Objectives

1. Thunderstorms Objectives • 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. Thunderstorms 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.

4. Thunderstorms 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. Thunderstorms How Thunderstorms Form

6. Thunderstorms 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. Thunderstorms Air-Mass Thunderstorms

8. Thunderstorms 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. 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. 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. 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 true true

17. End of Section 1

18. 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 100 000 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. Severe Weather Lightning The Power of Lightning • 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.

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. 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.

35. 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.

36. Severe Weather Tornadoes 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.

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. 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 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 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.

49. 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