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Name: Date: Period: Unit 4: Earth’s Fluid Spheres Section 2: Severe Weather Thunderstorms, tornadoes, or floods can happen almost anywhere. In some regions, these events occur very often, whereas in others they are unusual. However, at some time in your life you will probably experience some form of severe weather. In this chapter, you will learn what conditions create thunderstorms. How and Why Warm Air Rises (F 70) As air warms, it becomes less dense than its cooler surroundings and rises. Warm air is less dense than cold air because molecules in warm air are more active than the molecules in cold air. As molecules move around with increasing speed, the distance between them gets larger and their density decreases. Air masses can also be forced to rise when they flow over mountains or collide with other air masses. There are four main mechanisms that cause air to rise on a regional scale. 1. Convective Uplift – Convection refers to upward and downward motions of air caused by differences in air temperature (and therefore differences in density). As the Sun warms the ground, the ground warms the air immediately above it. Cooler, denser air from above sinks and forces the warmer air upward. The ascending warm air expands and cools and eventually sinks back to ground, completing the convective circulation. 2. Orographic Uplift – (orographic refers to mountains) When the wind encounters a mountain range, the mountain range acts as a barrier, and forces the air upward. 3. Frontal Wedging – When a cold air mass meets a warm air mass, the more dense cold air wedges or forces its way beneath the less dense warm air along a cold front. 4. Convergence – When winds blowing from different directions meet head-to-head, or converge, they have nowhere to go but up. Unit 4, Section 2: Severe Weather 1 What is a Front? A front is a narrow zone of transition between air masses that contrast in temperature and/or humidity. The map on the next page shows the “source regions” for the air masses that regularly move over North America. As indicated by the arrows, cold air usually flow southeastward and warm air masses flow northeastward. As air masses move out of their original regions, they change depending on the route they travel. Example: Cold air traveling southward will warm up before it reaches Florida so that is why Florida never gets as cold in the winter. Air masses are classified by their temperature and humidity (or moisture content), as follows… Continental: relatively dry air masses that form over land. Maritime: relatively humid air masses that form over the ocean. Polar: cold air masses that form at high latitudes. (Example: northern Canada) Tropical: warm air masses that form at low latitudes. (Example: Gulf of Mexico) General Types of air masses include: 1. continental polar – cold and dry 2. continental tropical – warm and dry 3. maritime polar – cold and humid 4. maritime tropical – warm and humid 5. artic air – exceptionally cold and dry Unit 4, Section 2: Severe Weather 2 When a front develops, whether it is a cold or warm front is determined by which air mass retreats or gives way to the other. If the colder (or drier) air advances while the warmer (or more humid) air retreats, the transition zone is considered a cold front. On the other hand, if the warm air advances while the cold air gives way, it is considered a warm front. Example of a cold front. Development of a Thunderstorm Thunderstorms can be caused by air rising along either warm fronts or cold fronts. However, the most intense thunderstorms develop from cold fronts. As a general rule, the greater the difference in temperature between the air masses, and the more rapidly the cold-air mass wedges under the warm air mass, the greater the chance a thunderstorm will form. Another way thunderstorms can form is through local convection of a warm and humid air mass. A thunderstorm is a relatively small, short-lived weather system. They usually affect an area the size of a city and for the duration of about an hour. A thunderstorm completes its “lifecycle” in three stages: cumulus, mature, and dissipating. The most severe weather occurs during the mature stage. At any given moment about 1800 thunderstorms are happening on Earth. This equates to 16 million thunderstorms per year worldwide. During the first stage, cumulus clouds build upward and laterally with a flat bases and towering tops. Cumulus clouds develop where air ascends as an updraft. As the air reaches the higher levels, it expands because the pressure is lower and that makes the air cool. The water vapor in the air condenses because of the colder temperatures producing clouds. No precipitation occurs during this stage and sometimes the cumulus clouds do not produce a thunderstorm. A thunderstorm that produces severe weather requires a very strong updraft that builds the developing cumulonimbus (nimbus means rain) cloud to great altitudes. Usually convection alone is not enough to produce a thunderstorm. Frontal wedging, orographic Unit 4, Section 2: Severe Weather 3 uplifting, or converging surface winds can strengthen an updraft and cause the cumulus clouds to build vertically. When a cumulus cloud gets tall enough, it will produce rain, hail, or even snow. Once the precipitation begins, the thunderstorm has reached its mature stage. Falling precipitation causes a downdraft next to the continuing updraft. During the mature stage, precipitation is the heaviest, lightening is most frequent, and hail or even tornadoes may develop. The higher the thunderstorm builds, the more severe the weather. Precipitation and the downdraft eventually spread throughout the thunderstorm as the system enters its dissipating stage. Precipitation tapers off and ends, clouds dissipate, and the chances for severe weather decrease dramatically. Review Questions 1. What happens as air warms? _______________________________________ ___________________________________________________________ 2. Why is warm air less dense than cold air? ______________________________ ___________________________________________________________ 3. What are two things that can force air masses to rise? a. _________________________________________________________ b. _________________________________________________________ 4. Describe what happens during convection. ______________________________ ___________________________________________________________ ___________________________________________________________ 5. What happens when a cold air mass meets a warm air mass? ___________________________________________________________ ___________________________________________________________ 6. In what direction do warm and cold air masses usually flow? ___________________________________________________________ 7. Why are winter temperatures warmer in Florida then they are in the Great Lakes region? ___________________________________________________________ ___________________________________________________________ 8. What are air masses classified by? _____________________________ and __________________________________ Unit 4, Section 2: Severe Weather 4 9. Describe each one of the five air masses. a. _________________________________________________________ b. _________________________________________________________ c. _________________________________________________________ d. _________________________________________________________ e. _________________________________________________________ 10. How does a cold front form? _______________________________________ ___________________________________________________________ 11. How does a warm front form? ______________________________________ ___________________________________________________________ 12. What causes a thunderstorm? ____________________________________________ ___________________________________________________________ ___________________________________________________________ 13. With what type of fronts do the most intense thunderstorms form? ________________ 14. What are the three stages in the lifecycle of a thunderstorm? a. ________________ b. _________________ c. _______________ 15. During which stage is the weather the most severe? __________________________ 16. How do cumulus clouds form and what do they look like? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 17. What are thunderstorm clouds called once they produce rain? ___________________ 18. What causes cumulus clouds to build into high altitude thunderstorm clouds? ___________________________________________________________ ___________________________________________________________ 19. What event signals the beginning of the mature stage?_________________________ 20. What is the relationship between the height of a thunderstorm and the weather it produces? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Unit 4, Section 2: Severe Weather 5 Name: Date: Period: How Clouds Form (F 80) All air contains water vapor, which is water that exists in the atmosphere as an invisible gas. Water vapor is not the same as clouds, fog, or steam, which is composed of tiny, visible droplets of water suspended in the air. There is a limit to how much water vapor can be concentrated in the air and when that limit is reached, the air is considered saturated. Warm air can hold more water vapor than cold air. When air is saturated with water vapor, some water vapor condenses into tiny droplets of water (or, if the temperature is really low, tiny ice crystals) that are visible in the form of clouds. The most common way clouds develop is by cooling of the air as it rises in the atmosphere. Ascending air cools because of expansion. A gas cools when it expands. Air coming out of a tire is cool because it is expanding as it escapes. The pressure is much less on the outside than on the inside the tire. Air Pressure Air pressure is the cumulative force of a multitude of air molecules colliding with a unit surface area of any object in contact with air. Think about a column of air pushing down on a specific surface area at the base of the column. Air pressure decreases with altitude, because the mass of air above you is getting smaller. Air that rises, like in an updraft, expands as it rises and loses heat. With that cooling, the air becomes saturated and excess water vapor condenses into droplets, which form clouds. This happens in the updraft of a thunderstorm. Conversely, if air is descending it is encountering growing air pressure and it warms and becomes compressed. During the dissipating stage of a thunderstorm the downdraft spreads through the thunderstorm cloud, and with compressional warming, the cloud vaporizes. Condensation Nuclei In addition to saturated air, cloud formation requires surfaces on which the water vapor can condense. Earth’s atmosphere contains an abundance of tiny solid and liquid particles that water vapor can condense on. These particles are called condensation nuclei. Nuclei are products of many different natural and human-related activities. Unit 4, Section 2: Severe Weather 6 Forest fires, volcanic eruptions, wind erosion of soil, saltwater spray, motor vehicle exhaust, and various industrial emissions are all sources of nuclei. Nuclei that promotes condensation is called condensation nuclei and nuclei that promotes the formation of ice crystals are called ice-forming nuclei. Types of Clouds Meteorologists classify clouds into three broad categories based upon shape: cirrus, stratus, and cumulus. Cirrus clouds are wispy, stratus clouds are layered, and cumulus clouds are puffy (like cottonballs). Clouds are further classified by their altitude: high, middle, low, or clouds with vertical development. And finally, clouds are classified by their composition: water droplets or ice crystals. Photographs of these clouds can be found on page F 82-83 of your text. The Mature and Dissipating Stages of a Thunderstorm (F 84) In the previous lesson, you learned that a thunderstorm begins when a cumulus cloud develops in an updraft of air. The more humid the air, the better chance a thunderstorm will form. In general, the more vigorous the updraft, the greater the altitude to which a thunderstorm cloud builds, and the more likely that the thundercloud can build. Consider the four different layers of the Earth’s atmosphere (listed from lowest to highest): troposphere, stratosphere, mesosphere and thermosphere. The boundaries between these layers are defined by air temperature. We live in the troposphere. On average, air temperature drops with increasing altitude up to the top boundary of the troposphere which is called the tropopause. The next layer up is the stratosphere, in which the air temperature is at first constant with altitude and then increases with altitude. A thunderstorm cloud that pushes above the tropopause and into the lower part of the stratosphere will be colder and denser than the surrounding air and will sink back down into the troposphere. Because of this reason, even a very intense thunderstorm cannot build much higher than the tropopause. This is also why cumulonimbus clouds develop their characteristic flat top. Thunderstorm precipitation falls through the updraft, weakening it and eventually dragging air downward, producing a downdraft alongside the updraft. The downdraft leaves the Unit 4, Section 2: Severe Weather 7 base of the cloud and flows along the ground ahead of the shaft of precipitation. The leading edge of this rain-cooled gusty air is like a miniature cold front and is known as a gust front. In an intense thunderstorm, a downdraft may strike the ground with wind speeds in excess of 100 km/hr (60 mph). An intense downdraft is known as a downburst. A downburst pushes ahead of the thunderstorm and can be strong enough to uproot trees and damage buildings. As precipitation spreads throughout the thunderstorm cloud, so does the downdraft. Eventually the downdraft overpowers the updraft. The updraft dies, precipitation comes to an end, and the cloud slowly vaporizes. This is the final, or dissipating, stage of the thunderstorm. Review Questions 21. What is water vapor? _____________________________________________ ___________________________________________________________ 22. What does the upper limit of water vapor in the atmosphere depend on? ___________________________________________________________ ___________________________________________________________ 23. Compare the amount of water vapor in warm and cool air. ___________________________________________________________ ___________________________________________________________ 24. What happens when air is saturated with water vapor? ___________________________________________________________ ___________________________________________________________ 25. How do clouds develop in the atmosphere? _________________________________ ___________________________________________________________ Unit 4, Section 2: Severe Weather 8 ___________________________________________________________ 26. What happens to ascending air? __________________________________________ ___________________________________________________________ ___________________________________________________________ 27. What happens to air pressure as altitude increases? ___________________________ 28. What happens to descending air?__________________________________________ ___________________________________________________________ ___________________________________________________________ 29. What are the sources of the nuclei? ___________________________________________________________ ___________________________________________________________ 30. What are the three things that meteorologist use to classify clouds? a. _______________________________________________________ b. _______________________________________________________ c. _______________________________________________________ 31. Describe each one of the three categories of clouds. a. _______________________________________________________ b. _______________________________________________________ c. _______________________________________________________ 32. What are the four layers of the Earth’s atmosphere from lowest to highest? a. ________________________ c. ________________________ b. ________________________ d. ________________________ 33. What defines the boundaries of these four layers?____________________________ ___________________________________________________________ ___________________________________________________________ 34. What is a downburst?___________________________________________________ ___________________________________________________________ ___________________________________________________________ 35. Describe the dissipating stage of a thunderstorm._____________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Unit 4, Section 2: Severe Weather 9 How Radar Works (F 90) Although most thunderstorms last for less than an hour, some can produce severe weather and damage for several hours such as flash flooding, destructive hail, and tornadoes. In this chapter, you will learn about radar and how it is used to track storm systems. 36. What is radar an acronym for? ______________________________________ 37. Describe how radar works. ________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 38. What type of energy does radar use? _________________________________ 39. What is radar echo? ____________________________________________ ___________________________________________________________ 40. How is echo strength calibrated? ____________________________________ ___________________________________________________________ 41. Why does the curve of the Earth limit the range of radar? ____________________ ___________________________________________________________ ___________________________________________________________ 42. What is radar’s maximum range? ____________________________________ 43. What is ground clutter? __________________________________________ ___________________________________________________________ ___________________________________________________________ 44. What do radar images track? ______________________________________ ___________________________________________________________ ___________________________________________________________ Unit 4, Section 2: Severe Weather 10 45. How are past positions, storm center movement, and future positions represented on a radar screen? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 46. Why do we sometimes have prolonged periods of severe weather? ___________________________________________________________ ___________________________________________________________ 47. Where and when are MMC’s most common? ___________________________________________________________ ___________________________________________________________ 48. How long does a typical MMC last? __________________________________ 49. What kind of thunderstorm is responsible for the most powerful tornadoes and the largest, most destructive hail? ____________________________ Unit 4, Section 2: Severe Weather 11 Name: Date: Period: Severe Weather Hazards: Flash Floods (F 98) Thunderstorms can produce torrential rains. Usually the rain does not last very long and cause no serious problems, however, sometimes heavy rains can last for hours. This can lead to a flash flood. A flash flood is a sudden rise in the water level of a stream, river, or a man-made drainage channel in response to extremely heavy rains. Flash floods can also occur when a brief but heavy rain falls over the entire area of a very small watershed. In some places, water might overflow stream banks and collect in low-lying places. The Link Between Flash Floods and Thunderstorms The life cycle of a thunderstorm cell is typically less than an hour. How could torrential thunderstorm rains last for many hours? Flooding usually results from more than one thunderstorm cell. Flooding thunderstorms are most likely in mountainous terrain where a persistent flow of humid air up a mountain slope can cause thunderstorm cells to develop and redevelop over and over. Outside of the mountains, another possible cause of flooding rains is a succession (or “train”) of slow-moving thunderstorm cells that mature over essentially the same geographic area. Thunderstorms move slowly when the steering winds in the middle and upper part of the troposphere are relatively weak. Watersheds A river plus all of its tributaries (smaller creeks that feed into the main river) drain a fixed geographical area, and that area is the drainage basin. Drainage basins are also called watersheds. Heavy rain falling on the upstream part of the drainage basin might cause flooding downstream in areas that received no rain. In other words, just because it is not raining where you are does not necessarily mean there is no flood danger. Where are flash floods most likely to occur? Flash floods may be more likely in an urban area than in the surrounding countryside. The reason for this is the rain in the country can seep into the soil. In the city, rain cannot seep into asphalt or concrete parking lots, roads, and driveways. Frozen soil is also impervious Unit 4, Section 2: Severe Weather 12 to water. Instead, the water runs off these surfaces into nearby streams or other drainage ways. Storm sewers also channel water from roads into streams. Streams receiving so much water at one time can overflow their banks quickly. Also, storm sewers sometimes clog or cannot handle excessive volumes of water and back up into the streets. The flash flood hazard is particularly serious in mountainous terrain where river valleys are narrow and deep. Stream level can rise very quickly because there are no broad areas next to the stream channel where the water can spread out. If the river valley also contains roads, campgrounds, or houses, a flood can be very destructive. When a river overflows its banks in a non-mountainous area, excess water spreads over a broad flat area adjacent to the river known as a floodplain. Floodwaters are usually shallower then they are in mountain valleys where floodplains are very narrow or nonexistent. Also, in non-mountainous areas, roads and buildings usually are not located as close to the river as they might be in the mountains. Safety Tips for Floods Flash floods can strike any time and any place with little or no warning. In mountainous or flat terrain, distant rain may be channeled into gullies and ravines, turning a quiet streamside campsite into a rampaging torrent in minutes. City streets can become rivers in seconds. Observe these flash flood safety rules. They could save your life. Keep alert for signs of heavy rain such as thunder and lightning, both where you are and upstream. Watch for rising water levels. Know where high ground is and get there quickly is you see or hear rapidly rising water. Don’t pitch your tent in a dry streambed. Be especially cautious at night; the danger is harder to recognize then. Do not attempt to cross flowing water that may be more than knee deep. If you have doubts, do not cross. Do not try to drive through flooded areas. If your vehicle stalls, abandon it and seek higher ground immediately. During threatening weather, listen to commercial radio or TV, or NOAA Weather Radio for weather watch and warning bulletins. Review Questions Unit 4, Section 2: Severe Weather 13 50. How long does one thunderstorm cell usually last? ____________________________ 51. How do thunderstorms cause flooding? ___________________________________________________________ ___________________________________________________________ 52. Where are flooding thunderstorms usually found? ___________________________________________________________ ___________________________________________________________ 53. Why are flash floods more likely in urban areas than in the countryside? ___________________________________________________________ ___________________________________________________________ 54. Why is it a problem if streams receive too much water at one time? ___________________________________________________________ ___________________________________________________________ 55. Why are flash flood hazards particularly serious in mountainous terrain? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 56. List four safety tips for floods. a. _________________________________________________________ b. _________________________________________________________ c. _________________________________________________________ d. _________________________________________________________ 57. When did the flash flood in the Big Thompson Canyon occur? ___________________ 58. What caused the flash flood in the Big Thompson Canyon? _____________________ ___________________________________________________________ ___________________________________________________________ 59. Why did the thunderstorm cells stay in the area? ______________________________ ___________________________________________________________ ___________________________________________________________ 60. How much rain did they receive to cause the flood? ___________________________ 61. What happened to people trying to escape in their cars? ________________________ ___________________________________________________________ Unit 4, Section 2: Severe Weather 14 62. What floodplain management strategies helped to prevent a greater loss of life during the 1997 Fort Collins flash flood? __________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Name: Date: Period: Severe Winds and Tornadoes (F 116) In the United States about 10,000 severe thunderstorms occur in an average year. About 10% of these produce tornadoes, and perhaps 1% of those tornadoes are violent and destructive. Downbursts and Tornadoes Downbursts that affect a relatively small area (4 km or less across) are known as microbursts. Microbursts cause sudden changes in wind speed and direction called wind shear, which can interfere with aircraft flight. Microbursts over short distances are particularly hazardous to aircraft taking off or landing because these winds typically affect only a part of the runway and are difficult to detect. Near the ground, downbursts winds blow outward from a central area, whereas winds produced by a tornado tend to swirl in circles. Microbursts can be deadly. Tornadoes threaten people and property mainly because of their exceptionally strong winds and powerful updrafts. Winds blowing hundreds of kilometers an hour knock over trees and power poles, toss cars around as if they were toys, and destroy even wellconstructed buildings. Another danger is flying glass and splintered lumber, which causes much of the injuries associated with tornadoes. In violent tornadoes, the updraft near the center of the system may top 160 km/hr (100 mph), strong enough to lift a house off its foundation. The most destructive tornadoes are large systems made up of two or more whirling masses of air (multi-vortex systems). The Fujita Scale Tornadoes intensity is rated on the Fujita scale (or F-scale) devised by the late Professor T. Theodore Fujita of the University of Chicago. The F-scale is based on rotational wind speeds estimated from property damage and classifies tornadoes as weak (F-0 or F-1), strong (F-2 or F-3), or violent (F-4 or F-5). Characteristics of Tornadoes Unit 4, Section 2: Severe Weather 15 Tornadoes come in variety of shapes, ranging from cylindrical cloud masses having nearly uniform lateral dimensions to long, slender rope-like pendants. Type of Tornado Weak Size of Tornado (width in meters) Wind Speed (km/h) Distance Traveled (km) Life Expectancy 100 m < 180 km/hr < 1.5 km 1-3 min > 100 m 500 km/hr 160 km + 2 hrs + Strong (Intense) Tornado paths are often erratic and produce a hopscotch pattern of destruction as the alternatively touch down and lift off of the ground. The average forward speed of a tornado is about 48 km/h (30 mph). Winds in the vast majority of Northern Hemisphere tornadoes blow in a counterclockwise direction when viewed from above. Strong tornadoes can occur in mountainous terrain. Each year, the United States can anticipate between 700 and 1100 tornadoes, with only about 1% rated as violent. By late February, the maximum tornado frequency is along the central Gulf states. In April, the maximum frequency shifts to the southeast Atlantic states. In May and June, the highest is usually over the southern Great Plains, and by early summer it has shifted to the northern Plains and the Great Lakes region. Tornado Development Most tornadoes develop in supercell thunderstorms. In a supercell thunderstorm, a powerful updraft interacts with horizontal winds to cause air to rotate about a vertical axis. The rotating air is called a mesocyclone. If conditions are right, the mesocyclone circulation narrows and grows downward towards Earth’s surface and the wind speed increases. Example: Ice skater bringing arms in during a spin increases spin speed. You can see tornadoes because humid air expands and cools as it is drawn inward toward the low-pressure center of the whirling system. Cooling of the air causes water vapor to condense into droplets which is what you are able to see. If the funnel cloud strikes the ground, dust and debris are drawn into its circulation, and the system is described as a tornado. About 60% of mesocyclones produce tornadoes. Unit 4, Section 2: Severe Weather 16 Most supercell thunderstorm develop as part of a squall line in the warm, humid air ahead of a well-defined cold front. A key requirement for bringing contrasting cold and warm air masses together is a strong cyclone. A cyclone is a stormy weather system that is plotted on a weather map as L or Low for the relatively low air pressure at its center. Here, cyclone does not refer to a hurricane or tornado, but rather a large weather system in which surface winds blow counterclockwise and inward. They can affect up to one third of the eastern half of the United States at one time. Where and When are Tornadoes Common? Although severe thunderstorms are tornadoes have been reported in every state, they are most frequent in a north-south belt in the center of the nation, stretching from east Texas northward through Oklahoma, Kansas, Nebraska, and into southeast South Dakota. This area is known as “tornado alley.” The frequency of tornadoes in that region is greater than anywhere else in the world! Although tornadoes have been reported in every month of the year, they are most common in spring and early summer. At this time of year, the temperature contrast in between air masses is greatest, meaning that the potential for severe thunderstorm development is highest. Predicting Tornadoes Tornadoes are hard to predict because they are such short-lived systems. When conditions in the atmosphere appear favorable for the development of a severe thunderstorm or tornado, the National Weather Service’s Storm Prediction Center in Norman, Oklahoma, issues a severe thunderstorm or tornado watch. When Unit 4, Section 2: Severe Weather 17 one has been spotted, the weather service issues an update letting people know what areas are affected. The National Weather Service radar uses the Doppler effect to determine how air is moving within a thunderstorm and can provide the public with advance warning of the development of tornadoes. The Doppler effect refers to a shift in frequency of sounds waves or electromagnetic waves when a source is moving. Example: Ambulance sirens sounds high pitched as they approach then lower as they move away. In this way, meteorologists can track particles in the atmosphere as they move and detect mesocyclones before they turn into tornadoes. Tornado Safety Tips If you have been warned of a tornado or see one approaching, take shelter immediately. Tornado shelter should have a steel-frame or a reinforced concrete building. If you are at home, go into the basement. If there is no basement, go to a small interior room such as a closet or hallway in the center of the house on the lowest floor. Seek shelter under a mattress or a rigid piece of furniture. Do not go near windows or outside walls and do not open windows or exterior doors. If you are in a large building, go to an interior hallway on the lowest floor. Avoid areas with wide free-span roofs such as auditoriums and gymnasiums. Never try to outrun a tornado in a car. It is hard to predict what direction they are going. Never seek shelter in a mobile home, car, or other motor vehicle. Review Questions 63. What type of damage can a tornado cause? ________________________________ ___________________________________________________________ ___________________________________________________________ 64. What speed can the updraft in a violent tornado reach? _______________________ 65. What is tornado intensity rated on? _______________________________________ 66. What is this scale based on? _______________________________________ ___________________________________________________________ Unit 4, Section 2: Severe Weather 18 67. What is a weak tornadoes path, life expectancy, and wind speed? ___________________________________________________________ ___________________________________________________________ 68. What is an intense tornadoes path, life expectancy, and wind speed? ___________________________________________________________ ___________________________________________________________ 69. In what direction does a tornado usually rotate? _____________________________ 70. Why do tornadoes create a “hopscotch” pattern of destruction? ___________________________________________________________ ___________________________________________________________ 71. What is the average forward speed of a tornado? _____________________________ 72. How many tornadoes can the U.S. anticipate every year? ______________________ 73. What causes the funnel-shaped cloud that extends down from the parent cumulonimbus cloud?___________________________________________________ ___________________________________________________________ ___________________________________________________________ 74. Where do most supercell thunderstorms develop? ___________________________________________________________ ___________________________________________________________ 75. Where are severe thunderstorms and tornadoes most frequent? ___________________________________________________________ ___________________________________________________________ 76. What is that frequent tornado area called? ___________________________________ 77. During what time of year are tornadoes most common and why? ___________________________________________________________ ___________________________________________________________ 78. Why are tornadoes hard to predict? ___________________________________________________________ ___________________________________________________________ 79. How does the National Weather Service use the Doppler effect? ___________________________________________________________ ___________________________________________________________ Unit 4, Section 2: Severe Weather 19 80. What does the Doppler radar monitor? ___________________________________________________________ ___________________________________________________________ 81. What are the colors displayed on a Doppler radar and what do they indicate? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 82. List three tornado safety tips. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Unit 4, Section 2: Severe Weather 20