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The other province within the Interior Plains, the Central Lowlands, extends eastward from the Great Plains to the upper Mississippi River divide. The land is generally level except where stream development has created a hilly topography. In the southeastern part of the basin is a 7 million acre area of hilly to mountainous land called the Ozark Plateaus of the Interior Highlands. The climate within the basin is determined largely by the interaction of four extensive air masses: warm, moist air from the Gulf of Mexico, cool, moist air from the northern Pacific Ocean, cold, dry air from the northern polar regions: and hot, dry air from the plateaus in north central Mexico. Because of the extreme differences in the characteristics of these four air masses, weather changes are sudden and severe. Local climates vary greatly with elevation, slope, and exposure within the mountains, 'as evidenced by substantial temperature and precipitation anomalies. Lifting of air masses over the Rocky Mountains often produces precipitation on the windward side of the mountains and dry, warm chinook winds on the leeward side. Primarily because of its mid-latitude and mid-continent location, the basin experiences weather that is known for its fluctuations and extremes. Averages are misleading because average weather seldom actually occurs. Instead, weather tends to fluctuate widely around the annual averages. Normal annual precipitation varies from west to east in the basin, averaging over 35 inches in the Rocky Mountains of the western boundary, about 14 inches on the Great Plains, about 26 inches on the Central Lowlands, and over 36 inches in the Interior Highlands. Figure 3 shows normal annual precipitation for the period 1931-1960. About 70 percent of the precipitation occurs as rainfall during the growing season. The extent of snowfall and snowpack in the higher elevations of the basin are important sources of surface water in the basin. One climatic factor of great importance because of the agricultural activity in the basin is the length of the freeze- free period. The freeze-free period is the average number of days each year between th~ last freezing temperature in the spring and the first freezing temperatures in the fall. While the freeze-free period does not completely define the growing season for all crops and grasses, it is a general indicator of the most favorable period. The length of the freeze-free period in the basin is about 30 days in the higher elevations of the -14-