and the Rockies all standing in their path, the Pacific westerlies are forced to rise, cool, and drop their moisture as they pass. By the time the winds flow down over the plains, they are almost devoid of rain. As they move across the western grasslands, they pick up humidity and carry it to the well-watered eastern forests. (Sometimes, as the winds swoop over the mountains, they whip themselves up into disturbances—known as Alberta Lows or Colorado Cyclones, depending on where they occur—that sweep across the plains, carrying a splattering of rain or a dusting of snow.)
Once the westerlies get past the mountains, there is nothing to stop them—except for the invisible resistance of other air masses. The prairies’ open spaces are a playground for the winds, drawing in not only mild, dry air from the Pacific but also colder and even drier air from the tundra and polar seas. As this Arctic air floods south, it meets moist, tropical air flowing north from the western Gulf of Mexico. A typical weather diagram for the Great Plains would show Arctic air pushing down from the north, tropical air swinging in from the south, and wrung-out Pacific air wedged between them like the point of an eastward-facing arrow. Arctic air dominates in the winter, sometimes forcing itself all the way to Texas as a stinging “blue norther” and, occasionally, pushing on south to the isthmus of Tehuantepec at the very tip of Mexico. In the spring, the balance of power is reversed, as the gulf air mass gains strength and surges north, sometimes carrying tropical heat and humidity all the way up to the Canadian prairies. The rained-out Pacific westerlies, with their meager stock of moisture, make themselves felt throughout the year, especially during the dry months of fall and winter. See Map 5: Major Air Masses Affecting the Great Plains.
In spring and summer, in particular, this picture is complicated by an influx of warm, dry-as-bone air that blows across the southern plains from the southwestern deserts. The interplay of these “four strong winds” produces the distinctive precipitation patterns of the Great Plains Grasslands. For instance, winter is a relatively arid season across most of the plains not only because cold air cannot hold much moisture but also because of the strong seasonal influence of dry air from the north and west. As a rule, less than one-third of the year’s precipitation falls between October and March, when these air masses exert their strongest influence. The other roughly 70 percent of the year’s moisture is received during the April-to-September growing season. Without this well-timed gift, the Great Plains would be a prickly expanse of cactus and other desert plants.
Much of the all-important spring-and-summer rainfall is generated when tropical air surges north and runs into Pacific and Arctic systems moving across the prairies to the east and south. Where the air masses collide, the lighter, warmer air from the tropics is forced up, cooling as it climbs, condensing to form clouds, and ultimately losing its moisture as general rain showers. Violent thunderstorms also frequently develop along these collision zones, or fronts, as the unstable tropical air rises up into towering, super-energized cumulonimbus clouds that glower over the landscape before releasing their humidity as hail or pounding downpours.
Because the contending air masses often meet in mid-continent, frontal thunderstorms are most common in the middle of the plains, in and around Colorado, Wyoming, South Dakota, Nebraska, and Kansas. But storms can also develop locally, without the clash of opposing weather systems to set them off. All it takes is a mass of warm, moist air and something to send that air spiraling up through the atmosphere. This lift-energy usually comes from the summer sun, which blazes down through cloudless prairie skies to heat the ground. Heat then radiates out of the soil into the surface air, causing it to rise, rotate, and mount upward to form a rain-filled thunderhead. In the dry western plains, one-third of the year’s precipitation can fall in a single hour from one of these spectacular cloudbursts.
GLOBAL “TELECONNECTIONS”
BY AND LARGE, the prairie climate is reliably unreliable. As the 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 climatic system seems to get stuck. “Wet spells,” for example, when the rain refuses to stop. “Dry spells” of months—or years—when the clouds seem dry as parchment and the air fills with dust.
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, between 1933 and 1940. Little more than a decade later, the central and southern plains—from the Mississippi to the Rockies and from Colorado to Texas—were again stricken by a severe drought that persisted from 1952 to 1957. The Canadian Prairie provinces were hit hard in 1961. Then, in the late 1980s, a three-year drought parched the entire northern plains and fueled disastrous forest fires in Yellowstone National Park. During the growing season of 1988, when the crisis was at its worst, many parts of the prairies were hotter and drier than they had been at any time during the Dirty Thirties.
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 the Upper Mississippi rivers spilling over their banks. By the time the waters receded, twenty-six people were dead.
Why do the prairies suffer these violent climatic spasms? Part of the answer to this question may 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 eventually affect the prairies begin to brew. Recent research suggests that there is a link between the surface temperature of the South Pacific and the amount of precipitation that ultimately falls on the Great Plains, particularly during the winter and early spring.
So far, no one knows exactly how all the complex linkages in this world-wide “teleconnection” work. And global influences, however stupendous, are not the only factors involved. Often, extreme conditions linger on the Great Plains long after the systems that triggered them have dispersed. A wet spell seems to breed more wet weather; a dry spell appears to breed more drought. But how could weather patterns possibly perpetuate themselves? The answer turns out to be surprisingly obvious. When precipitation is plentiful, water accumulates in the soil. As plants draw on this moisture to grow, they release water vapor into the air. This water vapor, in turn, combines with humidity that has evaporated directly from the earth, and these exhalations rise together to form clouds. Thus rain in the soil begets rain showers. What’s more, both rainfall and evapotranspiration (the release of water vapor from plants) have a cooling influence that helps to moderate temperatures and keep the evaporative demand within comfortable limits.
After a prolonged dry spell, by contrast, the cycle grinds to a stop. Plant growth slows and the rate of transpiration declines. So too does cooling evaporation from the soil. The ground and the surface layers of air sizzle in the sun, as a hot, dry land gets hotter and drier. (A case in point is the drought of the 1930s, which seems to have been intensified and prolonged by farming methods that left the soil exposed to the parching wind and robbed the system of what little moisture it held.) Eventually humid air from the south or the west returns to the scene, bringing welcome relief and restoring the climate to its own eccentric sense of normalcy.
GRADIENTS OF GRASS
THESE PROLONGED EPISODES of drought have been the making of the Great Plains Grasslands. Drought sucks moisture out of the soil, beginning at the surface and gradually burning farther down. If the dry spell is brief, the deep stores of moisture remain untapped, but if the evaporative demand persists, even the subsoil becomes parched and cracked. As a result, deeply rooted trees can cope without rain for several years by drawing water from underground, but they are doomed to defeat when drought reaches their root zone. Meanwhile, the grass lies patiently around the dying trunks, ready and able to spring back to life when the new rains finally come.
Long-term patterns
