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Warm air mass

Warm air mass

Anvil top

Cool air mass

Cold air mass

Warm fro

nt surfa

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Cold front surface

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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 and cold air driven largely by uneven heating of Earth’s surface by the sun (see Figure 6-4, p. 159). 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, with flat, anvil-like tops.

As a cold front passes through, it may cause high surface winds and thunderstorms. After it leaves the area, it usually results in cooler temperatures and a clear sky.

Near the top of the troposphere, hurricane-force winds circle Earth. These powerful 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.

Atmospheric pressure is greater near 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 slowly toward Earth’s surface and becomes warmer. Because of this warming, condensation of moisture usually does not take place and clouds usually do not form. Fair weather with clear skies follows for as long as the 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,

WEATHER BASICS

FIGURE 1 A warm front (left) occurs 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.

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Wind

Movement ofsurface water

Wind

Movement ofsurface water

Upwelling

Nutrients

UpwellingZooplankton

Fish

Phytoplankton

Zooplankton

Fish

Phytoplankton

Nutrients

Diving birdsDiving birds

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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 combine into larger drops or snowflakes heavy enough to fall from the sky, 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 merge.

Every Few Years Major Wind Shifts in the Pacific Ocean Affect Global Weather Patterns

Ocean currents can have a strong effect on the weather. Water moves vertically and horizontally from one area of the ocean to another. An upwelling, or upward movement of ocean water, can mix upper levels of seawater with lower levels, bringing cool and nutrient-rich water from the bottom of the ocean to the warmer surface where it supports large populations of phytoplankton, zooplankton, fish, and fish-eating seabirds.

Figure 6-3 on page 158 shows the oceans’ major upwelling zones. Upwellings far from shore occur when surface currents move apart and draw 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, normal shore upwellings in the Pacific Ocean (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 winds called tropical trade winds blowing east to west weaken or reverse direction. This allows 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.

When an ENSO lasts 12 months or longer, it can severely disrupt populations of plankton, fish, and seabirds in

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 Ahrens, Meteorology Today, 8E. © 2007 Cengage Learning.

FIGURE 3 A shore upwelling occurs when deep, cool, nutrient-rich waters are drawn up to replace surface water moved away from a steep coast by wind flowing along the coast toward the Equator.

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EQUATOR

AUSTRALIA

Warm water

Cold water

Thermocline

SOUTHAMERICA

AUSTRALIA SOUTHAMERICA

Warm water

Cold water

Thermocline

Warm water deepens offSouth America

Normal Conditions El Niño Conditions

Warm waterspushed westward

Surface windsblow westward

Drought inAustralia andSoutheast Asia

Winds weaken,causing updraftsand storms

Warm water �owstopped or reversed

Drought

Unusually high rainfall

Unusually warm periods

El Niño

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upwelling areas. A strong ENSO can also alter weather conditions over at least two-thirds of the globe (Figure 5)—especially in lands along the Pacific and Indian Oceans. Scientists do not know exactly what causes an ENSO, but they do 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 brings 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.

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 areas).

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.

Compiled by the authors using data from United Nations Food and Agriculture Organization.

FIGURE 5 Typical global weather effects of an El Niño–Southern Oscillation. How might an ENSO affect the weather where you live or go to school?

WEATHER BASICS

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Severe thunderstorm

Descending cool air

Rising warm air

Risingupdraft of air

Severe thunderstorms can trigger a number of smaller tornadoes

Tornado forms when cool downdraft andwarm updraft of air meet and interact

Warm moist air drawn in

Highest risk

Lowest risk

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Tornadoes, or twisters, are swirling, funnel-shaped clouds that form over land. They can destroy houses and cause other serious damage in areas where they touch down on Earth’s surface. The United States is the world’s most tornado-prone country, followed by Argentina and Bangladesh. Tornadoes occur on every continent except for Antarctica.

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 warm, 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 Great Plains and the Midwest.

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 6. Scientists hypothesize that the interaction of the

cooler air nearer the ground and the rapidly rising warmer air above causes a spinning, vertically rising air mass, or vortex.

Figure 7 shows the areas of greatest risk from tornadoes in the continental United States.

Tropical cyclones are spawned by the formation of low-pressure cells of air over warm tropical seas. Figure 8 shows the formation and structure of a tropical cyclone. Hurricanes

are tropical 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 to a depth of 46 meters (150 feet). A tropical cyclone forms when areas of low pressure over the warm

FIGURE 6 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 yet forecast exactly where tornadoes will form at any given time, but research on tornados and advanced computer modeling can help them identify areas at risk each day for the formation of these deadly storms.

FIGURE 7Comparison of the relative risk of tornados across the continental United States.

Compiled by the authors using data from NOAA.

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ocean draw in air from surrounding higher-pressure areas. Earth’s rotation makes these winds spiral counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere (see Figure 6-4). 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 8).

The intensities of tropical cyclones are rated in different categories, based on their sustained wind speeds in kilometers per hour, or kph: Category 1, 119–153 kph (74–95 miles per hour, or mph); Category 2, 154–177 kph (96–110 mph); Category 3, 178–209 kph (111–130 mph); Category 4, 210–249 kph (131–155 mph); and Category 5, greater than 249 kph (155 mph). The longer a tropical cyclone stays over warm waters, the stronger it gets.

Significant hurricane-force winds can extend 64–161 kilometers (40–100 miles) from the center, or eye, of a tropical cyclone.

Hurricanes and typhoons kill and injure people and damage property and agricultural production. Sometimes, however, the long-term ecological and economic benefits of a tropical cyclone exceed its short-term harmful effects.

For example, in parts of Texas along the Gulf of Mexico, coastal bays and marshes normally receive very limited freshwater and saltwater inflows because of barrier islands and unique natural formations. In August 1999, Hurricane Brett struck this coastal area. According to marine biologists, the storm 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 fresh seawater to flood the bays and marshes.

This flushing of the bays and marshes reduced brown tides consisting of explosive growths of algae feeding 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’s bays. Production of commercially important species of shellfish and fish also increased.

FIGURE 8 The formation of a tropical cyclone. Those forming in the Atlantic Ocean are called hurricanes; those forming in the Pacific Ocean are called typhoons.

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WEATHER BASICS

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Moist surface windsspiral in toward thecenter of the storm.

Warmmoist air

Gales circle the eye at speedsof up to 320 kilometers

(200 miles) per hour

The calm centraleye usually is about

24 kilometers(15 miles) wide.

Rising winds exitfrom the storm athigh altitudes.

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