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Hurricanes and Tornadoes Chapter 7 Vocabulary 1. Tropical Depression – a low pressure air mass that forms over warm water and has swirling winds can be as strong as 61 km per hour. 2. Tropical Storm – A low pressure air mass that forms over warm water and has swirling winds that are more than 61 km per hour, but less than 119 km per hour. 3. Hurricane – a dangerous storm made up of swirling bands of thunderstorms with wind speeds of at least 119 km per hour. 4. Storm surge – water pushed ahead onto shore by winds outside the eye wall of the hurricane. 5. Tornado – a rapidly spinning column of air that comes down out of a cloud and touches ground. 6. Vortex – an area where air or liquid spins, or spirals, in circles Chapter 7 Lesson 1 Notes What are hurricanes? -­‐Hurricanes are dangerous storms with wind speeds of at least 119 kilometers per hour. -­‐Many bands of thunderstorms make up a hurricane. -­‐The bands wrap around the hurricane’s center. -­‐Hurricanes get their energy from air moving upward over warm ocean water. How Hurricanes Form -­‐Thunderstorms grow out of a tropical depression. -­‐Air pressure at the ocean’s surface drops. Surface winds blow faster and begin to swirl. A tropical storm develops. -­‐Thunderstorms begin to move in spiral bands. Air pressure drops lower, and surface winds blow faster. The tropical storm is now a hurricane. Stages of Tropical Storms A tropical storm needs special conditions before it can form. 1. It needs a large area of warm ocean water. 2. It needs an area of low pressure at the ocean surface. A tropical disturbance develops warm, moist air rises and condenses into clouds. A tropical depression develops when the towering clouds in a tropical disturbance become thunderstorms. As the water vapor condenses, it releases heat energy. The heat makes the air in the thunderstorms warmer, causing the air to rise more and the storm to grow. A tropical depression becomes a tropical storm once wind speeds blow at 62 kilometers per hour, and the air pressure is even lower. Hurricane as a System A system is made up of two parts that affect each other or that work together. Two of Earth’s systems that work together to make a hurricane are the atmosphere and the ocean. Hurricanes are systems that form in the atmosphere and get their energy from the ocean. -­‐ Thunderstorms move in spiral bands toward the area of lowest air pressure. -­‐ More air moves out from the top of the storm than moves in at the ocean’s surface. -­‐ Air pressure drops even lower and surface winds blow faster toward the center. -­‐ When the winds reach 119 kilometers per hour, the tropical storm has become a hurricane. Hurricanes can change many of Earth’s land and living systems. The Hurricane’s Eye The eye of the hurricane is the spot in the middle. Winds are gentle in the eye, and it has no rain and few, if any, clouds. The entire hurricane spins around its eye.