The value generally decreases to about 195 days in the eastern panhandle, then more rapidly to 175 days in the western panhandle. The average length of the growing season (Figure 3), or freeze-free period, is at a maximum of 225 to 230 days in the southern tier of counties and in the Arkansas River Valley downstream of Tulsa. The lowest temperature on record is -31☏, set at Nowata on February 10, 2011. This value increases to about 110 days per year where the panhandle joins the rest of the state, and to 140 days in the western panhandle. Temperatures of 32☏ or less occur an average of 60 days per year in the southeast. In addition, Tishomingo observed 120☏ on July 26, 1943. This reading was first observed during the brutally hot summer of 1936: at Alva on July 18, at Altus on both July 19 and August 12, and at Poteau on August 10. The highest temperature ever recorded in the state was 120☏. Years without 100 deg F temperatures are rare, ranging from about one of every seven years in the eastern half of the state to somewhat rarer in the west.įigure 2: Map of the normal annual temperature (in degrees Fahrenheit) for Oklahoma using data from 1981 to 2000. Heat index values of 105 degrees or greater occur more than 40 times per year in the far southeast and less than 10 times per year in the far northwest. The increased humidity in the east, however, adds to that section of the state's summertime misery. Both the Panhandle and eastern Oklahoma average about 15 days above the century mark. With 30-40 days at or above 100 deg F, western Oklahoma experiences more extreme summer temperatures than elsewhere in the state. Temperatures of 100 deg F or higher occur, frequently during some years, from May through September, and very rarely in April and October. The average is about 115 days in southwest Oklahoma and about 85 days in the southeast. Temperatures of 90 deg F or greater occur, on average, about 60-65 days per year in the western panhandle and the northeast corner of the state. It then decreases westward to 56 deg F in Cimarron County. The mean annual temperature over the state ranges from 62 deg F along the Red River to about 58 deg F along the northern border (Figure 2). The following sections highlight some of these variables and associated events. Climatologists know how to work with changes in observing intervals, sensors, techniques, and locations to provide decision makers with an historical record to better understand climate normals, extremes, and variability. Even measurements of mundane variables such as temperature have become more common, with automated weather stations taking more measurements per day at more locations than in past decades. Hence, as Oklahoma's population increased over the years, human observations of rare events became more prevalent. Some weather events cannot be measured easily by automated methods (e.g., tornadoes) and must be documented by human observers. Our knowledge of climate is based on the variables that we measure, typically with surface observing stations, weather radar, satellites, weather balloons, and other instrumentation. Periods of extreme cold are infrequent, and those lasting more than a few days are rare. Winters are shorter and less severe than those of the more northern Plains states. Warm, moist air moving northward from the Gulf of Mexico often exerts much influence, particularly over the southern and eastern portions of the state, where humidity, cloudiness and precipitation are resultantly greater than in western and northern sections. The Washita and Kiamichi are the Red's principal tributaries in Oklahoma, and the Little River flows into the Red after it crosses into Arkansas.įigure 1: Elevation (in feet) above mean sea level across Oklahoma.Īccording to the Koppen climate classification, Oklahoma's climate ranges from humid subtropical in the east to semi-arid in the west. Principal tributaries of the Arkansas are the Verdigris, Grand (Neosho), Illinois, Cimarron, Canadian and North Canadian. The two main rivers in the state are the Arkansas River, draining the northern two-thirds of the state, and the Red River, which drains the southern third and is the state's southern border. Oklahoma lies entirely within the drainage basin of the Mississippi River. The western tip of the panhandle is part of the fractured terrain of the Black Mesa complex. Extreme northeastern counties are part of the Ozark Plateau, marked by steep, rocky river valleys between large areas of hills and rolling plains. Extreme east-central Oklahoma features the mountains of the Arkansas River Valley, rising several hundred feet above the plains. The Ouachita Mountains dominate southeast Oklahoma, with peaks rising as much as 2,000 feet above their base.
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