1. Objectives Vocabulary Identify the processes that form thunderstorms.
Compare and contrast different types of thunderstorms.
Describe the life cycle of a thunderstorm.
Vocabulary
air-mass thunderstorm
sea-breeze thunderstorm
frontal thunderstorm

2. Thunderstorms
Thunderstorms
At any given moment, nearly 2000 thunderstorms are occurring around the world.
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.

3. How Thunderstorms Form
For a thunderstorm to form, three conditions must exist.
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.
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4. How Thunderstorms Form
Limits to Growth
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.

5. How Thunderstorms Form

6. Air-Mass Thunderstorms
Thunderstorms are often classified according to the mechanism that caused the air to rise.
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.

7. Air-Mass Thunderstorms

8. Frontal Thunderstorms
Frontal thunderstorms are thunderstorms that are produced by advancing cold fronts and, more rarely, warm fronts.
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.

9. Thunderstorms
Stages of Development
A thunderstorm usually has three stages: the cumulus stage, the mature stage, and the dissipation stage.
The stages are classified according to the direction in which the air is moving.

10. Thunderstorms
Stages of Development

11. Stages of Development Cumulus Stage
Thunderstorms
Stages of Development
Cumulus Stage
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.

12. Stages of Development Mature Stage
Thunderstorms
Stages of Development
Mature Stage
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.

13. Stages of Development Dissipation Stage
Thunderstorms
Stages of Development
Dissipation Stage
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.

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

15. Thunderstorms
Section Assessment
2. What is the main cause of thunderstorm dissipation?
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.

16. Thunderstorms
Section Assessment
3. Identify whether the following statements are true or false.
______ 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.
true
false

17. End of Section 1

18. Objectives Vocabulary
Severe Weather
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.
Vocabulary
supercell
downburst
tornado
Fujita tornado intensity scale

19. Severe Weather
Severe Weather
Occasionally, weather events come together in such a way that there is a continuous supply of surface moisture.
This happens along a cold front that moves into warmer territory and can lift and condense a continuous supply of warm air.

20. Severe Weather
Severe Thunderstorms
Other factors also play a role in causing some storms to be more severe than others.
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.

21. Severe Weather
Severe Thunderstorms
Supercells are self-sustaining, extremely powerful severe thunderstorms, which are characterized by intense, rotating updrafts.
Only about ten percent of the roughly thunderstorms that occur each year in the United States are considered to be severe; even fewer become supercells.

22. Severe Weather
Lightning
Lightning is an electrical discharge caused by the friction of falling and rising ice crystals within strong drafts of a cumulonimbus cloud.
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.

23. Severe Weather
Lightning
When the stepped leader nears the ground, a channel of positively charged ions, called the return stroke, rushes upward to meet it.
The return stroke surges from the ground to the cloud, illuminating the channel with about 100 million V of electricity.

24. Severe Weather
Lightning

25. Lightning The Power of Lightning
Severe Weather
Lightning
The Power of Lightning
A lightning bolt heats the surrounding air to about °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.

26. Severe Weather
Lightning

27. 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.
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.

28. Severe Weather
Hail
Hail is precipitation in the form of balls or lumps of ice that can do tremendous damage.
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.

29. Severe Weather
Hail
The supercooled water droplets in the cloud freeze on contact with other ice pellets and are caught alternately in the updrafts and downdrafts.
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.

30. Severe Weather
Floods
When there are weak wind currents in the upper atmosphere, weather systems and resulting storms move slowly.
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.

31. Severe Weather
Tornadoes
A tornado is a violent, whirling column of air in contact with the ground.
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.

32. Severe Weather
Tornadoes
A tornado forms when wind speed and direction change suddenly with height, a phenomenon known as wind shear.
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.

33. Severe Weather
Tornadoes

34. Tornadoes Tornado Classification
Severe Weather
Tornadoes
Tornado Classification
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.
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35. Tornadoes Tornado Distribution
Severe Weather
Tornadoes
Tornado Distribution
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.
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36. Tornadoes Tornado Safety
Severe Weather
Tornadoes
Tornado Safety
In the United States, an average of 80 deaths and 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.

37. Severe Weather
Tornadoes

38. Severe Weather
Section Assessment
1. Match the following terms with their definitions.
___ supercell
___ macroburst
___ microburst
___ tornado B C D A
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

39. Severe Weather
Section Assessment
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.

40. Severe Weather
Section Assessment
3. Why do so many tornadoes form in “Tornado Alley”?
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.

41. End of Section 2

42. Objectives Vocabulary Identify where tropical cyclones originate.
Tropical Storms
Objectives
Identify where tropical cyclones originate.
Describe the life cycle of a tropical cyclone.
Recognize the dangers of hurricanes.
Vocabulary
tropical cyclone
eye
eyewall
Saffir-Simpson hurricane scale
storm surge

43. Tropical Storms
Tropical Storms
Tropical cyclones are large, rotating, low-pressure storms that form over water during summer and fall in the tropics.
The strongest of these cyclonic storms are known in the United States and other parts of the Atlantic Ocean as hurricanes.

44. Tropical Storms
Tropical Cyclones
Tropical cyclones thrive on the tremendous amount of energy in warm, tropical oceans.
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.

45. Tropical Storms
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.
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.

46. Tropical Cyclones Formation of Tropical Cyclones
Tropical Storms
Tropical Cyclones
Formation of Tropical Cyclones
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.

47. Tropical Storms
Tropical Cyclones

48. Tropical Cyclones Movement of Tropical Cyclones
Tropical Storms
Tropical Cyclones
Movement of Tropical Cyclones
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.
baby hurricane that is starting to form
Todler stage
Teen stage
Grown up stage

49. Tropical Cyclones Stages of Tropical Cyclones
Tropical Storms
Tropical Cyclones
Stages of Tropical Cyclones
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.

50. Tropical Storms
Tropical Cyclones
Stages of Tropical Cyclones

51. Tropical Cyclones Stages of Tropical Cyclones
Tropical Storms
Tropical Cyclones
Stages of Tropical Cyclones
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.

52. Tropical Cyclones Stages of Tropical Cyclones
Tropical Storms
Tropical Cyclones
Stages of Tropical Cyclones
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.

53. Classifying Hurricanes
Tropical Storms
Classifying Hurricanes
The Saffir-Simpson hurricane scale classifies hurricanes according to wind speed, air pressure in the center, and potential for property damage.
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.
Very important.

54. Classifying Hurricanes
Tropical Storms
Classifying Hurricanes
Running Out of Energy
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.

55. Tropical Storms
Hurricane Hazards
Hurricanes can cause a lot of damage, particularly along coastal areas.
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.

56. Hurricane Hazards Storm Surges
Tropical Storms
Hurricane Hazards
Storm Surges
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.

57. Tropical Storms
Hurricane Hazards
Storm Surges

58. Hurricane Hazards Hurricane Advisories
Tropical Storms
Hurricane Hazards
Hurricane Advisories
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.

59. Tropical Storms
Hurricane Hazards
Hurricane Advisories

60. Tropical Storms
Section Assessment
1. Match the following terms with their definitions.
___ tropical depression
___ hurricane
___ eyewall
___ storm surge A C B D
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

61. Tropical Storms
Section Assessment
2. What are the two main events that cause hurricanes to weaken?
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.

62. Tropical Storms
Section Assessment
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.

63. End of Section 3

64. Objectives Vocabulary
Recurring Weather
Objectives
Describe recurring weather patterns and the problems they create.
Identify atmospheric events that cause recurring weather patterns.
Vocabulary
drought
heat wave
cold wave
wind-chill factor

65. Recurring Weather
Floods and Droughts
Floods can occur when weather patterns cause even mild storms to persist over the same area.
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.

66. Floods and Droughts Heat Waves
Recurring Weather
Floods and Droughts
Heat Waves
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.

67. Floods and Droughts Heat Waves
Recurring Weather
Floods and Droughts
Heat Waves
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.

68. Recurring Weather
Floods and Droughts

69. Recurring Weather
Cold Waves
A cold wave is an extended period of below-normal temperatures.
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.

70. Recurring Weather
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.

71. Section Assessment 1. What is the primary cause of a drought?
Recurring Weather
Section Assessment
1. What is the primary cause of a drought?
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.

72. Recurring Weather
Section Assessment
2. What would the heat index be if the air temperature is 90ºF with a 60 percent relative humidity?
The heat index would be 100ºF.

73. Recurring Weather
Section Assessment
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.

74. End of Section 4

75. Chapter Resources Menu
Study Guide
Section 13.1
Section 13.2
Section 13.3
Section 13.4
Chapter Assessment
Image Bank
Chapter Resources Menu

76. Section 13.1 Study Guide
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.

77. Section 13.2 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.

78. Section 13.3 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.

79. Section 13.4 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.

80. Chapter Assessment
Multiple Choice
1. Which of the following states experiences the highest number of thunderstorm days annually?
a. Oklahoma c. Florida
b. Tennessee d. Iowa
Almost the entire state of Florida experiences more than 70 thunderstorm days annually.

81. Chapter Assessment
Multiple Choice
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.

82. 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
F0 and F1 are classified as weak tornadoes. F2 and F3 are classified as strong tornadoes. F4 and F5 are classified as violent tornadoes.

83. Chapter Assessment
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
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.

84. Multiple Choice 5. Cold waves are caused by ____.
Chapter Assessment
Multiple Choice
5. Cold waves are caused by ____.
a. high-pressure systems
b. low-pressure systems
c. mT air masses
d. cT air masses
Cold waves are brought on by large high-pressure systems that originate in the polar regions.

85. Chapter Assessment
Short Answer
6. Explain why cold-front thunderstorms can last through the night?
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.

86. Chapter Assessment
Short Answer
7. What is wind shear and why is it important in the formation of tornadoes?
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.

87. Chapter Assessment
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.
true
false

88. Image Bank
Chapter 13 Images

89. Image Bank
Chapter 13 Images

90. Image Bank
Chapter 13 Images

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