: Branches broken off trees; minor roof
damage
EFO
65-85 mph
F1
73-112 mph
Moderate damage: Trees snapped; mobile home
pushed off foundations; roofs damaged
EF1
86-110 mph
F2
113-157 mph
Considerable damage: Mobile homes demolished; trees EF2
uprooted; strong built homes unroofed
111-135 mph
F3
158-206 mph
Severe damage: Trains overturned; cars lifted off the
ground; strong built homes have outside walls blown
away
EF3
136-165 mph
F4
207-260 mph
Devastating damage: Houses leveled leaving piles of
debris; cars thrown 300 yards or more in the air
EF4
166-200 mph
F5
261-318 mph
Incredible damage: Strongly built homes completely
blown away; automobile-sized missiles generated
EF5
over 200 mph
20.3 Severe Storms
 Whirling tropical cyclones that produce
winds of at least 119 kilometers per hour
(74 mph) are known in the United States as
hurricanes.
 Occurrence of Hurricanes
• Most hurricanes form between about 5 and 20
degrees north and south latitude. The North
Pacific has the greatest number of storms,
averaging 20 per year.
Satellite View of Hurricane Floyd
This satellite image of Hurricane Floyd shows its position off
the coast of Florida a few days before the hurricane moved
onto land. Floyd eventually made landfall near the Cape Fear
River mouth in North Carolina (Southport, NC).
20.3 Severe Storms
 Development of Hurricanes
• Hurricanes develop most often in the late
summer when water temperatures are warm
enough to provide the necessary heat and
moisture to the air.
• The eye is a zone of scattered clouds and calm
averaging about 20 kilometers(13 mi.) in
diameter at the center of a hurricane.
• The eye wall is a doughnut-shaped area of
intense cumulonimbus development and very
strong winds that surrounds the eye of a
hurricane.
20.3 Severe Storms
 Hurricane Intensity
• The intensity of a hurricane is described using
the Saffir-Simpson scale.
• A storm surge is the abnormal rise of the sea
along a shore as a result of strong winds.
Saffir-Simpson Hurricane Scale
74-95
96-110
111-130
131-155
>155
Concept Map (page 57)
Two air
masses
meet to form
front
can be
warm
cold
stationary
occluded