Objectives• Identify the processes that form thunderstorms.

Thunderstorms

• Compare and contrast different types of thunderstorms.

• Describe the life cycle of a thunderstorm.

– air-mass thunderstorm

– sea-breeze thunderstorm

– frontal thunderstorm

Vocabulary

• Some are capable of producing hail the size of baseballs, swirling tornadoes, and surface winds of more than 160 km/h.

• All thunderstorms, regardless of intensity, have certain characteristics in common.

Thunderstorms• At any given moment, nearly 2000 thunderstorms

are occurring around the world.

Thunderstorms

How Thunderstorms Form• For a thunderstorm to form, three conditions

must exist.

Thunderstorms

1. There must be an abundant source of moisture in the lower levels of the atmosphere.

2. Some mechanism must lift the air so that the moisture can condense and release latent heat.

3. The portion of the atmosphere through which the cloud grows must be unstable.

How Thunderstorms Form

Limits to Growth

Thunderstorms

– The air in a thunderstorm will keep rising until:

1. It meets a layer of stable air that it cannot overcome

2. The rate of condensation, which diminishes with height, is insufficient to generate enough latent heat to keep the cloud warmer than the surrounding air

– Typical thunderstorms last only about 30 minutes and individual storms are only about 24 km in diameter.

How Thunderstorms Form

Thunderstorms

Air-Mass Thunderstorms• Thunderstorms are often classified according to

the mechanism that caused the air to rise.

Thunderstorms

• An air-mass thunderstorm is a thunderstorm that results from the air rising because of unequal heating of Earth’s surface within one air mass.

– Mountain thunderstorms occur when an air mass rises as a result of orographic lifting, which involves air moving up the side of a mountain.

– Sea-breeze thunderstorms are local air-mass thunderstorms caused, in part, by extreme temperature differences between the air over land and the air over water.

Air-Mass Thunderstorms

Thunderstorms

Frontal Thunderstorms• Frontal thunderstorms are thunderstorms that

are produced by advancing cold fronts and, more rarely, warm fronts.

Thunderstorms

• Cold-front thunderstorms get their initial lift from the push of the cold air which can produce a line of thunderstorms along the leading edge of the cold front.

• Because they are not dependent on daytime heating for their initial lift, cold-front thunderstorms can persist long into the night.

Stages of Development• A thunderstorm usually has three stages: the

cumulus stage, the mature stage, and the dissipation stage.

Thunderstorms

• The stages are classified according to the direction in which the air is moving.

Stages of Development

Thunderstorms

Stages of Development

Cumulus Stage

Thunderstorms

– In the cumulus stage, air starts to rise nearly vertically upward.

– Transported moisture condenses into a visible cloud and releases latent heat.

– As the cloud droplets coalesce, they form larger droplets, which eventually fall to Earth as precipitation.

Stages of Development

Mature Stage

Thunderstorms

– As precipitation falls, it cools the air around it which becomes more dense than the surrounding air, so it sinks creating downdrafts.

– The updrafts and downdrafts form a convection cell.

– In the mature stage, nearly equal amounts of updrafts and downdrafts exist side by side in the cumulonimbus cloud.

Stages of Development

Dissipation Stage

Thunderstorms

– The supply of warm, moist air runs out because the cool downdrafts cool the area from which the storm draws energy.

– Without the warm air, the updrafts cease and precipitation can no longer form.

– The dissipation stage is characterized primarily by lingering downdrafts.

Section Assessment

1. Why does there need to be an abundant source of moisture in the lower levels of the atmosphere for thunderstorms to form?

Thunderstorms

The moisture feeds into a thunderstorm’s updrafts, releasing latent heat when it condenses.

Section Assessment

2. What is the main cause of thunderstorm dissipation?

Thunderstorms

The downdrafts created by a thunderstorm eventually cut off the flow of warm, moist air into the storm. Without the warm updrafts, precipitation can no longer form and the convection stops.

______ Latent heat is crucial in maintaining the upward motion of a cloud.

______ Thunderstorms are more likely to develop along a warm front instead of a cold front.

______ A mountain thunderstorm is an example of an air-mass thunderstorm.

______ In the mature stage of a thunderstorm, updrafts are roughly equal to downdrafts.

Section Assessment

3. Identify whether the following statements are true or false.

Thunderstorms

true

false

true

true

Objectives• Explain why some thunderstorms are more severe

than others.

• Recognize the dangers of severe thunderstorms, including lightning, hail, high winds, and floods.

• Describe how tornadoes form.

– supercell

– downburst

– tornado

– Fujita tornado intensity scale

Vocabulary

Severe Weather

Severe Weather• Occasionally, weather events come together in

such a way that there is a continuous supply of surface moisture.

Severe Weather

• This happens along a cold front that moves into warmer territory and can lift and condense a continuous supply of warm air.

Severe Thunderstorms• Other factors also play a role in causing some

storms to be more severe than others.

Severe Weather

• Cold fronts are usually accompanied by upper-level, low-pressure systems that are marked by pools of cold air, which cause the air to become more unstable.

• When the strength of the storm’s updrafts and downdrafts intensifies, the storm is considered to be severe.

Severe Thunderstorms• Supercells are self-sustaining, extremely

powerful severe thunderstorms, which are characterized by intense, rotating updrafts.

Severe Weather

• Only about ten percent of the roughly 100 000 thunderstorms that occur each year in the United States are considered to be severe; even fewer become supercells.

Lightning• Lightning is an electrical discharge caused by the

friction of falling and rising ice crystals within strong drafts of a cumulonimbus cloud.

Severe Weather

• Some atoms lose electrons and become positively charged ions, while other atoms receive the extra electrons and become negatively charged ions.

• This creates regions of air with opposite charges.

• To relieve the electrical imbalance, an invisible channel of negatively charged air, called a stepped leader, moves from the cloud toward the ground.

Lightning• When the stepped leader

nears the ground, a channel of positively charged ions, called the return stroke, rushes upward to meet it.

Severe Weather

• The return stroke surges from the ground to the cloud, illuminating the channel with about 100 million V of electricity.

Lightning

Severe Weather

Lightning

The Power of Lightning

Severe Weather

– A lightning bolt heats the surrounding air to about 30 000°C.

– Thunder is the sound produced as this superheated air rapidly expands and contracts.

– Each year in the United States, lightning accounts for about 7500 forest fires, which result in the loss of millions of acres of forest.

– Lightning strikes in the United States cause a yearly average of 300 injuries and 93 deaths to humans.

Lightning

Severe Weather

The Fury of the Wind• Instead of dispersing over a large area underneath

a storm, downdrafts sometimes become concentrated in a local area.

Severe Weather

• Downbursts are violent downdrafts that are concentrated in a local area and can contain wind speeds of more than 160 km/h.

– Macrobursts can have wind speeds of more than 200 km/h, can last up to 30 minutes, and cause a path of destruction up to 5 km wide.

– Microbursts affect areas of less than 3 km wide but can

have winds exceeding 250 km/h.

Hail• Hail is precipitation in the form of balls or lumps of

ice that can do tremendous damage.

Severe Weather

• Hail forms because of two characteristics common to thunderstorms.

– Water droplets exist in the liquid state in the parts of a cumulonimbus cloud where the temperature is actually below freezing.

– The abundance of strong updrafts and downdrafts existing side by side within a cloud.

Hail• The supercooled water droplets in the cloud freeze

on contact with other ice pellets and are caught alternately in the updrafts and downdrafts.

Severe Weather

• The ice pellets are constantly encountering more supercooled water droplets and growing.

• Eventually they become too heavy for the updrafts to keep aloft and fall to Earth as hail.

Floods• When there are weak wind currents in the upper

atmosphere, weather systems and resulting storms move slowly.

Severe Weather

• Flooding can occur when a storm dumps its rain over a limited location.

• If there is abundant moisture throughout the atmosphere, the processes of condensation, coalescence, and precipitation are much more efficient and thus produce more rainfall.

• Floods are the main cause of thunderstorm-related deaths in the United States each year.

Tornadoes• A tornado is a violent, whirling column of air in

contact with the ground.

Severe Weather

• Before a tornado reaches the ground, it is called a funnel cloud.

• Tornadoes are often associated with supercells.

• The air in a tornado is made visible by dust and debris drawn into the swirling column, or by the condensation of water vapor into a visible cloud.

Tornadoes• A tornado forms when wind speed and direction

change suddenly with height, a phenomenon known as wind shear.

Severe Weather

– Under the right conditions, this can produce a horizontal rotation near Earth’s surface.

– A thunderstorm’s updrafts can tilt the twisting column of wind from a horizontal to a vertical position.

– Air pressure in the center drops as the rotation accelerates.

– The extreme pressure gradient between the center and the outer portion of the tornado produces the violent winds associated with tornadoes.

Tornadoes

Severe Weather

Tornadoes

Tornado Classification

Severe Weather

– The Fujita tornado intensity scale classifies tornadoes according to their path of destruction, wind speed, and duration.

– The scale ranges from F0, which is characterized by winds of up to 118 km/h, to the violent F5, which can pack winds of more than 500 km/h.

– Most tornadoes do not exceed the F1 category.

– Only about one percent ever reach the violent categories of F4 and F5.

Tornadoes

Tornado Distribution

Severe Weather

– While tornadoes can occur at any time or place, some places are more conducive to their formation.

– Most tornadoes form in the spring during the late afternoon and evening, when the temperature contrasts between polar air and tropical air are the greatest.

– Tornadoes occur most frequently in a region called “Tornado Alley,” which extends from northern Texas through Oklahoma, Kansas, and Missouri.

Tornadoes

Tornado Safety

Severe Weather

– In the United States, an average of 80 deaths and 1500 injuries result from tornadoes each year.

– The National Weather Service issues tornado watches and warnings before a tornado actually strikes.

– The agency stresses that despite advanced tracking systems, advance warnings may not be possible.

– Signs of an approaching or developing tornado include the presence of dark, greenish skies, a towering wall of clouds, large hailstones, and a loud, roaring noise similar to that of a freight train.

Tornadoes

Severe Weather

Section Assessment

1. Match the following terms with their definitions.

___ supercell

___ macroburst

___ microburst

___ tornado

Severe Weather

A. a violent, whirling column of air in contact with the ground

B. self-sustaining, extremely powerful thunderstorms that are characterized by intense, rotating updrafts

C. downburst causing a path of destruction up to 5 km wide

D. downburst causing a path of destruction up to 3 km wide

B

C

D

A

Section Assessment

Severe Weather

2. Does cloud-to-ground describe lightning? Why or why not?

Lightning is the illumination that you see when the return stroke surges from the ground to the cloud, lighting the channel of the stepped leader. It would be better to say ground-to-cloud.

Section Assessment

3. Why do so many tornadoes form in “Tornado Alley”?

Severe Weather

Large temperature contrasts occur most frequently in the Central United States, where cold continental polar air collides with maritime tropical air moving northward from the Gulf of Mexico.

– tropical cyclone

– eye

– eyewall

– Saffir-Simpson hurricane scale

– storm surge

Objectives• Identify where tropical cyclones originate.

• Describe the life cycle of a tropical cyclone.

• Recognize the dangers of hurricanes.

Vocabulary

Tropical Storms

Tropical Storms• Tropical cyclones are large, rotating, low-

pressure storms that form over water during summer and fall in the tropics.

Tropical Storms

• The strongest of these cyclonic storms are known in the United States and other parts of the Atlantic Ocean as hurricanes.

Tropical Cyclones• Tropical cyclones thrive on the tremendous

amount of energy in warm, tropical oceans.

Tropical Storms

• This latent heat from water that has evaporated from the ocean is released when the air begins to rise and water vapor condenses.

• Rising air creates an area of low pressure at the ocean surface.

• The cyclonic rotation of a tropical cyclone begins as warm air moves toward the low-pressure center to replace the air that has risen.

Tropical Cyclones• As the moving air approaches the center of the

growing storm, it rises, rotating faster and faster as more energy is released through condensation.

Tropical Storms

• Air pressure in the center of the system continues to decrease, while surface wind speeds increase—sometimes in excess of 240 km/h.

• As long as atmospheric conditions allow warm air to be fed into the system at the surface and to be removed from the system in the upper atmosphere the process will continue.

Tropical Cyclones

Formation of Tropical Cyclones

Tropical Storms

– Tropical cyclones require two basic conditions to form:• An abundant supply of very warm ocean water

• Some sort of disturbance to lift warm air and keep it rising

– These conditions exist in all tropical oceans except the South Atlantic Ocean and the Pacific Ocean west of the South American Coast.

– They occur most frequently in the late summer and early fall, when Earth’s oceans contain their greatest amount of stored heat energy.

Tropical Cyclones

Tropical Storms

Tropical Cyclones

Movement of Tropical Cyclones

Tropical Storms

– Tropical cyclones move according to the wind currents that steer them.

– In the deep tropics, tropical cyclones are often caught up in subtropical high-pressure systems that are usually present.

– They move steadily toward the west, then eventually turn poleward when they reach the far edges of the high-pressure systems.

– There, they are guided by prevailing westerlies and begin to interact with midlatitude systems.

Tropical Cyclones

Stages of Tropical Cyclones

Tropical Storms

– Tropical cyclones usually begin as disturbances that originate either from the ITCZ or as weak, low-pressure systems called tropical waves.

– Only a small percentage these ever develop into hurricanes because conditions throughout the atmosphere must allow rising air to be dispersed into the upper atmosphere.

Tropical Cyclones

Stages of Tropical Cyclones

Tropical Storms

Tropical Cyclones

Stages of Tropical Cyclones

Tropical Storms

– When a disturbance over a tropical ocean acquires a cyclonic circulation around a center of low pressure, it is known as a tropical depression.

– When wind speeds around the low-pressure center of a tropical depression exceed 65 km/h, the system is called a tropical storm.

– If air pressure continues to fall and winds around the center reach at least 120 km/h, the storm is officially classified as a hurricane.

Tropical Cyclones

Stages of Tropical Cyclones

Tropical Storms

– Once a hurricane, the development of a calm center of the storm, called an eye, takes place.

– The eyewall is a band immediately surrounding the eye that contains the strongest winds in a hurricane.

Classifying Hurricanes• The Saffir-Simpson hurricane scale classifies

hurricanes according to wind speed, air pressure in the center, and potential for property damage.

Tropical Storms

• The Saffir-Simpson hurricane scale ranges from Category 1 hurricanes to Category 5 storms, which can have winds in excess of 155 mph.

• Most of the deadliest hurricanes that strike the United States were classified as major hurricanes.

Classifying Hurricanes

Running Out of Energy

Tropical Storms

– A hurricane will last until it can no longer produce enough energy to sustain itself. This usually happens when:

• The storm moves over land and no longer has access to the warm ocean surface from which it draws its energy.

• The storm moves over colder water.

Hurricane Hazards• Hurricanes can cause a lot of damage, particularly

along coastal areas.

Tropical Storms

• Much of this damage is associated with violent winds of the eyewall, the band about 40 to 80 km wide that surrounds the calm eye.

Hurricane Hazards

Storm Surges

Tropical Storms

– A storm surge occurs when hurricane-force winds drive a mound of ocean water, sometimes as high as 6 m above normal sea level, toward coastal areas where it washes over the land.

– In the northern hemisphere, a storm surge occurs primarily on the right side of a storm relative to its eye, where the strongest onshore winds occur.

– Floods are an additional hurricane hazard, particularly if the storm moves over mountainous areas, where orographic lifting enhances the upward motion of air.

Hurricane Hazards

Storm Surges

Tropical Storms

Hurricane Hazards

Hurricane Advisories

Tropical Storms

– The National Hurricane Center, which is responsible for tracking and forecasting the intensity and motion of tropical cyclones in the western hemisphere, issues a hurricane warning at least 24 hours before a hurricane strikes.

– The center also issues regular advisories that indicate a storm’s position, strength, and movement.

Hurricane Hazards

Hurricane Advisories

Tropical Storms

Section Assessment

1. Match the following terms with their definitions.

___ tropical depression

___ hurricane

___ eyewall

___ storm surge

Tropical Storms

A. a tropical cyclone with wind speeds of at least 65 km/h

B. the band that has the highest wind speeds in a hurricane

C. a tropical cyclone with wind speeds of at least 120 km/h

D. a mound of wind-driven water that washes over coastal lands

A

C

B

D

Section Assessment

2. What are the two main events that cause hurricanes to weaken?

Tropical Storms

Hurricanes will weaken when they lose their energy source or warm ocean water. This happens when the hurricane moves over land or an area with cooler water.

Section Assessment

Tropical Storms

3. What are the three main threats that a hurricane poses?

The three main threats that a hurricane poses are extreme winds, storm surges that cause coastal flooding, and heavy rains that cause inland flooding.

– drought

– heat wave

– cold wave

– wind-chill factor

Objectives• Describe recurring weather patterns and the problems

they create.

• Identify atmospheric events that cause recurring weather patterns.

Vocabulary

Recurring Weather

Floods and Droughts• Floods can occur when weather patterns cause

even mild storms to persist over the same area.

Recurring Weather

• Droughts are extended periods of well-below-normal rainfall.

• Droughts are usually the result of shifts in global wind patterns that allow large high-pressure systems to persist for weeks or months over continental areas.

Floods and Droughts

Heat Waves

Recurring Weather

– Heat waves, which are extended periods of above-normal temperatures, can be formed by the same high-pressure systems that cause droughts.

– As the air under a large high-pressure system sinks, it warms by compression and causes above-normal temperatures.

– The high-pressure system also blocks cooler air masses from moving into the area, so there is little relief from the heat.

Floods and Droughts

Heat Waves

Recurring Weather

– If the air is humid, it slows the rate of evaporation, which diminishes the body’s ability to regulate internal temperature.

– Because of the danger, the National Weather Service routinely reports the heat index.

– The heat index assesses the effect of the body’s increasing difficulty in regulating its internal temperature as relative humidity rises.

Floods and Droughts

Recurring Weather

Cold Waves• A cold wave is an extended period of below-

normal temperatures.

Recurring Weather

• Cold waves are brought on by large, high-pressure systems of continental polar or arctic origin.

• Winter high-pressure systems are much more influenced by the jet stream than are summer systems and therefore rarely linger over one area.

• Several polar high-pressure systems can follow the same path and subject the same areas to bout after bout of numbing cold.

Cold Waves• The wind-chill factor is measured by the wind-

chill index, which estimates the heat loss from human skin caused by the combination of cold air and wind.

Recurring Weather

Section Assessment

1. What is the primary cause of a drought?

Recurring Weather

Droughts are usually the result of shifts in global wind patterns that allow high-pressure systems to persist for weeks or months over continental areas.

Section Assessment

2. What would the heat index be if the air temperature is 90ºF with a 60 percent relative humidity?

Recurring Weather

The heat index would be 100ºF.

Section Assessment

Recurring Weather

3. Which type of air masses are usually responsible for cold waves?

Cold waves are caused by air masses of continental polar or arctic origin.

Chapter Resources Menu

Chapter Resources Menu

Study Guide

Section 13.1

Section 13.2

Section 13.3

Section 13.4

Chapter Assessment

Image Bank

Section 13.1 Main Ideas• For a thunderstorm to occur, there must be abundant

moisture in the lower levels of the atmosphere and a mechanism to lift the moisture so it can condense. In addition, the air must be unstable so that the growing cloud will continue to rise.

• Thunderstorms are classified according to the mechanism that caused the air to rise. In an air-mass thunderstorm, the cloud rose because of unequal heating of Earth’s surface within one air mass. In a frontal thunderstorm, the air rose because it was pushed up by an advancing air mass.

Section 13.1 Study Guide

Section 13.2 Main Ideas• Lightning is produced when an advancing stepped leader

unites with an upward-moving return stroke. Thunder is the sound made by the rapid expansion of air around the lightning bolt as a result of extreme heating of the lightning channel.

• Thunderstorms can damage property and cause loss of life. The hazards of thunderstorms include lightning, violent winds, hail, floods, and tornadoes.

• The Fujita tornado intensity scale classifies tornadoes according to wind speed, path of destruction, and duration.

Section 13.2 Study Guide

Section 13.3 Main Ideas• Tropical cyclones derive their energy from the evaporation

of warm ocean water and the release of heat.

• The Saffir-Simpson hurricane scale classifies hurricanes according to intensity.

• Hurricane hazards include violent winds, floods, and storm surges. The National Hurricane Center tracks hurricanes and issues advance warnings to help reduce loss of life.

Section 13.3 Study Guide

Section 13.4 Main Ideas• Examples of persistent weather events include floods,

droughts, cold waves, and heat waves.

• The heat index assesses the impact of humidity combined with excessive heat on the human body. The wind-chill index estimates the heat loss from human skin caused by a combination of cold air and wind.

Section 13.4 Study Guide

1. Which of the following states experiences the highest number of thunderstorm days annually?

a. Oklahoma c. Florida

b. Tennessee d. Iowa

Multiple Choice

Chapter Assessment

Almost the entire state of Florida experiences more than 70 thunderstorm days annually.

2. The ____ causes the illumination that you see as lightning.

a. stepped leader c. channel

b. return stroke d. thunder

The stepped leader is the invisible channel of negatively charged air that moves from the cloud toward the ground. The return stroke rushes upward from the ground to meet it, illuminating the channel with about 100 million V of electricity.

Multiple Choice

Chapter Assessment

Multiple Choice

3. Which classification on the Fujita tornado intensity scale represents a strong tornado?

a. F0 c. F3

b. F1 d. F5

Chapter Assessment

F0 and F1 are classified as weak tornadoes. F2 and F3 are classified as strong tornadoes. F4 and F5 are classified as violent tornadoes.

Multiple Choice

4. Which of the following areas is least likely to be hit by a hurricane or typhoon?

a. western Africa

b. eastern United States

c. southern Japan

d. eastern India

Chapter Assessment

As a general rule, the most likely areas to be hit by a hurricane are on the eastern side of continents. Australia is the exception; both its east and west coasts are vulnerable.

Multiple Choice

5. Cold waves are caused by ____.

a. high-pressure systems

b. low-pressure systems

c. mT air masses

d. cT air masses

Chapter Assessment

Cold waves are brought on by large high-pressure systems that originate in the polar regions.

Short Answer

6. Explain why cold-front thunderstorms can last through the night?

Chapter Assessment

Cold-front thunderstorms get their initial lift from the push of cold air. They are not dependent on daytime heating. The thunderstorm can persist as long as the flow of moist, warm air into it is not disrupted.

Short Answer

7. What is wind shear and why is it important in the formation of tornadoes?

Chapter Assessment

Wind shear is when wind speed and direction change suddenly with height. This can produce a horizontal rotation near Earth’s surface. If this occurs close to the thunderstorm’s updrafts the twisting column of wind can be tilted from a horizontal to vertical position.

True or False

8. Identify whether the following statements are true or false.

______ Tornadoes can occur virtually anywhere on Earth.

______ Typical thunderstorms last about two hours.

______ High instability in the atmosphere limits thunderstorms.

______ Air-mass thunderstorms generally occur during

mid-afternoon.

______ Tropical disturbances have a cyclonic circulation.

Chapter Assessment

true

false

true

true

false

Image Bank

Chapter 13 Images

Image Bank

Chapter 13 Images

Image Bank

Chapter 13 Images

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