Oklahoma offers a wide variety of climates. Annual precipitation, based on data collected between 1971 through 2000, averages approximately thirty-six inches across the state. Annual precipitation decreases both in frequency and amount from southeast to northwest, ranging from fifty-six inches in the extreme southeast to seventeen inches in the western Panhandle. The statewide-averaged temperature of 60° F varies locally from 64° along the southern border to 55° in the western Panhandle. An axis of relative warmth extends from extreme south central and southwestern Oklahoma northward through the central part of the state.
The four seasons are distinct, but the impact of each varies by region. The climate is somewhat temperate in all regions and is controlled by the meandering of the midlatitude jet stream and the seasonal migration of a large, semipermanent area of high pressure centered over the Atlantic Ocean near Bermuda. Summers are generally hot. Daytime temperatures in the nineties are commonplace, and thermometer readings of 100° F or more are frequent, especially in the southwest. Winters are cold in northern Oklahoma but quite mild in the south. Temperatures seldom fall below 10°, and rarely do they remain that low for long. Spring, the season of greatest precipitation, is noted for the severe thunderstorms that produce the most tornadoes (per unit area) of any place in the world. Autumn, while featuring some of the state's most pleasant weather, presents a secondary maximum of precipitation over most of Oklahoma.
Moisture for precipitation largely comes from the Gulf of Mexico, carried into the state by the south-to-southeast winds that prevail over most of the state most of the year. Southeastern Oklahoma's proximity to moisture from the gulf and lifting of the air over the Ouachita Mountains enhance its precipitation total. A second precipitation maximum is observed north of the Arkansas River over the Ozark Plateau. April through June is the season of maximum precipitation over most of the state, with a secondary maximum occurring in September and October. The Panhandle enjoys a later May-through-August precipitation maximum. The cold-weather months are the driest everywhere in the state. Snowfall averages around thirty inches per year in the western Panhandle, declining to approximately ten inches in central parts of the state. Snowfall is rare in the southeast, but storms producing six inches or more occasionally occur. Freezing rain is a significant hazard during winter throughout the state.
Precipitation frequency declines westward from 115 days per year near the Arkansas border to forty-five days in parts of western Oklahoma. Thunderstorm frequency (approximately fifty-five days per year) is highest over the Ouachita Mountains, the Ozark Plateau, and the extreme western Panhandle. Thunderstorms are least frequent over the southwestern corner of the state (fewer than forty-five days per year). Western and central Oklahoma are frequent breeding grounds for the severe thunderstorms that supply much of the state's spring and autumn precipitation and from which most of the state's tornadoes and other severe weather events arise. Oklahoma experiences an average of fifty-four tornadoes per year, most of them occurring between April and June. Floods are a hazard along several of the state's rivers, a danger that has been reduced in recent years by a variety of flood-prevention programs. Flash floods, particularly along creeks that traverse urban areas, occur in response to locally heavy rainstorms throughout the warm season. The greatest one-day precipitation total, as reported by an official rain gauge, is 15.68 inches, recorded at Enid on October 11, 1973.
Year-to-year precipitation is quite variable. Localized droughts extending through a growing season or two are relatively common. The longest periods of drought since the establishment of a comprehensive observing network have been 1909 through 1918, 1930 to 1940, 1952 to 1958, and 1962 through 1972. Each of those episodes was interrupted at least once by a year with greater-than-normal precipitation.
Summer heat and drought are closely related. Oklahoma's hottest summers coincide with significant precipitation shortfalls. The state's highest recorded temperature (120° F) has been reported six different times, most recently on June 27, 1994, at Tipton. Four of those record-high temperatures occurred during the brutal summer of 1936. The lowest temperatures typically occur on clear, calm nights after a snowstorm. Thermometers have registered as low as 27° twice, at Vinita on February 13, 1905, and at Watts on January 18, 1930. Winter precipitation and cold are not well correlated, as Oklahoma's winter precipitation can be delivered as snow and ice or as rain.
BIBLIOGRAPHY: Howard L. Johnson and Claude E. Duchon, The Atlas of Oklahoma Climate (Norman: University of Oklahoma Press, 1995). Kenneth S. Johnson, et al., Earth Sciences and Mineral Resources, Educational Publication 9 (Norman: Oklahoma Geological Survey, 2004). John W. Morris, Charles R. Goins, and Edwin C. McReynolds, Historical Atlas of Oklahoma (Norman: University of Oklahoma, 1986).
Howard L. Johnson
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