Tropical Storms & Recurrent Weather

1. Tropical Storms & Recurrent Weather

2. Biblical Reference Before very long, a wind of hurricane force, called the Northeaster, swept down from the island. Acts 27:14

3. Tropical Storms • Tropical cyclones are large, rotating, low-pressure storms that form over water during summer and fall in the tropics. • The strongest cyclonic storms are hurricanes that occur in the Atlantic Ocean. • Tropical cyclones thrive on the tremendous amount of energy in warm, tropical oceans. • The 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.

4. Tropical Storms • The cyclonic rotation of a tropical cyclone begins as warm air moves toward the low-pressure center and replaces the air that has risen. • 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 warm air is fed into the system at the surface and removed from the system in the upper atmosphere, the process will continue.

5. Formation of 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.

6. The Formation of a Hurricane

7. The word, hurricane, comes from the Spanish word, huracan, which means “tempest”.

8. 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.

9. Stage 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.

10. Stages of a Tropical Cyclone

11. Stages of a Tropical Cyclone • 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.

12. Stages of a Tropical Cyclone • 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.

13. 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.

14. 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.

15. 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.

16. Storm Surges • A storm surge occurs when hurricane-force winds drive a mound of ocean water, 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.

17. Storm Surges

18. Hurricane Advisories • The National Hurricane Center is responsible for tracking and forecasting the intensity and motion of tropical cyclones in the western hemisphere. • It 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.

19. Hurricane Safety

20. Pop Quiz 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

21. Pop Quiz 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.

22. Pop Quiz 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.

23. 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.

24. Heat Waves • Heat waves(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.

25. 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.

26. The Heat Index

27. 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 summer systems and thus 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.

28. Wind Chill Factor • The wind-chill factor is measured by the wind-chill index, an estimate of the heat loss from human skin caused by cold air and wind.

29. Pop Quiz 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.

30. Pop Quiz What would the heat index be if the air temperature is 90F with a 60% relative humidity? The heat index would be 100ºF.

31. Pop Quiz Which type of air masses are usually responsible for cold waves? Cold waves are caused by air masses of continental polar or arctic origin.