Chapter 5 Earth’s Weather
Lesson 4: Severe Weather - Notes
Main Idea:
Storms are caused by the collision of air masses that have different temperatures and
humidities.
Vocabulary:
Air mass (265) – a large region of air that has similar temperature and humidity
Front (265) – meeting place between air masses
Thunderstorm (266) – a rainstorm that includes lightning and thunder
Tornado (268) – a rotating funnel shaped cloud that contains winds that blow up to 299 miles
(480km) per hour
Low pressure closure (268) – an area of low pressure air is surrounded by an air of high
pressure air
Hurricane (270) – a tropical storm with winds of 117 km (73 miles) per hour or greater
Storm surge (271) – bulge of water in the ocean caused by waves whipped up by a hurricane
Cyclone (271) – any storm with a low pressure closure that cuases a circular pattern of wind to
form
Monsoon (272) - seasonal winds that bring heavy rains
What causes severe weather?
Main Idea:
When air masses with different temperature and humidity meet, they cause changes in air
pressure than can lead to severe weather.
Supporting Details
1. Weather is affected by the air mass passing though an area
2. Air mass
a. is a large region of air with similar temperature and humidity
b. can cover thousands of square kilometer / miles
c. is continuously moving
d. Types of air masses
Cold dry air mass
Northern air mass formed over land
Cold moist air mass
Northern air mass formed over water
Warm dry air mass
Southern air mass formed over land
Warm moist air mass
Southern air mass formed over water
3. Changes in weather occur when two different air masses meet.
4. A front is where the two air masses meet – the boundary marks the front edge of the
oncoming air mass.
a. When a cold and dry air mass runs into warm moist air mass,
i. the cold air (which is more dense) moves under and pushes the warm
moist air up
ii. the warm moist air condenses, towering cumulus clouds form and storms
may follow
b. When a warm moist air mass runs into a cold dry air mass
i. The warm moist air slides over the cold dry air
ii. Light cirrus clouds appear in the sky
What causes thunderstorms?
Main Idea:
A thunderstorm is caused by rapid rise of warm moist air. The storms occur most commonly in
the latter part of the afternoon and quickly end when the land is cooled.
Supporting Details
1. A thunderstorm is a rainstorm that includes lightning and thunder
a. Fronts
i. A cold front moves in pushing warm humid air upward.
ii. As the warm humid air rises, it expands and cools
b. Thunderheads
i. As the warm humid air cools, some of the water vapor condenses.
ii. This releases energy and warms the air around it.
iii. The warm air rises further forming a thunderhead.
iv. The clouds flattens out when the winds at the higher altitudes blow the
condensation away
c. Precipitation
i. The water droplets combine and grow
ii. When the droplets become too big for the wind to keep them in the
clouds, the droplets fall to the Earth’s surface.
2. Lightning
a. Is the spark caused when the electricity in a thunderhead discharges
b. A theory about the cause of lightning
i. Moving air causes particles of rain and ice moving upward to rub against
particles of rain and ice moving downward
ii. The particles moving down lose and electron while the particles moving
up gain an electron
iii. As this continues to occur, an electrical charge, similar to static electricity,
builds up and is discharged as lightning
3. Thunder
a. Lightning warms up the air around it to more than 5 times the temperature of
the surface of the sun.
b. The burst of heat makes the air expand violently.
c. Thunder is the sound of rapidly expanding air.
4. Thunderstorms most likely occur during the latter part of the afternoon when the land is
the warmest.
5. Thunderstorms end quickly after the rain has cooled down the land below the storm.
What are tornadoes?
Main Idea:
Tornados can occur when cold dry air collides with warm moist air.
Supporting Details
1. A tornado is a rotating funnel shaped cloud that contains winds that blow up to 480
km/h (299 mph)
2. Under the right conditions, a thunderstorm can turn into a tornado.
a. Warm air moves upward in a thunderhead creating an area of low pressure
i. The low air pressure draws air inward and upward
ii. The low pressure area is quickly surround by high pressure air creating a
low pressure closure
b. As air flows into the low pressure closure, it rotates around in a circle faster and
faster
c. From the ground, the shape of the cloud looks like a large funnel
i. Warm air rises up the center of the cloud
ii. Rain falls outside the cloud
d. When the tip of the funnel touches the ground, it becomes a tornado
i. Only a small section of a tornado actually touches the ground.
3. Tornado alley
a. Tornadoes can occur all over the US, but they generally occur in an area called
tornado alley where cold dry air from Canada meets warm moist air from the
Gulf Of Mexico
b. Oklahoma. Kansas, Nebraska, Iowa, and parts of Colorado and Texas are in
tornado alley
What are hurricanes?
Main Idea:
Hurricanes begin as tropical storms that form over the equator. When wind speeds reach 73
mph, the storm is classified as a hurricane.
Supporting Details
1. A tropical storm
a. forms over the ocean near the equator
b. has rotating winds with low pressure at its center
c. wind speed is between 35 – 72 miles per hour
2. Formation of a tropical storm
a. Water evaporates into warm air that rises
i. As cool air flows towards the space where the warm air used to be, a low
pressure closure forms
ii. The low pressure closure causes more water to evaporate which lowers
the air pressure even more
b. Movement of air from the surrounding circle of high pressure to the low
pressure center causes winds that blow towards the center
3. A hurricane
a. A tropical storms is reclassified as a hurricane when its wind speed reaches 117
km / hr (73 miles / hr)
b. Spinning of a hurricane is related to the Earth’s rotation
i. Winds that blow from the North Pole to the equator curve right
ii. When they reach the equator, they push the low pressure centers in
hurricanes to the right in the Northern Hemisphere. (Coriolis effect)
c. Appearance of a hurricane
i. A spiral of clouds with a hole in its middle called the eye
ii. The hole is the low pressure area
iii. A hurricane’s fastest winds and heaviest rain occur near the eye
iv. There is no wind or precipitation inside the eye
d. Hurricanes whip up large waves.
i. The waves create a bulge of water called a storm surge
ii. As the storm moves over the coast, the storm surge can cause water
levels to suddenly rise
e. Any storm with a low pressure closure that causes a circular pattern of wind to
form is a cyclone. Hurricanes and tornadoes are types of cyclones.
What are other forms of severe weather?
Main Idea:
Other types of severe weather include monsoons and fog.
Supporting Details
1. Monsoon
a. A seasonal wind that brings heavy rain
b. Occur in southeast Asia and sometimes in southwest US
2. Dense Fog
a. Ground fog
i. Forms when cool clear nights occur after a sunny day
ii. A low layer of warm moist air begins to cool after sunset
iii. The water vapor in the air condenses
b. Advection fog
i. warm humid air flows over cold ground or water
ii. the warm air meets the cold air over the ocean, the air cools
iii. the water vapor condense
iv. the air is blown toward the land during the day