Climate of the Great Plains Region of the United States

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln

Great Plains Quarterly Great Plains Studies, Center for

1987

Climate of the Great Plains Region of The United States

Norman J. Rosenberg University of Nebraska's Center for Agricultural Meteorology and Climatology

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Rosenberg, Norman J., "Climate of the Great Plains Region of The United States" (1987). Great Plains Quarterly. 344. https://digitalcommons.unl.edu/greatplainsquarterly/344

This Article is brought to you for free and open access by the Great Plains Studies, Center for at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Great Plains Quarterly by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. CLIMATE OF THE GREAT PLAINS REGION OF THE UNITED STATES

NORMAN ]. ROSENBERG

T he climate of the Great Plains is extreme ture. Thus a soil surface will heat quickly when and variable. A wide range of weather condi­ exposed to the midday sun but water tempera­ tions can occur within the period of a day, ture rises only slowly. Conversely, at night, the from one day to the next, from season to soil cools rapidly as it loses heat to the season, and from year to year. There are two atmosphere but water cools slowly. The pres­ key reasons for this situation: (1) the greatest ence of large quantities of water vapor in the portion of the Plains is remote from any major air over maritime regions provides yet another body of water and (2) air masses of differing buffering effect, since water vapo~ absorbs characteristics alternate frequently in their thermal radiation and creates a greenhouse dominance of the region. type of condition by trapping energy emitted at the surface. Where water vapor content is great the heat emitted by earth is effectively A CONTINENTAL CLIMATE absorbed and the rate of cooling is slowed. Water bodies are the great buffers in the Much of the Great Plains region is remote exchange of energy between earth and atmo­ from the one major body of water that sphere and exert a modifying and moderating influences it-the Gulf of Mexico. The fact of influence on adjacent land areas as well. Water its remoteness is the reason that the climate is will absorb five times as much heat as will soil described as continental. Continentality dic­ with the same attendant change in tempera- tates that the diurnal range of temperature (night to day) and the annual range of Norman J. Rosenberg is George Holmes Professor temperature (winter to summer) will be great. and Director of the University of Nebraska's The Great Plains (particularly the northern Center for Agricultural Meteorology and Clima­ portion) has the most distinctly continental tology. An authority on the incidence and impact climate in the United States. of drought worldwide, he has also conducted field research on modifying the microclimate of crops. Continentality and Temperature in the Great Plains

[GPQ ( (\X'intcr 1986): 22-32.J The map in Figure 1a shows the normal 22 CREA T PLAINS C U1--.1A TE 23 daily minimum temperature in January for the summer. The northern Great Plains are h otter United States. Note the daily minimum in than the northeastern part of the country. At Amarillo in the T exas Panhandle is the same Steele, North Dakota, for example, the maxi­ as it is near Detroit, Michigan, fi ve hundred mum summer temperature recorded is 121° F. miles to the north. Settlers who moved The lowest temperature recorded is _50 0 F. for westward onto the plains encountered climates an annual range of 171 ° F. In many parts of that were considerably colder in the winter. the northern Great Plains an annual tempera­ Figure 1b, on the other hand, shows that ture range of 140 0 F is common. maximum temperatures in the northern Great Plains are as high in summer as they are in the Air Masses southeastern United States. The normal daily maximum temperature in north central South Large bodies of air are conditioned contin­ Dakota is about 90° F in July-the same as it is ually by the surfaces over which the air moves. in Jacksonville, Florida. Air that has been in contact with tropical seas Winters in the northern Great Plains are will be warm and mo ist; air over polar or near­ the coldest in the contiguous forty-eight states. polar regions will be cool and dry, since the The southern Great Plains are as hot as the capacity of air to h old water vapor is small at southeastern part of the United States in low temperatures. Further, little evaporation

100 90

Normal Daily Minimum Temperature (0 F), January

FIG. lao Nonnal daily minimum temperature (OF) January. (Climatic Atlas of U.S. lower ho.lf of p. 12) ~4 liKE.A T r LAl:\S \.2 l'ARTERL Y, Wl:-nER 19S ,

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Daily Maximum Temperature (0 F), July

FIG. lb. Nannal daily maximum t"mperature (OF) Jul y. (Climatic Atlas of U.S. upper half af p. 13)

occurs where the sun provides little energy to Condensation and precipitation occur and the convert liquid or frozen water to vapor. air mass dries out. When it descends the The region between the Rocky Mountains eastern slopes of the Rocky Mountains to the and the Appalachian chain is open to motion western Great Plains, this maritime Pacific air of air masses that originate in the north is warm and quite dry. Thus we say that the (continental polar air masses) and over the western Plains lie in the "rainshade" of the Gulf of Mexico (maritime tropical air masses). Rockies. Still a fourth source of air sometimes The orientation of the North American moun­ dominates the Plains, air warmed over the dry tain chains does not separate the air masses southwestern deserts of the United States and from one another as do the Alps in Europe northern Mexico. Frequent outbursts of this and the Himalayas in Asia. Instead, the air warm, dry air in spring and summer create masses meet and clash with great frequency. A strong evaporative stress on crops growing in third type of air mass enters the Great Plains the region. periodically, air that is conditioned over the Pacific Ocean-moist but cooler than Gulf air Air Masses, Fronts, and Precipitation because of its more northerly origin. Maritime Pacific air masses cool as they rise to cross the Rapid changes in the temperature, humid­ western mountain ranges, and the capacity of ity, cloudiness, windspeed, and wind direction the air to hold water vapor decreases sharply. that occur frequently in the Great Plains are GREAT PLAINS CLl~lA TE 25 due to the passage of air masses with different western Montana, it is twelve inches. characteristics: where two or more air masses The advance of frontal systems may be meet, fronts are formed. Fronts are defined as accompanied by light general rains or by the transition zones or interfaces between two violent storms in which torrential rains fall for air masses of different density. The passage of hours or sometimes days. Thus, it is the fronts across the Plains is irregular. Continen­ variability in moisture content of the maritime tal polar air masses moving south encounter tropical air, the path of its movement, and the maritime tropical air masses advancing north­ intensity of the impact with other air masses ward from the Gulf. The maritime air is that produce the great variability in spacial lighter, because of its higher temperature, and and temporal distribution of precipitation in more moist. It is forced aloft and, cooling as it the Great Plains. rises, loses water by precipitation. Thus, most Warming of ground surfaces under un­ of the precipitation decreases with distance stable moist air may also lead to precipitation from the source of the vapor-the Gulf of in isolated thunderstorms. These occur after Mexico. Figure 2 shows how sharply the total formation of towering cumulus clouds in annual precipitation drops off with distance summer and are characteristically accompa­ from the Gulf. Near the Gulf Coast annual nied by thunder and lightning. The distribu­ precipitation is forty-eight inches. In north- tion of rainfall from this type of storm is more

~,2 TO 48

Normal Annual Total Precipitation (inches)

FIG. 2. Normal annual total precipitation (inches) (Climatic Atlas of u.s. p. 43) 2(, GREAT PL.A.INS Ql.'ARTERL Y, \V[l\'TER 1987

random than that from the frontal systems. country by the National Weather Service. The annual patterns of precipitation in the Four locations are compared in the table. On Great Plains change with the seasons. The an annual basis, evaporation at Mead in distribution of snow in the north and rain in eastern Nebraska is less than at Phoenix, the south is fairly uniform from east to west in Arizona, or at Davis, California, but is more winter, but winter precipitation contributes than at Coshocton, Ohio. The western loca­ only a small portion of the annual total. The tions have longer growing seasons, and Phoe­ bulk of the annual precipitation fall s during nix is essentially a desert location. Coshocton the growing season from April to September. is a bit farther south than Mead and in a more Autumn weather is usually drier in most of the humid region. Of the four locations, the Great Plains than it is farther east-a definite standard deviation of the annual evaporation advantage in the harvest. (a measure of variability) is greatest at Mead. It is important to realize that the foregoing This is due to the high degree of day-to-day describes a normal or average annual pattern and year-to-year variability in Great Plains of rainfall distribution. Rainfall may be scant weather caused by the continual interchange during one season and excessive in the next. of air masses. Drought may occur one year and floods the Another interesting feature of Table 1 is next. This irregular distribution of rainfall the data on maximum reported daily evapo­ over space and time in the Great Plains is one transpiration (ET). Evaporranspirarion is the of the major constraints to the development of sum of water evaporated directly from the soil the region. surface and water evaporated at the leaves of plants through which the water has passed Evaporarion-an Indicaror 0/ Climaric from the soil (transpiration). Evapotranspira­ Variabiliry tion rates are as high in the east central Great Plains as in the southwestern desert location. Rapid frontal movements through the The rate at Mead, Nebraska, is also greater plains region account for the rapid and fre­ than it is in the Mediterranean climate of quent changes in cloudiness, air temperature, Davis or the inland climate of Coshocton. humidity, windspeed, and wind direction. In Thus, we see that, at least at times, the turn, each of these factors determines rate of integrated effect of the Great Plains climate­ evaporation. Thus, study of the evaporation high temperatures, low humidity, strong rates gives an indication of the variability in winds, and clear skies-leads to a very rapid the overall climatic condition. Table 1 gives consumption of water by growing plants. data on the annual amount of evaporation Figure 3 shows the mean annual evapora­ from special pans maintained throughout the tion from lakes in the United States. The lines

TABLE 1 PREDICTED EVAPORATION FROM CLASS A PANS AND REpORTED MAXIMUM ET AT FOUR LOCATIONS IN THE CONTINENTAL UNITED STATES (mm) (AFTER ROSENBERG, 1969)

Location: Mead Coshocton Davis Phoenix (Nebr.) (Ohio) (Calif.) (Ariz.)

Annual pan evaporation 1,524 1,117 1,778 2,267 Stand. deviation (annual) 264 86 107 173 May-October evaporation 1,173 861 1,333 1,840 Daily mean May-October 6.41 4.70 7.28 10.05 Maximum reported daily ET 12.02 9.14 11.56 12.20 GREAT PLAIN S C UMATE 27

100 90

Mean Annual Lake Evaporation (inches)

FIG. 3. Mean annual lake evaporation (inches). (Climatic Atlas of U.S. p. 63-plare 2) representing areas of equal evaporation have a amount of water available through precipita­ northwest to southeast orientation in the tion. northern Great Plains from Montana through All of these factors point out that the Great Nebraska. South of Nebraska, the evaporation Plains is a region of deficit water supply. The increases sharply but in a different regional atmospheric demand for water from growing pattern. By comparing this figure with Figure plants is strong snd generall y exceeds the 2, we may see how, on an annual basis, the supply of natural precipitation. It is for this evaporative demand exceeds the rainfall al­ reason that irrigation has been of such major most everywhere in the Great Plains region. importance in the Great Plains region. Only At Lincoln, Nebraska, for example, the aver­ through the development of irrigation can age annual rainfall is about thirty-four inches. farmers have a reasonable degree of confidence The annual evaporation from free water is in their ability to produce consistently high about forty-two inches. Therefore, if water yields of crops and to survive periodic drought. were always available, evaporation would exceed rainfall by eight inches. At Scottsbluff, CUMATIC HAZARDS AND Nebraska, the annual rainfall is about sixteen ADAPT A TIONS IN THE PLAINS inches. The annual evaporation from free water is estimated at forty-four inches. In Thus one can readily see that the Great western Nebraska, as in the western portion of Plains posed for settlers severe climatic prob­ most of the Great Plains, the atmospheric lems that to this day restrict the economic and demand for water vapor far exceeds the social development of the region. 28 CREAT l'L\I0JS QUi\RTERL y, \\'1:\TER IL):i7

Extreme T emperatHres winter creates other hazards to human and animal life. G ood shelter and fuel for h eating The frequency with which very hot days have always been necessary in the northern occur in all parts of the plains is considerably Pl ains. Even in southern Texas, occasional greater than it is at the same latitudes in the northers drop temperatures to below freezing eastern portion of the United States. Forty for a few days at a time, d amaging sensitive days a year of temperatures higher than 90° F subtropical crops such as grapefruit. Blizzards, occur at Lincoln, Nebraska. At New York although infrequent, create other dangers. City (about the same latitude) only five to ten People caught out-of-doors can lose all sense of such days occur each year. Great heat imposes direction during a blizzard and some have a strong evaporative demand on growing frozen to death only yards away from shelter. crops, which not even irrigation can fully Herds of cattle drift aimlessly in blizzards and alleviate. Human comfort is affected b y high serious losses occur as the animals wander into temperatures and domestic animals are also dangerous terrain. sensitive. Large losses of livestock are reported in the Plains each year during hot spells, Short Growing Season particularly when humidity is also high. The polar climate of the Grear Plains Figure 4 maps the average length of the

100 90

I ~L.~ ------FIG. 4. Mean length of the freeze-free period. Da ys between last 32°F temperature in spring and first 32°F temperature in autumn. (Climatic Atlas of U.S. p. 31) (,REr\ T r L Al~S CL1~lA TE 29 freeze-free period in the continental United (or more than five Years. Weakly fo und tree­ States. This is the number of days between the ring p:ltterrl:-: that indicated the occurrence of last occurrence of 32° F temperatures in spring one drought period of thirty-eight Years. and the first such occurrence in fall. Growing Long-term records of annual precipitation season in the northernmost part of the Great \'irtually anywhere in the Plains shol\' extreme Plains is limited to between three and four \"Car to vear \ari3bilitv. Such records also show months. The choice of crops is necessarily very that drv vea r:; h:we tended to occur very often restricted. In the southern plains, the growing in series of t\\'o. three or more vears-although season may last eleven months each year. these series are often interspersed with occa­ Except for the area adjacent to the Gulf and sional \\"l't \'Cars. Thus, for example, in the except for the mountainous areas in the period since system atic weather records have Appalachian chain, it is correct to say that the been kept in the Great Plains, a number of growing season is shorter anywhere in the drought periods of greater and lesser severity Plains than it is at the same latitude in the h:l\'C hee n cxperienced. W. c:. Palmer has, eastern United States. Crops adapted to th rough \\"eKansas is typical. Drought that had adapted to the shorter growing season. major impact on the agricultural production in Even after crops have been planted in western Kamas occurred in 1894, when as spring and before the harvest in fall, frosts may manv as 90 percent of the settlers ahandoned occur and cause considerable damage. The their farms in some areas. Drought occurred extensive cropping systems (large areas, low again in 1913, following the dry years of 1910 capital input) of the Plains region do not and 1911 and the abnormally wet year of 1912. generally justify expensive measures for frost Between August 1932 and Octoher 1940, protection, so frost is just one more hazard thirtv-four months of extreme drought oc­ with which the farmer must learn to live. curred in western Kansas. In 1952 and 1953, all thirty-one counties in western Kansas were RCCl!rrent Drol1ght declared :1 drought disaster area bv the federal government, and the dry conditions extended Drought can be defined in various ways. through 1955. Similar histories can be recited Any extended period of dry weather that leads for other parts of the Great Plains during these to a measurable loss of crop production can sa me periods. Recurrent drought is part of the correctly be called dTOl1ght. When such periods Great Plains environment and its inevitability become so long and the shortage of soil is, unfortunately, forgotten when the weather moisture so critical as to cause abandonment is good. of fields already sown, or when crop cover becomes so sparse, because of lack of moisture, Strong and Persistent Wind as to permit the erosion of the soil surface by wind, drought can be catastrophic in the The Great Plains is the windiest region of Great Plains region. the country-open as it is to the free sweep of Drought in the Plains is not just a modern air masses from north and south. Because the phenomenon. H. E. Weakly's dendrochrono­ terrain is relatively flat and smooth and the logical studies (tree-ring measurement and land is largely cropped or in pasture grass, the interpretation) on specimens of red cedar and frictional (orces that reduce wind speed in ponderosa pine have identified short periods of other regions are less active in the Great dry years and, less frequently, droughts lasting Pl ains. In winter, northerly winds predominate ),1 CiREA.T [,L:\I~S QL1.ARTERL y, WINTER 1987 a:; far south as Texas. In summer, southerly sweeper of the atmosphere. The Great Plains \\'inds predominate as far north as Montana. owes the relative cleanliness of its air to the Hot southerl y winds during the growing fact that manmade and natural pollutants are season create stress o n plants and the low moved out of the region with great rapidity humidity of southwest winds increases the because of the almost continual action of the e\'aporativc demand on crops still more. winds. Strong \\'inds from any direction can cause mec hanical damage to crops such as breakage Other Ha zards or blowing over (lodging). In winter, northerly \\inds increase the loss of heat from buildings. Frontal storms over the Great Plains as well Animals also seek shelter from the strong as other regions of the country frequently northerly winds during winter. For these trigger severe wind and, occasionally, torna­ re,bons, earl y settlers used European tech­ does . The region of most intense tornadic niques of windbreak plantations to protect activity is centered in east central Kansas and their fa rm steads, animals, and fields. In the north central Oklahoma. The frequency of ]930s and ea rl y 19405 , the Shelterbelt Project hailstorms is greatest in an area centered in the was initiated by the federal government to southwestern corner of the Nebraska Panhan­ protect the entire Great Plains region by the dle and in adjacent parts of Wyoming and planting of thousa nds of miles of tree wind­ Colorado. As many as nine days with hail­ breaks. It was thought at the time that, as well storms occur each year, on the average. The as providing immediate protection for fields frequency of hailstorm days drops to about five and farms, this extensive network of trees per year in the Wyoming, Kansas, Colorado, might beneficially change the climate of the and Nebraska areas surrounding the "hail Plains. The Shelterbelt Project was abandoned center." The eastern edge of the Great Plains during World War II for lack of funds and commonly has three to four days with hail­ labor to complete it, although windbreak storms each year. The high frequency of hail in plantings continue less systematically today. the western plains region explains the fact that Wind erosion has been a serious problem hail insurance rates in that region are the on the Pla ins, probably from times well before highest in the United States. Hail can be settlement. During the drought of the 1930s, particularly devastating to wheat growers in particularly, farming methods that left the so il the western High Plains because the storms are bare at certain times of the year led to serious common during late spring and early summer wind erosion and dust storms. The periodic when the crop is ripening and the impact of a plowing of marginal lands in the semiarid belts hailstorm will shatter heads of wheat, making of the western Great Pl ains also contributed to harvesting virtually impossible. Cultivation of the problem. Modern methods of soil erosion the sugar beet is one means of adaptation to control (strip-cropping, stubble-mulching, in­ the great hail hazard in western Nebraska, creasing surface roughness) have been effective eastern Colorado, and Wyoming. Sugar beets in minimizing wind erosion damage, but these recover remarkably well from hailstorms, improved practices are not used universally. which sometimes strip the plant of virtually all The Great Plains settler treated the wind as its leaves. an enemy, but it is also a source of power. Windmills once dotted the Plains to pump Aov ANT AGES OF THE water, and, before rural electrification, to drive GREAT PLAINS CLIMATE simple generators. With the energy shortage before us and with engineering improvements While it is true that drought, severe in windmill design, a windmill renaissance may weather, strong winds, and extreme heat and be coming on the plains. Wind is also the cold impose serious restrictions on the devel- GREAT PLAI0JS CLIMATE 11 opmental possibilities of the Great Plains constraints to the stability of the agricultural region, it is also true that the region enjoys production in the region. Late spring and early distinct climatic advantages. The intensity of fall frosts are frequent but unpredictable. solar radiation at any given latitude increases There is a significant risk that crops will not from the East Coast to the Rocky Mountains. have a long enough season for optimum This trend occurs most clearly across the growth. Droughts of greater or lesser severity plains region because of the increasing eleva­ are a regular feature of the plains climate. The tion of the land and the reduced frequency of history of the region and its flow of population cloud cover. The air in the Great Plains 111 and out are closely linked to the incidence regions, because of a generally low humidity of drought. content, is also less turbid (opaque). Thus, Adaptation to the plains climate has re­ crops that require intense illumination for quired ingenuity, persistence, and fortitude on development are well adapted to the western the part of the peoples who have ventured into Great Plains. it. Earthen shelters and sod houses for protec­ Another advantage of the Great Plains tion from the cold; shelterbelts for protection climate is the relatively dry autumn season. of homes, animals, and fields from the damag­ The probability of significant rainfall drops off ing effects of severe wind; use of windmills for rapidly in September and October in most power to pump water; introduction of short parts of the Plains. This makes the use of season crop varieties or varieties that extend mechanized harvesting equipment more effi­ the season by overwintering; soil terracing, cient than it is, generally, in the more humid stubble-mulching and minimum tillage prac­ East. There are times, of course, when rain and tices to minimize water and soil erosion; the early snowfall make harvesting very difficult, introduction of irrigation-all arc important but generally the harvest progresses easily. adaptations that have been made. Humidity decreases from east to west across Optimists in the past have proposed that the United States to the Great Plains and man's works would alter the plains climate­ decreases with distance from the Gulf. The low but rainfall docs not "follow the plow" as Dr. relative humidity suppresses many types of Samuel Aughey of the University of Nebraska plant fungal diseases that are common in the proposed in the early 1870s, nor do shel­ East. The potato crop in western Nebraska, for terbelts modify and moderate the climate as example, is more easily protected against "late­ President Franklin D. Roosevelt's Committee blight" (a fungal disease) than it is in more on the Future of the Great Plains suggested in easterly locations. 1936. Neither does it seem reasonable to expect that the influence of large-scale irriga­ SCM\1ARY tion projects will extend very much beyond the irrigated region. Some believe that cloud The climate of the Great Plains is charac­ seeding for rainfall augmentation will have a terized by: (a) a great range in daily, seasonal, significant effect on the climate of the region. and annual temperature; (b) strong atmospher­ Others (myself included) doubt that this will ic potential for evaporation because of the be the case. ample solar radiation; (c) strong windiness and The climate of the Plains will be in the usual dryness of the air; (d) wide difference in foreseeable future what it has been in the past, the alinual totals of precipitation received from although the possibility of its becoming warm­ the east to the west, and (e) frequent severe er and drier as a result of increasing the carbon weather including damaging winds, hail­ dioxide concentration in the atmosphere (the storms, and tornadoes. greenhouse effect) seems more and more likely Limited length of the growing season and as new scientific evidence on this question irregularity of the precipitation are the major accumulates. Man's works are too puny in (IREi\! PLAINS \.2Li\RTERLY, WINTER IYH7 scale to modify the weather, but improved Rosenberg, N.]. 1982. The Increasing CO, concen­ crop varieties, better irrigation and dryland tration ... , II. Effects through CO.-induced farming techniques, and modifications of the climatic change. Climatic Change 4:239~54. U.S. Department of Commerce, Environmental environment of individual fields will all con­ Science Services Administration, Environmen­ tribute to increased stability of agricultural tal Data Service. 1968. Climatic Arias of the production in the region, United States. Superintendent of Documents, The Creat Plains has been, in modern U.S. Government Printing Office. times, a major producer of food and feed Weakly, H. E. 1943. A tree ring record of precipita­ tilm in western Nebraska. Journal of Forestry gr;]ins, fodder, and livestock products for the 4l:S16~19. nation and the world. Critical shortages in the Weakly, H. E. 1963. History of drought in Nebras­ world's food supply, experienced during the b. Journal of Soil and Water Conservation 10705, have been alleviated and the economics 17:271~73, 275. of food production in the Great Plains is at hest confused in the late 10805. It seems likely that, with continuing growth in world popula­ tion, demands for Great Plains food produc­ RECOMMENDED READI)\!CS tion will again increase, but the economic success of production agriculture will require, Dale, R. F. 1967. The climate of the Great Plains. In: C. E. Zimmerman and S. Russel, cds. Sympo­ more than ever hefore, a degree of manage­ sium on the Great Plains of North America. pp. ment skill that minimizes the stresses imposed 15~48. North Dakota Institute for Regional hy the climate of the region and makes the best Studies. possible usc of its heneficent features. Flora, S. D. 1958. Tornadoes of the United States. University of Oklahoma Press. 194 pp. Nuttonson, M. Y. 1965. Global Agroclimatic An­ alogies of the Northern Great Plains Region of REFERENCES the United States and an Outline of its Physiog­ raphy, Climate and Farm Crops. American CJre,l( Plains Committee, Future of the Great Plains, Institute of Crop Ecology, Washington, D.C. III February, 1936, U.S. Government Document Rosenberg, N.]. 1966. What windbreaks really do. 144. Crops and Soils-June-July issue. American Palmer, W. C. 1965. Meteornlogical drought. U.S. Society of Agronomy. Department of Commerce Weather Bureau Thornthwaite, C. W. 1941. Climate and settlement Research Paper 45, 58 pp. in the Cjreat Plains. In: Climate and Man. 1941 Rosenberg, N. J. 1969. Advective contribution of Yearbook of Agriculture, U.S. Department of cncrgv utilized in evapotranspiration by alfalfa in Agriculture. pp. 177~87. the east central Creat Plains. Agricultural Meteo­ T rewartha, G. T. 1941. Climate and settlement of rology 6: 179--iH. the subhumid lands. In: Climate and Man. 1941 Rosenberg, N. J. 1981. The Increasing CO. concen­ Yearbook of Agriculture. U.S. Department of tration in the atmosphere and its implication on Agriculture. pp. 167~76. agricultural productivity, I. Effects on photosyn­ Stout, G. E. and S. A. Changnon, Jr. 1968. Clima­ thesis, transpiration and water usc efficiency. tography of hail in the central United States. Climatic Change ):265~79. Illinois State Water Survey.