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Storms
Chapter 13
Section 4
Essential Questions
Chap 13 Sec 4
1. What are the main kinds of storms, and
how do they form?
2. What measures can you take to ensure
safety in a storm?
Storms
Storms
 A storm is a violent disturbance in the
atmosphere
 It is involves sudden changes in air
pressure, which can cause rapid air
movements
 There are several types of storms
Types of Storms
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

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Thunderstorms
Tornadoes
Snow Storms
Hurricanes
Thunderstorms
Thunderstorms
 Thunderstorms are small storms with heavy
precipitation and frequent thunder &
lightning
 Many thunderstorms form in the spring and
summer in southern states and on the
Western Plains
 Thunderstorms form in large cumulonimbus
clouds (either on hot, humid afternoons or
when cold fronts pass through)
How thunderstorms form:
 Warm, humid air rises rapidly
 The air cools, forming dense thunderheads
(cumulonimbus clouds)
 Heavy rain and sometimes hail falls
 The clouds contain strong upward and
downward winds (called updrafts and
downdrafts)
 Thunderstorms contain thunder & lightning
Stages in the Development
of a Thunderstorm
Thunder & lightning
 Lightning is a sudden spark (an electrical
discharge) as positive and negative charges
jump between parts of a cloud, between nearby
clouds, or between a cloud and the ground
 Thunder is caused by the superfast heating of
the air by lightning
– Lightning heats the air around it to 30,000° C (much
hotter than the surface of the sun)
– The rapidly heated air expands suddenly and
explosively
– Thunder is the sound of the explosion
 Because light travels much faster than sound,
you see lightning before you hear thunder
Calculate the distance of lightning:
 Count the number of seconds between the
moment you see the lightning and when you
hear the thunder
 Divide that number by 5 to get the
approximate distance in miles
– Example: 15 seconds divided by 5 = 3 miles
 Do this inside!!!!!!!
Damage & Danger from
Thunderstorms:
 Heavy rains can cause flooding
 Lightning can strike and damage trees,
homes, electronics in our homes, animals,
and people
 Lightning can start fires
 When it strikes people and animals, it can
cause unconsciousness, serious burns, or
even heart failure
Floods
 Floods are caused by too much water pouring
into streams and rivers at once
 In cities floods can be caused by lack of ground
to absorb the water (covered with roads,
buildings, etc)
 If the ground is already soaked because of
previous rains it is more likely to flood
 Floods can destroy human structures and
wildlife habitats
 Floods can be good! They can provide rich new
soil for agriculture
Safety during Thunderstorms
 Go inside
 Avoid touching telephones, electrical appliances,
or plumbing fixtures (all conduct electricity)
 Cars are generally safe if they have a hard top
because the electricity will flow along the metal
surface of the car to the ground
 Avoid metal objects, tall structures, and bodies
of water such as:
– Trees, poles, high hills, fences, being in pools or lakes
(even in a boat)
 Get in a low area as close to the ground as
possible
Tornadoes
All About Tornadoes
 A frightening and destructive storm
 A rapidly whirling, funnel-shaped cloud that
reaches down from a storm cloud to touch Earth’s
surface
 A waterspout is a tornado over a lake or ocean
 Tornadoes usually touch the ground for 15 minutes
or less and are usually only a few hundred meters
across
 Winds in the tornado reach up to 500 kilometers
per hour (311 mph)
How Tornadoes Form
 Tornadoes usually develop in cumulonimbus
clouds just like thunderstorms, but can
develop in any situation that produces
severe weather
 They are most likely to occur in the spring
and early summer, often late in the
afternoon when the ground is warm (just like
thunderstorms)
Formation continued
 A special situation on the Great Plains creates
perfect weather for tornadoes to form
– A warm, humid air mass moves north from the Gulf of
Mexico into the lower Great Plains
– A cold, dry air mass moves south from Canada
– When the air masses meet, the cold air moves under
the warm air, forcing it to rise
– A squall line (narrow band of thunderstorms) is formed
and the storms travel from southwest to northeast
– A single squall line can produce ten or more tornadoes
Tornado Alley
 The US has more tornadoes per year than
any other country (about 800)
 Tornadoes occur in nearly every part of the
US
 Because of the warm air from the Gulf of
Mexico and the cold air from Canada,
there is Tornado Alley
– It includes Texas, Oklahoma, Kansas,
Nebraska, and Iowa (and a very small area of
South Dakota). See page 453 for a map.
Tornado Safety
 The safest place to be is in a storm shelter or the
basement of a well-built building
 Stay away from windows and doors and avoid
flying debris
 Lie on the floor under a sturdy piece of furniture
(such as a strong table)
 Outside – lie flat in a ditch
 Don’t take cover under an overpass or bridge
(the forces are actually strengthened here)
 There is usually little warning of a tornado – take
shelter immediately if you hear of a warning
20.3 Severe Storms
Tornadoes
 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
Snowstorms
Snowstorms
 All year, most precipitation begins in clouds as
snow!! If the air is colder than 0° C all the way to
the ground, the precipitation falls as snow.
 A snowstorm has high winds blowing the snow
and even picking up snow from the ground and
blowing it back into the air
 It is easy to become lost in a snowstorm because
of poor visibility
 Strong winds cool a person’s body rapidly
Snowstorm Safety
 Try to find shelter from the wind
 Cover exposed parts of your body and try to
stay dry
 If in a car, only run the engine if the exhaust
pipe is clear of snow
Hurricanes
Satellite View of Hurricane Floyd
Hurricane
 A tropical cyclone (remember from last
week) that has winds of 119 km/h
(kilometers per hour) or (74 mph)
 A typical hurricane is about 600 km (373
miles) across
 Hurricanes form in the Atlantic, Pacific, and
Indian oceans
– Called typhoons in the western Pacific Ocean
– Called cyclones in the Indian Ocean
How Hurricanes Form
 A hurricane begins over warm ocean water
as a low-pressure area (called a tropical
disturbance)
 If the tropical disturbance grows in size and
strength it becomes a tropical storm, then
can become a hurricane
 Hurricanes that strike the US usually form in
the Atlantic Ocean north of the equator near
Africa in August, September, or October
Formation Continued
 Warm, humid air at the ocean’s surface rises and forms
clouds
 The rising air draws more warm, moist air into the system
 Inside the storm are bands of very high winds and heavy
rain
 Winds spiral inward toward the area of lowest pressure at
the center
 The lower the air pressure at the center of a storm, the
faster the winds blow toward the center
 Hurricane winds may be as strong as 320 km/h (199 mph)
Structure of the Hurricane
 The hurricane has bands of very high winds and
heavy rains
 The narrow band around the center of the storm
has the strongest winds
 At the center of the storm is a ring of clouds called
the eyewall which encloses a quiet “eye”
– The winds get stronger as the eye approaches
– When the eye arrives, the weather suddenly changes to
calm air and possibly clear skies
– After the eye passes, the storm resumes, but the wind
blows from the opposite direction
Cross Section of a Hurricane
How Hurricanes Move
 Hurricanes last much longer than other storms
(usually a week or more)
 They travel a far distance over the ocean before
making “Landfall”
 Hurricanes formed in the Atlantic Ocean are
steered by trade winds toward the Caribbean
Islands and the southeastern US (that is us!)
 After a hurricane moves over land, it loses its
supply of warm, humid air and gradually loses
strenght
 Even after losing strength, it can carry heavy
rainfall for long distances over several days
Hurricane Damage
 Hurricanes bring high waves, severe flooding,
damaging winds, and even tornadoes as they
come ashore
 In the area of the hurricane while over the
ocean, low pressure and high winds can raise
water up to 6 meters (20 feet) above normal sea
level
– As the hurricane makes landfall, a huge dome of
water called a storm surge sweeps across the coast
where the hurricane lands
– Storm surge is the most deadly part of a hurricane
where it makes landfall
– Storm surges can cause great damage, washing
away beaches, destroying buildings along the coast,
and eroding coastlines
Hurricane Safety
 Prior to the 1950s people had little warning
about hurricanes because of a lack of
technology
 Today, we watch hurricanes move across
the ocean for days
 Cities will issue hurricane warnings when
hurricane conditions are expected with 24
hours
– Citizens are often told to evacuate which
means they should leave the area
immediately
20.3 Severe Storms
Hurricanes
 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
Essential Questions Answered
Chap 13 Sec 4
1. What are the main kinds of storms, and
how do they form?




Thunderstorms – form when warm, humid air
rises quickly
Tornadoes – form in cumulonimbus clouds
Hurricanes – form in low-pressure areas over
the ocean
Snowstorms – form when it is really cold,
windy, and snowing
Essential Questions
Chap 13 Sec 4
2. What measures can you take to ensure
safety in a storm?
Thunderstorm – stay inside, avoid water &
electricity
Tornadoes – storm cellar, basement, avoid cars,
windows, and doors
Hurricanes – evacuate
Snowstorms – find shelter, no exposed body parts
Section 5 – Predicting the
Weather
Essential Questions (Ch 13 S 5)
1. How do weather forecasters
predict the weather?
Collect Data
 Collect data about clouds, air pressure, and
other weather readings
 Analyze the data to predict what will happen
next
 Meteorologists are scientists that study the
causes of weather and try to predict it
– They use maps, charts, and computers to analyze
weather data and to prepare weather forecasts
– Data is collected by local weather observers,
instruments carried by balloons, satellites, and
weather stations around the world
– The National Weather Service owns much of the
equipment and provides data to the meteorologists
You can collect data too:
 Observe air pressure. Falling air pressure
usually indicates an approaching low-pressure
area, possibly bringing rain or snow
 Observe clouds. Thin cirrus clouds high in the
sky indicate a warm front may be approaching.
Cumulus clouds growing taller and taller on a
warm afternoon can become cumulonimbus
clouds and produce thunderstorms
 A red sky at sunrise might indicate an
approaching low-pressure area and stormy
weather
 A red sky at sunset often indicates storms are
moving away and clear weather is ahead
Weather Maps
 The National Weather Service assembles
weather maps using data from all over the
country
 Isobars are lines often found on weather maps.
These lines show areas having the same air
pressure. Iso means equal and bar means
pressure. Measurement may be in inches or
mercury, millibars, or both.
 Isotherms show areas have the same
temperature.
 Weather maps have symbols to show fronts,
areas of high and low pressure, types of
precipitation, and temperatures
 See the weather map on page 463
Limits of Weather Forecasts
 Computers, satellites, and radar have made
weather monitoring and predicting much
better
 However, we will probably never be able to
predict the weather a month in advance
because just a small change today can
create big changes several days later
The Butterfly Effect
 A scientist once suggested that even the
flapping of a butterfly’s wings causes a tiny
disturbance in the atmosphere. This tiny
event might cause a larger disturbance that
could grow into a large storm.
Essential Questions Answered
(Ch 13 S 5)
1. How do weather forecasters
predict the weather?
They make observations, collect
weather data, and interpret the
data to make their predictions
Review
1. What is the name of a scientist that studies
and predicts weather?
2. Which type of storm develops over the
ocean over warm, humid air?
3. Which type of storm usually develops
inside a cumulonimbus cloud over land
and has strong, swirling winds?
4. Which type of storm produces heavy rain,
thunder, and lightning?