Weather Basics: El Niño, Tornadoes, and Tropical Cyclones (Chapters 4, 7, 11, 19)

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Weather Basics: El Niño, Tornadoes, and Tropical Cyclones (Chapters 4, 7, 11, 19) SUPPLEMENT 8 Weather Is Affected by Moving Masses of Warm and Cold Air Weather is the set of short-term atmospheric

Examples of atmospheric conditions include temperature, pressure, moisture content, precipitation, sunshine, cloud cover, and wind direction and speed.

1 Weather Basics: El Niño, Tornadoes, and Tropical Cyclones (Chapters 4, 7, 11, 19) SUPPLEMENT 8 Weather Is Affected by Moving Masses of Warm and Cold Air Weather is the set of short-term atmospheric conditions typically those occurring over hours or days for a particular area. Examples of atmospheric conditions include temperature, pressure, moisture content, precipitation, sunshine, cloud cover, and wind direction and speed. Meteorologists use equipment mounted on weather balloons, aircraft, ships, and satellites, as well as radar and stationary sensors, to obtain data on weather variables. They then feed these data into computer models to draw weather maps. Other computer models project the weather for a period of several days by calculating the probabilities that air masses, winds, and other factors will change in certain ways. Much of the weather we experience results from interactions between the leading edges of moving masses of warm or cold air. Weather changes as one air mass replaces or meets another. The most dramatic changes in weather occur along a front, the boundary between two air masses with different temperatures and densities. A warm front is the boundary between an advancing warm air mass and the cooler one it is replacing (Figure 1, left). Because warm air is less dense (weighs less per unit of volume) than cool air, an advancing warm front rises up over a mass of cool air. As the warm front rises, its moisture begins condensing into droplets, forming layers of clouds at different altitudes. Gradually the clouds thicken, descend to a lower altitude, and often release their moisture as rainfall. A moist warm front can bring days of cloudy skies and drizzle. A cold front (Figure 1, right) is the leading edge of an advancing mass of cold air. Because cold air is denser than warm air, an advancing cold front stays close to the ground and wedges underneath less dense warmer air. An approaching cold front produces rapidly moving, towering clouds called thunderheads. As a cold front passes through, we may experience high surface winds and thunderstorms. After it leaves the area, we usually have cooler temperatures and a clear sky. Near the top of the troposphere, hurricaneforce winds circle the earth. These powerful Figure 2 A jet stream is a rapidly flowing air current that moves west to east in a wavy pattern. This figure shows a polar jet stream and a subtropical jet stream in winter. In reality, jet streams are discontinuous and their positions vary from day to day. (Used by permission from C. Donald Ahrens, Meteorology Today, 8th ed. Belmont, Calif.: Brooks/ Cole, 2006) winds, called jet streams, follow rising and falling paths that have a strong influence on weather patterns (Figure 2). Weather Is Affected by Changes in Atmospheric Pressure Changes in atmospheric pressure also affect weather. Atmospheric pressure results from molecules of gases (mostly nitrogen and oxygen) in the atmosphere zipping around at very high speeds and hitting and bouncing off everything they encounter. Anvil top Warm air mass Warm front surface Cool air mass Cold air mass Cold front surface Warm air mass Figure 1 Weather fronts: a warm front (left) arises when an advancing mass of warm air meets and rises up over a mass of denser cool air. A cold front (right) forms when a moving mass of cold air wedges beneath a mass of less dense warm air. SUPPLEMENT 8 S47

Atmospheric pressure is greater near the earth s surface because the molecules in the atmosphere are squeezed together under the weight of the air above them.

2 Atmospheric pressure is greater near the earth s surface because the molecules in the atmosphere are squeezed together under the weight of the air above them. An air mass with high pressure, called a high, contains cool, dense air that descends toward the earth s surface and becomes warmer. Fair weather follows as long as this high-pressure air mass remains over the area. In contrast, a low-pressure air mass, called a low, produces cloudy and sometimes stormy weather. Because of its low pressure and low density, the center of a low rises, and its warm air expands and cools. When the temperature drops below a certain level where condensation takes place, called the dew point, moisture in the air condenses and forms clouds. If the droplets in the clouds coalesce into larger drops or snowflakes heavy enough to fall from the sky, then precipitation occurs. The condensation of water vapor into water drops usually requires that the air contain suspended tiny particles of material such as dust, smoke, sea salts, or volcanic ash. These so-called condensation nuclei provide surfaces on which the droplets of water can form and coalesce. Every Few Years Major Wind Shifts in the Pacific Ocean Affect Global Weather Patterns An upwelling, or upward movement of ocean water, can mix the water, bringing cool and nutrient-rich water from the bottom of the ocean to the surface where it supports large populations of phytoplankton, zooplankton, fish, and fish-eating seabirds. Figure 7-2 (p. 142) shows the oceans major upwelling zones. Upwellings far from shore occur when surface currents move apart and draw Movement of surface water water up from deeper layers. Strong upwellings are also found along the steep western coasts of some continents when winds blowing along the coasts push surface water away from the land and draw water up from the ocean bottom (Figure 3). Every few years in the Pacific Ocean, normal shore upwellings (Figure 4, left) are affected by changes in weather patterns called the El Niño Southern Oscillation, or ENSO (Figure 4, right). In an ENSO, often called simply El Niño, prevailing tropical trade winds blowing east to west weaken or reverse direction. This allows Wind Diving birds Upwelling Fish Zooplankton Figure 3 A shore upwelling occurs when deep, cool, nutrient-rich waters are drawn up to replace surface water that has been moved away from a steep coast by wind flowing along the coast toward the equator. Phytoplankton Nutrients Surface winds blow westward Drought in Australia and Southeast Asia Winds weaken, causing updrafts and storms EQUATOR AUSTRALIA Warm waters pushed westward SOUTH AMERICA AUSTRALIA Warm water flow stopped or reversed SOUTH AMERICA Warm water Thermocline Cold water Warm water Warm water deepens off South America Thermocline Cold water Normal Conditions El Niño Conditions Figure 4 Normal trade winds blowing east to west cause shore upwellings of cold, nutrient-rich bottom water in the tropical Pacific Ocean near the coast of Peru (left). A zone of gradual temperature change called the thermocline separates the warm and cold water. Every few years a shift in trade winds known as the El Niño Southern Oscillation (ENSO) disrupts this pattern. Trade winds blowing from east to west weaken or reverse direction, which depresses the coastal upwellings and warms the surface waters off South America (right). When an ENSO lasts 12 months or longer, it severely disrupts populations of plankton, fish, and seabirds in upwelling areas and can alter weather conditions over much of the globe (Figure 5). S48 SUPPLEMENT 8

3 El Niño Drought Unusually high rainfall Unusually warm periods Figure 5 Typical global weather effects of an El Niño Southern Oscillation. During the ENSO, huge waves battered the coast in the U.S. state of California and torrential rains caused widespread flooding and mudslides. In Peru, floods and mudslides killed hundreds of people, left about 250,000 people homeless, and ruined harvests. Drought in Brazil, Indonesia, and Australia led to massive wildfires in tinder-dry forests. India and parts of Africa also experienced severe drought. A catastrophic ice storm hit Canada and the northeastern United States, but the southeastern United States had fewer hurricanes. (Data from United Nations Food and Agriculture Organization) Data and Map Analysis 1. How might an ENSO affect the weather where you live or go to school? 2. Why do you think the area to the west of El Niño suffers drought? November 10, 1997 Warm water of El Niño the warmer waters of the western Pacific to move toward the coast of South America, which suppresses the normal upwellings of cold, nutrient-rich water (Figure 4, right). The decrease in nutrients reduces primary productivity and causes a sharp decline in the populations of some fish species. A strong ENSO can alter the weather of at least two-thirds of the globe (Figure 5) especially in lands along the Pacific and Indian Oceans. Scientists do not know for sure the causes of an ENSO, but they know how to detect its formation and track its progress. La Niña, the reverse of El Niño, cools some coastal surface waters, and brings back upwellings. Typically, La Niña means more Atlantic Ocean hurricanes, colder winters in Canada and the northeastern United States, and warmer and drier winters in the southeastern and southwestern United States. It also usually leads to wetter winters in the Pacific Northwest, torrential rains in Southeast Asia, lower wheat yields in Argentina, and more wildfires in Florida. Figure 6 uses satellite data to show changes in the locations of masses of warm and cold water in the Pacific Ocean during an El Niño (top) and a La Niña (bottom). February 27, 1999 Cool Cool water of La Niña Warm Figure 6 Locations of flowing masses of warm and cold water in the Pacific Ocean during an El Niño (top) and a La Niña (bottom). (Data from Jet Propulsion Lab, NASA) SUPPLEMENT 8 S49

4 Tornadoes and Tropical Cyclones Are Violent Weather Extremes Sometimes we experience weather extremes. Two examples are violent storms called tornadoes (which form over land) and tropical cyclones (which form over warm ocean waters and sometimes pass over coastal land). Tornadoes or twisters are swirling funnelshaped clouds that form over land. They can destroy houses, cause other serious damage, and kill people in areas where they touch down on the earth s surface. The United States is the world s most tornado-prone country, followed by Australia. Tornadoes in the plains of the midwestern United States usually occur when a large, dry, cold-air front moving southward from Canada runs into a large mass of humid air moving northward from the Gulf of Mexico. Most tornadoes occur in the spring and summer when fronts of cold air from the north penetrate deeply into the midwestern plains. As the large warm-air mass moves rapidly over the more dense cold-air mass, it rises swiftly and forms strong vertical convection currents that suck air upward, as shown in Figure 7. Scientists hypothesize that the rising vortex of air starts spinning because the air near the ground in the funnel is moving more slowly than the air above. This difference causes the air ahead of the advancing front to roll or spin in a vertically rising air mass or vortex. Figure 8 shows the areas of greatest risk from tornadoes in the continental United States. Severe thunderstorm Tornado forms when cool downdraft and warm updraft of air meet and interact Warm moist air drawn in Descending cool air Highest risk Lowest risk Tropical cyclones are spawned by the formation of low-pressure cells of air over warm tropical seas. Figure 9 shows the formation and structure of a tropical cyclone. Hurricanes are tropical Rising warm air Severe thunderstorms can trigger a number of smaller tornadoes Rising updraft of air Figure 7 Formation of a tornado or twister. Although twisters can form at any time of the year, the most active tornado season in the United States is usually March through August. Meteorologists cannot tell us with great accuracy when and where most tornadoes will form. Figure 8 States with very high and high tornado risk in the continental United States. (Data from NOAA) Data and Map Analysis 1. How many states have areas with a risk factor of 25 or higher? How many have areas with a risk factor of 20 or higher? 2. What is the level of risk where you live? If you live in a far western state, does this mean you are guaranteed never to see a tornado in your area? cyclones that form in the Atlantic Ocean; those forming in the Pacific Ocean usually are called typhoons. Tropical cyclones take a long time to form and gain strength. As a result, meteorologists can track their paths and wind speeds and warn people in areas likely to be hit by these violent storms. For a tropical cyclone to form, the temperature of ocean water has to be at least 27 C (80 F) to a depth of 46 meters (150 feet). A tropical cyclone forms when areas of low pressure over the warm ocean draw in air from surrounding higher-pressure areas. The earth s rotation makes these winds spiral counterclockwise in the northern hemisphere and clockwise in the southern hemisphere (Figure 7-3, p. 142). Moist air warmed by the heat of the ocean rises in a vortex through the center of the storm until it becomes a tropical cyclone (Figure 9). The intensities of tropical cyclones are rated in different categories based on their sustained wind speeds: Category 1: kilometers per hour (74 95 miles per hour); Category 2: kilometers per hour ( miles per hour); Category 3: kilometers per hour ( miles per hour); Category 4: kilometers per hour ( miles per hour); and Category 5: greater than 249 kilometers per hour (155 miles per hour). The longer a tropical cyclone stays over warm waters, the stronger it gets. Significant hurricane-force winds can extend kilometers ( miles) from the center, or eye, of a tropical cyclone. Figure 10 shows the change in the average surface temperature of the global ocean between 1871 and Note the rise in this temperature since These higher temperatures, especially in tropical waters, may explain why S50 SUPPLEMENT 8

5 4 Rising winds exit from the storm at high altitudes. The calm central eye usually is about 24 kilometers (15 miles) wide. Gales circle the eye at speeds of up to 320 kilometers (200 miles) per hour 3 2 Warm moist air 1 Moist surface winds spiral in toward the center of the storm. Figure 9 Formation of a tropical cyclone. Those forming in the Atlantic Ocean usually are called hurricanes; those forming in the Pacific Ocean usually are called typhoons. the average intensity of tropical cyclones has increased since With the number of people living along the world s coasts increasing, the danger to lives and property has risen dramatically. The greatest risk from hurricanes in the continental United States is along the gulf and eastern coasts, as shown in Figure 11 (p. S52). Hurricanes and typhoons kill and injure people and damage property (Figure 8-18, p. 177) and agricultural production. Sometimes, however, the long-term ecological and economic benefits of a tropical cyclone exceed its shortterm harmful effects. For example, in parts of the U.S. state of Texas along the Gulf of Mexico, coastal bays and marshes normally are closed off from freshwater and saltwater inflows. In August 1999, Hurricane Brett struck this coastal area. According to marine biologists, it flushed out excess nutrients from land runoff and swept dead sea grasses and rotting vegetation from the coastal bays and marshes. It also carved out 12 channels through the barrier islands along the coast, allowing huge quantities of seawater to flood the bays and marshes. This flushing of the bays and marshes reduced brown tides consisting of explosive growths of algae that had fed on excess nutrients. It also increased growth of sea grasses, which serve as nurseries for shrimp, crabs, and fish and provide food for millions of ducks wintering in Texas bays. Production of commercially important species of shellfish and fish also increased. Temperature ( C) Year Temperature ( F) Figure 10 Change in global ocean temperature from its average baseline temperature from 1880 to (Data from National Oceanic and Atmospheric Administration) Data and Graph Analysis 1. In 1900, the global ocean temperature dropped from its baseline temperature by how many degrees? (Give answer in both Centigrade and Fahrenheit.) 2. Since about what year after 1980 have global ocean temperatures consistently increased (with all changes being positive)? What has been the highest temperature increase since then, and in about what year did it happen? SUPPLEMENT 8 S51

6 Figure 11 The number of hurricanes expected to occur during a 100-year period in the continental United States (based on historical data from U.S. Geological Survey) Data and Map Analysis 1. What is the degree of risk where you live or go to school? 2. How many states include an area with some risk of hurricanes? Number of hurricanes expected to occur during a 100-year period More than Hawaii Alaska Puerto Rico S52 SUPPLEMENT 8

Warm Up Vocabulary Check

Warm Up Vocabulary Check Surface current Coriolis Effect global winds upwelling Gulf Stream deep current climate El Nino convection current continental deflection 1.The apparent curving of the path of