How Can You Predict When Severe Weather Will Occur?

4.7 Explain How Can You Predict When Severe Weather Will Occur? In this Learning Set, you have been exploring how winds and oceans affect weather and climate. You then used what you know to explain how all the factors that you have been investigating can help you predict weather. However, for the Big Challenge, you will need to be able to predict in advance if severe weather will occur. Unless you know that there will be severe weather, you cannot prepare for it adequately. Before you can make any predictions about severe weather, you need to know a little more about the different types of severe weather. The following reading provides you with information about severe weather events. As you read, make notes on the following: the type of severe weather event that can occur the factors that determine if this is a severe weather event how fronts are involved the type of clouds that are associated with the severe weather the dangers involved with the severe weather event WW 230

4.7 Explain Severe Weather Severe weather can cause tremendous damage and loss of life. Consider Hurricane Katrina, which struck the central Gulf Coast states in August 2005. Katrina was the most destructive hurricane to strike the United States to date, devastating the cities of New Orleans, Louisiana; Mobile, Alabama; Gulfport, Mississippi; and surrounding areas. The storm s winds reached 190 km (120 mi) from the storm s center. Wind speeds were as high as 210 km/h (130 mph). By the time it was over, an estimated 1836 people had lost their lives. Damages were estimated at $81.2 billion. This storm had tremendous social, economic, and political effects on the country. The loss of life and the damage would have been even greater if meteorologists had not been able to predict that the massive storm would strike near New Orleans. Because they predicted that a storm was coming, many people left the affected areas before the storm arrived. Meteorologists know that severe weather often results when there are large differences in the air pressure of colliding air masses. Because the atmosphere is complex, however, they cannot always make exact predictions. Small variations in pressure and changes in wind can cause unexpected effects. The predictions that meteorologists make are based on data collected over many years. They know that certain conditions make severe weather likely. When conditions indicate that severe weather is possible, the National Weather Service will issue a watch alert, for example a tornado watch. When severe weather has been spotted or is considered likely to occur soon, the alert is upgraded to a warning, for example a tornado warning. These alerts are issued by NOAA weather radio reports and by announcements at the NOAA and National Weather Service Internet sites. WW 231 WEATHER WATCH

Snowstorms, like rainstorms, are caused when moist air is pushed upward. Rainstorms and Snowstorms You know that rainstorms and high winds can occur when air masses collide at fronts. Storm clouds also form when two fronts collide at an occluded front. When a warm front meets a cold front, rainstorms or snowstorms can result. In the summer, light or moderate rain can fall for several days at a stationary front. On the other hand, if a cold front is moving rapidly, it can push huge masses of warm air rapidly into the higher levels of the troposphere. When this happens, heavy rains can occur suddenly, dropping large amounts of water in a very short time. As this water collects in low areas, it may not drain away as more water flows in. The National Weather Service sometimes issues flash flood alerts when conditions indicate that a sudden storm might occur. Meteorologists look at water table levels to predict flash floods. A water table indicates the depth of groundwater that moves under the ground. When the water table is low, much of the rain that falls can be absorbed by the ground, so a severe amount of rain may not produce flooding. But large amounts of rain combined with high levels of the water table can lead to flash floods. For coastal storms, meteorologists also consider the tides. At high tide, coastal flooding is much more likely than at low tide. Another alert that the National Weather Service might issue is a winter weather advisory or warning. Snowstorms are similar to rainstorms that occur at lower temperatures. Like rainstorms, snowstorms are caused when moist air is pushed upward. The rising water vapor cools and forms crystals of ice. These crystals often fall as snow. Depending on temperatures in different parts of the atmosphere, precipitation can also fall as hail, sleet, or freezing rain. Each form of precipitation can cause problems if it occurs in great amounts very quickly. In the winter, heavy snowfalls can produce dangerous travel conditions and lead officials to close schools and businesses. WW 232

4.7 Explain Meteorologists use several factors to determine when a snowstorm will arrive and how much snow will accumulate. Some fronts move at a steady, predictable speed. The speed of the front can be used to calculate when the storm will arrive. A fast-moving front will pass through an area quickly, so there may be less precipitation compared to a storm produced by a slow-moving front. However, if a storm covers a larger area, it may produce large amounts of rain or snow even though the storm is moving quickly. Blizzards A blizzard is a severe weather event involving low temperatures, strong winds, and heavy snow. The key factor that separates a blizzard from other snowstorms is a steady wind of 56 km/h (35 mph). Blizzards occur in winter when large pressure differences produce strong winds combined with heavy precipitation. In a blizzard, snow carried by the wind reduces visibility so that you can see no farther than 400 m (.25 mi) for at least 3 hours. Sometimes, an event similar to a blizzard can occur without precipitation because the wind blows snow that is already on the ground. These events usually occur in places that have large, open, flat areas of land. Blizzards are dangerous for several reasons. Traveling by vehicle is difficult because the blowing snow makes it hard to see other cars and trucks. Ice and snow make roads and sidewalks slippery. Also, blizzards pose a risk for people who do not have access to shelter. The combination of low temperatures and high winds can cause your body to lose heat rapidly. Any exposed skin can quickly be damaged by frostbite. Blizzards, like ice storms, can bring down power lines and cause power failures. This is most likely when the snow is wet and heavy. Repair crews may have trouble reaching damaged areas until roads are safe for travel. Ice storms occur when rain falls and then freezes immediately on trees, on cars, and on power lines. Because so many power lines can be damaged, it may take weeks to restore power to hundreds of thousands of people. Blizzards can dump more than a foot of snow in less than 24 hours. WW 233 WEATHER WATCH

One type of weather pattern meteorologists look for in predicting blizzards is called a Nor easter (or Northeaster). This is a large storm system off the east coast of the United States with winds that come from the northeast. The winds bring cold air from the north, and they also pick up moisture from the Atlantic Ocean and bring it inland. As the storm rotates counterclockwise, the ocean keeps feeding the storm with additional moisture that leads to heavy accumulations of snow. Meteorologists watch the track of the storm. If the storm moves inland, it will produce less snowfall because it runs out of moisture. This kind of storm can produce precipitation for 24 hours or longer. Stop and Think 1. What factors cause rainstorms and snowstorms? 2. What factors cause damage in a rainstorm? 3. How are blizzards different from snowstorms? 4. How do meteorologists predict that a blizzard is coming? 5. What are three dangers posed by blizzards? Lightning is more likely to strike in some areas of the country. On this map, constructed from satellite images and data, the areas in red are most likely to experience lightning strikes. WW 234

4.7 Explain Thunderstorms Thunderstorms are heavy rainstorms with lightning and thunder. They usually develop at a cold front where a fast-moving cold air mass pushes a warm air mass up very rapidly. Cumulus clouds are formed. As more warm air rises, the developing cumulonimbus cloud grows larger and taller. The rising air, called an updraft, feeds more and more moisture into the cloud. Eventually, the cloud can no longer contain all the water droplets and ice crystals. Heavy rain or hail falls to Earth s surface. As the precipitation falls, it causes air to rush downward in downdrafts. Because air is rushing both up and down, the water droplets and ice crystals collide frequently. This shifts electrical charges within the cloud. A cloud has a positive area (near the top) and a negative area (near the bottom). When too much negative charge builds up in one area of the cloud, it can travel to a place with a less negative charge as lightning. The sound associated with lightning is called thunder. Lightning bolts can travel from cloud to cloud. They can also travel from a cloud to the ground. Lightning can even travel from the ground to a cloud! Although a storm may be local, lightning can travel great distances before striking the ground. Lightning also can occur after the rain stops if excess electrical charges have not been released. These are just a few reasons that lightning can be very dangerous and unpredictable. A single cloud can produce a thunderstorm, but cumulonimbus clouds often occur in a cluster of clouds that stretches along a cold front. The thunderstorm produced by a single cloud might last only 30 minutes, but when a cluster of clouds passes through an area, there may be thunderstorms for many hours. Thunderstorms are difficult to predict precisely because the clouds form so quickly, so the storm cannot be seen coming from miles away. These storms are most likely to occur in spring and summer, and they are also more likely in the afternoon or early evening. WW 235 WEATHER WATCH

The mammatus clouds pictured above are often associated with tornadoes. Tornadoes Sometimes violent thunderstorms result in tornadoes. Tornadoes form in conditions known as cyclones. A cyclone is an area of low pressure in which surface winds rotate (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). A cyclone has updrafts which can contain warm, rising air. As cooler air moves in to take the place of the warm rising air, the air begins to spin. Some of the rotating air gets trapped between two horizontal layers of air coming in from opposite directions. This starts the air spinning in a horizontal plane. The updrafts associated with thunderstorms can cause the spinning column of air to tip out of the horizontal plane. The spinning column, becomes vertical, and a tornado is formed. Under the right conditions, a strong tornado can develop within minutes. As the air spins faster, the pressure decreases further. The low pressure draws in more air from higher-pressure areas, feeding the force of the tornado. Tornadoes usually last only a few minutes, but these can be the most destructive minutes in anyone s life. Winds inside the funnel can reach speeds of 500 km/h (300 mph) and are capable of picking up incredibly large objects, including automobiles and even houses. The average speed of the tornado as it moves across the ground is around 50 km/h (30 mph), but it can move at speeds up to 110 km/h (70 mph). As a tornado moves across the landscape, it can change direction quickly and unpredictably. Geography and seasonal factors help meteorologists to make forecasts. Tornadoes occur in many parts of the world, and they are common in three parts of the United States the Great Plains, the Mississippi Valley, and Florida. Because tornadoes rely on big differences in the air pressure of air masses, they occur most frequently in the spring and summer. WW 236

4.7 Explain Tornadoes are difficult to predict for the same reasons that thunderstorms are difficult to predict. Tornadoes, like thunderstorms, do not last a long time, and they also cover a small area. To make predictions, meteorologists use radar to detect strong updrafts in thunderstorms. They also look for certain types of clouds, such as the mammatus clouds shown in the photograph on the previous page. When meteorologists see conditions that may produce tornadoes, they issue alerts and warnings to help the public prepare in case these devastating events do take place. Hurricanes Hurricanes form over the ocean. For a hurricane to develop, the ocean waters must have a surface temperature of at least 81 F. At this temperature, warm moist air can rise very quickly, creating a lowpressure area. You have read that air tends to move from areas of high pressure to areas of low pressure. Cool high-pressure air rushes in to the low-pressure region formed by the rising warm air. The cool air causes water vapor to condense, which typically leads to the formation of a ring of thunderstorms. Hurricanes are most common during summer and early fall, when the right conditions are most likely. The conditions that lead to hurricanes include a lot of solar heating and a lot of moisture in the air. That is why hurricanes form over tropical oceans. Warm seawater evaporates and the warm air above the water rises because it is now less dense than the air above it. A column of low-pressure air forms. Winds increase as air rushes into the low-pressure area. The Coriolis effect causes Hurricanes are generally about 500 km (300 mi) across, but they can be as large as 1100 km (675 mi) in diameter. WW 237 WEATHER WATCH

the storm to start to rotate. As the low-pressure area in the center of the circling winds passes over more warm water, more water evaporates and rises into the storm, which keeps the pressure low and the winds strong. Hurricanes in the Northern Hemisphere usually begin by traveling from east to west. As the storms approach the coast of North America, they shift to a more northerly direction. In the Southern Hemisphere, the storms also may travel westward at first, but then they generally shift toward the south. The path of an individual hurricane is irregular and often difficult to predict. Hurricanes eventually move toward higher latitudes, where there is cooler air and water and less moisture. Under those conditions, the storm weakens. If the hurricane moves over land, it will die out quickly because it no longer has a steady source of heat and water vapor. Heavy rains, thunderstorms, and even tornadoes may continue even after the storm weakens and is no longer classified as a hurricane. Meteorologists take many factors into account to make predictions about hurricanes. They use computer simulations to compare the current storm to tropical storms and hurricanes that developed in the past under similar conditions. They plot the track of the storm and determine its size. They enter other data into the computer simulations, including air temperature, ocean temperatures, air pressure, humidity, and wind speeds. Because hurricanes affect such large areas, hurricane forecasts can save more lives than forecasts for any other type of severe weather event. Stop and Think 1. What kinds of clouds are associated with thunderstorms? How do updrafts contribute to the growth of these clouds? 2. Why do thunderstorms produce lightning? 3. What dangers do thunderstorms pose? 4. Under what conditions do tornadoes form? 5. Under what conditions do hurricanes form? How does the formation of hurricanes differ from the formation of tornadoes? 6. How are thunderstorms, tornadoes, and hurricanes alike? How do they differ? 7. How do meteorologists know if a thunderstorm, tornado, or hurricane is coming? WW 238

4.7 Explain Reflect 1. What weather factor is common to all the forms of severe weather? 2. How might meteorologists know when severe weather is likely to occur? 3. Describe one experience you have had with severe weather. Were there high winds, lots of precipitation, or thunder and lightning? 4. What types of severe weather occur near where you live? Why? 5. In what ways might a hurricane be less predictable than a snowstorm? In what ways might a hurricane be more predictable than a snowstorm? 6. If a particular type of severe weather event is less predictable, how do you think this should affect the forecasts that the National Weather Service issues? 7. What different types of severe weather would you expect in different seasons? In different areas of the country? Explain You have already made claims about the causes of weather and weather changes. Now you will work with your group to develop the answer to this question: How can you predict when severe weather will occur? Use a new Create Your Explanation page, and work with your group. Begin by recording the question you are answering. Then work with your group to develop a claim that answers the question. Begin by thinking about your claim. You now know something about the atmospheric conditions that result in several types of severe weather. Add what you know now about predicting severe weather. Then add evidence and science knowledge that support your revised claim. Make sure your claim, evidence, and science knowledge are consistent with one another. Then, with your group, revise your explanation statement. It should include how you know which atmospheric conditions are the right ones to use for predicting each type of severe weather. Keep track of any group disagreements. Record evidence to support your claim, and record science knowledge that supports your claim. Develop an explanation statement. Your explanation statement should present the science that tells why your claim is trustworthy. WW 239 WEATHER WATCH

Communicate Share Your Explanation When it is your group s turn to share your explanation, have different members from your group report each of the following: the group s claim, the evidence that supports it, the science knowledge that supports your claim, the explanation statement, and any disagreements your group had. As you listen, notice any evidence or science knowledge other groups used that is relevant to answering the question, How can you predict when severe weather will occur? Be sure you understand what others are reporting and that you agree. If you do not understand something or think a group has not been accurate, respectfully ask a question or offer advice. Revise Your Explanation As a class, develop a class claim and explanation statement. Record the evidence and scientific knowledge that support this claim and explanation statement. Then identify its strengths and weaknesses, and identify what else you need to know to make your claim and explanation statements more complete and accurate. What s the Point? Like any weather event, severe weather is caused by differences in temperature and pressure in the atmosphere. Uneven heating of Earth s surface causes these differences. Severe weather occurs when air masses with very different pressures meet. If the conditions are right, strong winds form. Usually, some kind of precipitation occurs at the same time as warm air moves upward and cools. Because of the strength of the wind and the amount of precipitation, severe weather events can be very hazardous. It is important to be aware of the conditions that can lead to severe weather in your area. When these conditions occur, you must be alert and prepared to protect yourself and your family from the hazards. WW 240