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2-4 I I I I I I I I I I I I I I I I I I I 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 Mexicol cool, moist air from the northern Pacific Oceanl cold, dry air from the northern polar regionsl 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 can be greatly influenced by the Rocky 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-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 2-2 shows normal annual precipitation for the period 1931-1960. About 70 percent of the precipitation occurs as rainfall during the growing season.