Wet and Dry Periods on the Great Plains

 

Paragraph 1: By and large, the climate of the North American prairie is unreliable. As rival air masses interact with each other over the plains, they keep the atmosphere in a state of more-or-less constant flux, so that the weather oscillates from extreme to extreme. But there are also times when the climate system seems to get stuck — wet spells (periods) for example, when the rain refuses to stop; or dry spells of months — or years — when the clouds seem dry as parchment and the air fills with dust. 

 

1. The word “rival” in the passage is closest in meaning to 

 

O unstable 

 

O competing

 

O rising 

 

O growing 

 

 

 

2. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. 

 

O But the dry spell following a particularly rainy period can last for months, or even years. 

 

O Extended periods of rain are called wet spells while prolonged periods of drought are called dry spells. 

 

O Although dry periods can last for months or even years, the wet periods seem to go on even longer. 

 

O At times, however, the weather stays very wet or very dry for long periods of time.

 

 

 

 

 

Paragraph 2: These persistent weather patterns also tend to be widespread, affecting significant parts of the Great Plains for prolonged periods. The droughts of the 1930s, for example, occasionally flared out to singe the entire continent, but they were at their most intense across the Great Plains grasslands. Some parts of the High Plains in Oklahoma and Texas experienced eight consecutive years of drought, from 1933 to 1940. More recently, in the late 1980s, a three-year drought parched the entire northern plains and fuelled disastrous forest fires in Yellowstone National Park. Yet five years later, some of these same areas were in full flood, as torrential rains pounded the western Midwest and sent both the Missouri and upper Mississippi rivers spilling over their banks. 

 

 

 

3. According to paragraph 2, all of the following statements about the persistent weather patterns that affect the Great Plains region are true EXCEPT: 

 

O They usually affect more than a small portion of the region. 

 

O They are limited to the Great Plains region.

 

O They are characterized by either very little or very heavy rainfall. 

 

O They can have disastrous effects on the region. 

 

 

 

 

 

Paragraph 3: Why do the prairies suffer these violent climatic spasms? The answer to this question appears to lie halfway around the world, in a region somewhere between Australia and Peru. There, in the equatorial waters of the South Pacific Ocean, weather patterns that will ultimately affect the prairies begin to brew. According to recent research (much of it conducted during and after the 1980s drought), there appears to be a powerful connection between the surface temperature of the South Pacific and the amount of rain that ultimately falls on the Great Plains. 

 

 

 

 

 

Paragraph 4: It is well-known that when surface currents in the equatorial South Pacific are unusually warm — during a so-called El Nino event — huge volumes of water evaporate from the ocean into the air. When this massive upwelling of warm, moist air spirals into the atmosphere, it stirs up the circulation so energetically that the disturbance quickly spreads and the airflow over North America is disrupted. As the air masses swirl into new alignment across the continent, a persistent low-pressure system may eventually develop somewhere over the Great Plains. (Exactly where this system sits determines which parts of the continent are most affected.) Since air flows toward areas of low pressure, this low-pressure system acts as a kind of pump that sucks in humid air from the Gulf of Mexico. Moist and warm, heavy with rain, this tropical influx dumps tons of precipitation onto the plains. This is what happened in 1993, when an intense El Nino off the coast of Peru translated into a persistent low-pressure system over the Great Plains and catastrophic flooding along the Red and Missouri rivers. 

 

 

 

4. According to paragraph 4, an El Nino event increases the amount of rain on the Great Plains by 

 

O increasing the amount of evaporation from rivers and lakes across North America 

 

O disrupting the air currents that normally bring dry air from neighboring regions 

 

O creating a persistent low-pressure system that pulls in moist, tropical air

 

O increasing the temperature and humidity of air over the Gulf of Mexico 

 

 

 

5. It can be inferred from paragraph 4 that in 1993, off the coast of Peru, the 

 

O humidity was unusually low 

 

O waters were unusually warm

 

O air was unusually cool 

 

O air currents were unusually calm 

 

 

 

 

 

Paragraph 5: When the surface currents in the equatorial South Pacific are exceptionally cold — when a La Nina sets in — the effects are reversed. This time, the cooldown of the water sharply reduces evaporation from the ocean into the air. And just as the influx of moist air during an El Nino sends the atmosphere into a spin, so does the relative lack of this input during a La Nina. Again, the atmospheric readjustment rapidly extends across North America, but this time, as the air masses flow into their new alignment, a massive whorl of high (rather than low) pressure develops somewhere over the mid-continent. If a low-pressure system is a semi-vacuum that draws air into itself, a high-pressure system is an atmospheric downspout that forces air out across the land. Pacific air that nudges onto the plains from the west is deflected north, where it drops its meager rainfall on the boreal forest, while air from the Gulf of Mexico is sent careening north and east, carrying its precious supply of moisture to Atlantic Canada. Parts of the prairies, meanwhile, are left to scorch in the sun until, somewhere out in the South Pacific, conditions shift and the climatic spell is broken. 

 

 

 

6. The word “meager” in the passage is closest in meaning to 

 

O cool 

 

O little

 

O welcome 

 

O remaining 

 

 

 

7. According to paragraph 5, what happens to the moisture from the Gulf of Mexico during a La Nina? 

 

O It remains in the gulf until climatic conditions shift. 

 

O It contributes to the buildup of a low-pressure system. 

 

O It falls on Atlantic Canada rather than on the Great Plains.

 

O It is pushed to the north, where it falls on the boreal forest. 

 

 

 

8. Which of the following best describes the organization of the passage? 

 

O Certain patterns are identified, and the processes that create these patterns are described.

 

O Two patterns are contrasted, and they are shown to be caused by the same process. 

 

O A phenomenon is identified, and two competing explanations of that phenomenon are presented. 

 

O A proposed explanation of a certain phenomenon is challenged by presenting a case for which the explanation does not work. 

 

 

 

 

 

Paragraph 3: Why do the prairies suffer these violent climatic spasms? ■The answer to this question appears to lie halfway around the world, in a region somewhere between Australia and Peru. ■There, in the equatorial waters of the South Pacific Ocean, weather patterns that will ultimately affect the prairies begin to brew. ■According to recent research (much of it conducted during and after the 1980s drought), there appears to be a powerful connection between the surface temperature of the South Pacific and the amount of rain that ultimately falls on the Great Plains.■ 

 

 

 

9. Look at the four squares [■] that indicate where the following sentence could be added to the passage. 

 

When the temperature is unusually high, rainfall is heavy, and when the temperature is unusually low, there is drought.

 

Where would the sentence best fit? Click on a square [■] to add the sentence to the passage. 

 

 

 

 

 

10. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the 3 answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Drag your choices to the spaces where they belong. To review the passage, click on View Text. 

 

The Great Plains region of North America suffers from extended periods of extreme weather.

 

 

 

Answer Choices 

 

O Much of the research that led to an understanding of the weather patterns on the Great Plains was conducted after a severe drought in the 1980s. 

 

O The climatic phenomena known as El Nino and La Nina explain the persistent low- or high-pressure systems that develop over the Great Plains.

 

O A long-lasting low-pressure system over the Great Plains causes heavy rainfall, while a long-lasting high-pressure system causes a drought.

 

O The weather patterns on the Great Plains depend on the amount of evaporation from the equatorial waters of the South Pacific Ocean.

 

O A low-pressure system develops when the Pacific Ocean is cooler than the Gulf of Mexico, and a high-pressure system develops when the opposite conditions prevail. 

 

O When the atmospheric turbulence dies down and the weather stabilizes, that means that conditions have changed from an El Nino to a La Nina. 

 

 

 

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