Unit 6: The Atmosphere and the Oceans Chapter 13 The Nature of Storms
NC ESES Unit 6: The Atmosphere and the Oceans NC Earth Science Essential Standards EEn. 2.5 - Understand the structure of and processes within our atmosphere. EEn. 2.5.1 - Summarize the structure and composition of our atmosphere. EEn. 2.5.2 - Explain the formation of typical air masses and the weather systems that result from air mass interactions. EEn. 2.5.3 - Explain how cyclonic storms form based on interaction of air masses. EEn. 2.5.4 - Predict the weather using available weather maps and data (including surface, upper, atmospheric winds and satellite imagery). EEn. 2.5.5 - Explain how human activities affect air quality.
NC ESES Unit 6: The Atmosphere and the Oceans Reading Assignment: Read Chapter 13; pages: 328-351 Objective: -Identify the process that form thunderstorms; -Compare and contrast different types of thunderstorms; -Describe the life cycle of a thunderstorm; -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; -Identify where tropical cyclones originate; -Describe the life cycle of a tropical cyclone; -Recognize the dangers of hurricanes; -Describe recurring weather patterns and the problems they create; -Identify atmospheric events that cause recurring weather patterns. Vocabulary: Air-mass -thunderstorm Sea-breeze -thunderstorm Frontal -thunderstorm Super cell Downburst Tornado Fujita tornado -intensity scale tropical cyclone Eye Eyewall Storm surge Saffir-Simpson -hurricane scale drought Heat wave Cold wave Wind-chill factor
How Thunderstorms Form Under the right conditions, convection can cause a cumulus cloud to grow into a cumulonimbus cloud, which could produce thunderstorms. 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) There must be some type of mechanism to lift the air so that the moisture can condense and release latent heat. 3) The atmosphere through which the cloud grows must be unstable. In other words, the air must continue to cool with increasing altitude for the growing cloud to stay warmer than the surrounding air.
Limits to Growth If the previous conditions are met, the air will keep rising, causing more moisture to condense and create more latent heat. The process will continue until the rising air meets a layer of stable air which that it cannot overcome or until the rate of condensation, which diminishes with height, is insufficient to generate enough latent heat to keep the cloud warmer than the surrounding air (this factor limits cumulonimbus clouds to a height of around 18,000 m). Typical thunderstorms last only 30 minutes and individual storms are only 24 km in diameter.
Air-Mass Thunderstorms Thunderstorms are often classified according to the mechanism that caused the air to rise. Air-mass thunderstorms (which are the most common) result from air rising due to unequal heating of Earth s surface within one air mass and reaches its maximum during mid-afternoon; Mountain thunderstorms and Sea-breeze thunderstorms are the two common types of air-mass thunderstorms. 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 Sea-breeze thunderstorms are common along coastal areas during the summer, especially in the tropics and subtropics. 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. During the day, cool air over the ocean moves inland and creates a sea breeze. The cool air forces warm air over the land to rise. The rising air cools and sinks, creating a convection cell. These conditions can produce strong updrafts that result in thunderstorms.
Frontal Thunderstorms The second main classification of thunderstorms are frontal thunderstorms, which are produced by advancing cold fronts and, more rarely, warm fronts. In a cold front, cold air pushes warm air rapidly up the steep cold-front boundary. This rapid upwards motion can produce a line of thunderstorms, sometimes hundreds of kilometers long, along the leading edge of a cold front. Cold-front thunderstorms get their initial lift from the push of the cold air. Because they are not dependent on daytime heating for their initial lift, cold-front thunderstorms can persist long into the night.
Frontal Thunderstorms Less frequently, thunderstorms can develop along the advancing edge of a warm front. In a warm front, a warm air mass slides up and over a cold air mass. The boundary between two air masses is not steep; thus, the air gradually rises. However, if the warm air behind the warm front is unstable and moisture levels sufficiently high, a relatively mild thunderstorm can develop.
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. In the cumulus stage, air starts to rise nearly vertically upward, creating updrafts, transporting moisture to upper reaches of the cloud; the moisture condenses into visible cloud droplets and releases latent heat. As cloud droplets coalesce, they form larger and larger droplets, which eventually fall to Earth as precipitation.
Mature Stage In the mature stage, precipitation in a thunderstorm is composed of water droplets that formed at high, cool levels of the atmosphere. As precipitation falls, it cools the air around it; The newly cooled air is more dense than surrounding air, so it sinks rapidly to the ground along with the precipitation. This crates downdrafts. As the updrafts and downdrafts form a convection cell that produces the gusty surface winds of the storm. In mature stage, nearly equal amounts of updrafts and downdrafts exist side by side in cumulonimbus clouds.
Dissipation Stage In the dissipation stage, the production of downdrafts is the thunderstorm s undoing. The convection cell s can only exist if there is a steady supply of warm, moist air; once the supply of warm, moist air runs out, the updrafts slow and eventually stop; without the warm air, the updrafts cease and precipitation can no longer form. The dissipation stage is characterized primarily by lingering downdrafts; that will last until the cloud runs of previously formed raindrops.
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. In this case, a line of thunderstorms that can last for hours or even days as they continually regenerate themselves with the new, warm air that is introduced into the updrafts. 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.
Severe Thunderstorms This cold, high air increases the temperature differences between the upper and lower parts of the storm, which causes the air to become more unstable. As the instability of the air increases, the strength of the storm s updrafts and downdrafts intensifies; the storm is then considered to be severe. Severe thunderstorms can produce some of the most violent weather conditions on Earth. They may develop into self-sustaining, extremely powerful storms called supercells, which are characterized by intense, rotating updrafts.
Supercells Supercells: self-sustaining, extremely powerful severe thunderstorms, which are characterized by intense, rotating updrafts. Only about 10% of the roughly one hundred thousand thunderstorms that occur each year in the U.S. are considered to be severe; even fewer become supercells. The violent, furious storms can last for several hours and can have updrafts as strong as 240 km/h.
Lightning Lightning is electricity caused by the rapid rush of air in a cumulonimbus cloud. A lightning bolt forms when friction between updrafts and downdrafts within a cumulonimbus cloud separates electrons from their atoms either in the cloud or near the ground. The atoms that lose electrons become positively charged ions, other atoms receive 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. The return stroke surges from the ground to the cloud, illuminating the channel with about 100 million volts of electricity. When a stepped leader nears an object on the ground, a powerful surge of electricity from the ground moves upward to the cloud, and lightning is produced.
The Power of Lightning A lightning bolt heats the surrounding air to about 30 000 C; five times hotter than Sun s surface. Thunder is the sound produced as this superheated air rapidly expands and contracts. Each year in the U.S., 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.
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.
Hail Hail forms due to two characteristics in thunderstorms: 1) Water droplets in cumulonimbus clouds are supercooled water droplets and when they encounter ice pellets, the water droplets freeze on contact and cause the ice pellets to grow larger. 2) With the abundance of strong updrafts and downdrafts existing side by side within a cloud; growing ice pellets are caught alternately in these updrafts and downdrafts; which keeps them growing. The ice pellets keep growing until they are too heavy 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. 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 Are a violent, whirling column of air in contact with the ground and are often associated with supercells. Before a tornado reaches the ground, it is called a funnel cloud. 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. 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.
Tornadoes 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. The Enhanced Fujita scale (EF-Scale) rates the intensity of tornadoes in the United States and Canada based on the damage they cause. A change in wind direction and speed creates a horizontal rotation in the lower atmosphere. Strong updrafts tilt the rotating air from a horizontal to a vertical position. A tornado forms within the rotating winds.
Tornado Classification Fujita tornado intensity scale classifies tornadoes according to their path of destruction, wind speed, and duration (named after Dr. Theodore Fujita) 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. Those that do can lift entire buildings from their foundations and toss automobiles and trucks around like toys.
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. Although more than 700 tornadoes touch down each year in the United States, many of the tornadoes occur most frequently in a region called Tornado Alley, which extends from northern Texas through Oklahoma, Kansas, and Missouri.
Tornado Safety 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.
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 U.S. and other parts of the Atlantic Ocean as hurricanes. 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.
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.
Formation of Tropical Cyclones Tropical cyclones require two basic conditions to form: An abundant supply of very warm ocean water and 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, because the ocean water is cooler and the ITCZ is positioned farther north. Tropical cyclones occur in the large expanse of warm waters in the western Pacific, where they are known as typhoons. Hurricanes most frequently occur in the late summer and early fall, when Earth s oceans contain their greatest amount of stored heat energy.
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 mid-latitude systems. The interaction of the various wind and weather systems makes the storms unpredictable.
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. These disturbances are common during the summer and early fall. 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. In this hurricane cross section, the rising moist air-indicated by small red arrows-forms clouds in bands around the eye.
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 lowpressure 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. 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. Notice that a tropical cyclone and or hurricane both have a counter clockwise rotation, due to the fact they are formed from a low pressure systems.
Four Stages of a Hurricane Tropical Disturbance: The birth of a hurricane, having only a slight circulation with no closed isobars around an area of low pressure. Tropical disturbances commonly exist in the tropical trade winds at any one time and are often accompanied by clouds and precipitation. Tropical Depression: If sustained winds increase to at least 20 knots, a disturbance is upgraded to a tropical depression. Surface wind speeds vary between 20 and 34 knots and a tropical depression has at least one closed isobar that accompanies a drop in pressure in the center of the storm. Tropical Storm: If sustained wind speeds increase to at least 35 knots, a tropical depression is upgraded to a tropical storm. Surface wind speeds vary between 35 and 64 knots and the storm becomes more organized. Tropical storms resemble the appearance of hurricane due to the intensified circulation.
Four Stages of a Hurricane Hurricane: As surface pressures continue to drop, a tropical storm becomes a hurricane when sustained wind speeds exceed 64 knots. A pronounced rotation develops around the central core as spiral rain bands rotate around the eye of the storm. The heaviest precipitation and strongest winds are associated with the eye wall. Watches and Warnings: Modern technology provides forecasters with the ability to accurately determine the position and intensity of hurricanes. This information is used to provide advanced warnings to those populations at risk. If it appears that a particular area is in potential danger of being struck by a hurricane, a "hurricane watch" is issued, sometimes up to several days in advance of the storm's predicted arrival. When there is a high probability that a hurricane will strike an area within 24 hours, a "hurricane warning" is issued. Unfortunately, despite the advanced warning systems, hurricanes still claim the lives of hundreds, even thousands of people each year.
Classifying Hurricanes Saffir-Simpson hurricane scale classifies hurricanes according to wind speed, air pressure in the center, and potential for property damage. The 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 U.S. were classified as major 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. During its life cycle, a hurricane can undergo several fluctuations in intensity as it interacts with other atmospheric systems. 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. Outside the eyewall, winds taper off with distance from the center; winds of more than 60 km/h can extend as far as 400 km from the center.
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.
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. Awareness, combined with proper safety precautions has greatly reduced death tolls associated with hurricanes.
Floods, Droughts and Heat Waves 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. 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 highpressure 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.
Heat Waves If the air is humid, it slows the rate of evaporation, which diminishes the body s ability to regulate internal temperature, this can lead to serious health problems such as heatstroke, sunstroke, and even death. 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.
Cold Waves and Wind-Chill A cold wave is an extended period of below-normal temps. and 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 highpressure systems can follow the same path and subject the same areas to bout after bout of numbing cold. 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. This index estimates how cold the air actually feels to the human body. As the heat index, the National Weather Service records the wind-chill index in U.S. units for convenience. Limitations: the wind-chill index does not account for individual sensitivity to cold, or the effects of physical activity, or humidity.
NOAA Wind Chill Chart
Unit 6: The Atmosphere and the Oceans End
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.
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.
Thunderstorms Section Assessment 3. Identify whether the following statements are true or false. true Latent heat is crucial in maintaining the upward motion of a cloud. false Thunderstorms are more likely to develop along a warm front instead of a cold front. true A mountain thunderstorm is an example of an air-mass thunderstorm. true In the mature stage of a thunderstorm, updrafts are roughly equal to downdrafts.
Severe Weather Section Assessment 4. Match the following terms with their definitions. B supercell C macroburst D microburst A tornado 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
Severe Weather Section Assessment 5. 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.
Severe Weather Section Assessment 6. 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.
Tropical Storms Section Assessment 7. Match the following terms with their definitions. A tropical depression C hurricane B eyewall D storm surge 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
Tropical Storms Section Assessment 8. 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.
Tropical Storms Section Assessment 9. 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.
Recurring Weather Section Assessment 10. 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.
Recurring Weather Section Assessment 11. What would the heat index be if the air temperature is 90ºF with a 60 percent relative humidity? The heat index would be 100ºF.
Recurring Weather Section Assessment 12. 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 Assessment Multiple Choice 13. 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.
Chapter Assessment Multiple Choice 14. 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.
Chapter Assessment Multiple Choice 15. 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.
Chapter Assessment Multiple Choice 16. 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.
Chapter Assessment Multiple Choice 17. 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.
Chapter Assessment Short Answer 18. 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.
Chapter Assessment Short Answer 19. 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.
Chapter Assessment True or False 20. Identify whether the following statements are true or false. true Tornadoes can occur virtually anywhere on Earth. false Typical thunderstorms last about two hours. true High instability in the atmosphere limits thunderstorms. true Air-mass thunderstorms generally occur during mid-afternoon. false Tropical disturbances have a cyclonic circulation.
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.
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.
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.
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.