Chapter 3: Weather Patterns
Section 2: Storms
Storms: a violent disturbance in the atmosphere; involve sudden changes in air pressure, causing
rapid air movement
*storms move due to the prevailing westerlies
Thunderstorms:
 How Thunderstorms form?
o Warm, humid air is forced upward at a cold front
o Warm humid air rises rapidly and cools
o Forms large cumulonimbus clouds or thunderheads
o Rain/hail/heavy wind (produce strong upward and downward winds- updrafts and
down drafts
 Lightening: a sudden spark, or energy discharge, caused when positive and negative charges
build up and move between parts of a cloud.
o Lightening can be as hot as 30,000 degrees
o Lightening moves between clouds and from clouds to the ground
 Thunder: sound of the explosion causes by rapidly heated air expanding
o You see lightening before you hear thunder because light travels faster than sound
 wind shear- sudden burst of air caused when a downward draft strikes the ground, and
spreads out in all directions
Facts:
 Thunderstorms occur all over the world
 Does not always happen at a cold front- can occur when large amounts of moist air rises
quickly
 Usually occur in spring and summer months, but can occur any time of year
 Thunderstorm Safety
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Avoid touching metal objects (they can conduct electricity into the body)
Stay out of open areas
Crouch with your head down and hands on your knees
Avoid touching telephones or electrical appliances
In a car with a hard top? Stay inside- but do not touch any metal in the car
Tornadoes:
 Tornado: Rapidly whiling, funnel-shaped cloud that reaches down from a storm cloud to
touch Earth’s surface
 Water Spout: Tornado occurring over a lake or ocean
 How do they occur?
o Cold, dry air mass meets warm, humid air mass
o Cold air moves under warm air, which rises quickly
o As warm air rushes past cold air, creates a vortex, with high winds
 When do they occur?
o When warm, humid air masses (maritime Tropical from the Gulf of Mexico) meet
cold, dry air masses (continental polar from Canada)
o Spring and summer
o ¾ of all tornadoes occur in the US between March and July
 Where do they occur?
o Tornado Alley- Great Plain States- runs from north-central Texas across central
Oklahoma, Kansas and Nebraska
o Occur more often in the U.S. than any other country
Facts:
 Brief but deadly- strong winds, flying debris
 Wind speeds may approach 480km/hr
 Can cause hail
 Tornadoes are measured
o Fujita Scale (F0-F5)- based on wind speed and damage
o Enhanced Fujita Scale (EFO-EF5)- more accurate as of 2007
 Tornado Wind speed
o F1 73mph
o F5 260+ mph
 Tornado Tunnel Speed
o Stationary
o Over 60 mph
 Tornado Safety
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Tornado Watch- tornadoes are possible in the area
Tornado Warning- a tornado has been seen in the sky
Move to a safe area- basement of a well built building
Stay away from windows and doors
Lie on the floor under a sturdy piece of furniture
Outside? Move to a ditch or building
Hurricanes:
 Hurricane: tropical storm with winds 119km per hour or higher, approximately 600km
across
 Hurricanes in the Western Pacific Ocean are called typhoons
 Destructive
 Bring large amounts of rain
How Hurricanes Form:
o hurricanes begin over warm water as a low-pressure area, or tropical disturbance
o tropical disturbance
tropical storm
hurricane
o last a week or more
o gets its energy from the warm, humid air at the ocean’s surface
o wind spirals inward into the areas of low pressure
o bands of heavy wind and heavy rain
o lowest air pressure and warmest temperatures are at the center of the hurricane
o the lower the air pressure at the center, the faster the winds blow
o winds can be up to 320km per hour
Eye of a Hurricane:
o center of hurricane is made of a ring of clouds surrounding the “eye”
o the wind gets stronger as the eye approaches
o when in the eye weather changes suddenly
o weather in the eye is calm and skies are clear
o after the eye passes the storm continues BUT the wind is blowing in the opposite
direction
How Hurricanes Move
o over a week or more
o passing over land draws energy from a hurricane and causes it to weaken
 Hurricane Damage
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High waves
Severe flooding
Wind damage
Storm surge: a dome of water that sweeps across the coast where a
hurricane lands
Destruction of buildings, coastlines
 Hurricane Safety
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Be aware of warnings/evacuations
Hurricane watch: hurricane conditions are possible in your area within the
next 36 hours- be prepared to evacuate
Hurricane warning: hurricane conditions are expected in your area within
the next 24 hours- evacuate immediately if told
Winter Storms:
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Snow falls when humid air cools below freezing
Can block roads, trap people in their homes, make it hard for emergency vehicles to move
Extreme cold can damage crops and cause water pipes to freeze and burst
High winds
 Dangerous temperatures
 Blowing snow, low visibility
Lake Effect Snow:
o In the fall and winter the land near the Great Lakes cools faster than the water in the
lakes
o Cold, dry air masses pick up water vapor and heat from the warmer lakes
o When the air mass reaches the other side of the lake, the air rises and cools againforming clouds and causing snow around the lakes to fall
 Snowstorm Safety
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Find shelter from the wind
Cover exposed parts of the body
Stay dry
In a car? Keep the engine running and clear snow away from exhaust