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Arrival of first killing frost varies; map shows the average daMissouri After the 100-degree dog days of August, it may seem hard to think about cooler weather and the arrival of autumn's first killing frost. "Where you live can make a big difference when to expect your first frost," said Pat Guinan, University of Missouri Extension climatologist. Missouri's latitudinal variation, the Ozark Plateau, river bottomlands and hills and valleys are all factors contributing to frost potential. A map showing the average date of the first frost (32 degrees) in Missouri can be found at http://agebb.missouri.edu/weather/frost2.htm. These are dates in the fall before which there is a 50 percent chance of a light killing frost. "Typically, the first fall frost occurs over northern and central Missouri by the second and third week of October, respectively," he said. Frosts are more likely to be experienced earlier in the fall over the Ozarks when compared to central Missouri. This is due to the higher elevation of the Ozark Plateau, which causes cooler temperatures in the Ozark region than would be expected, he said. In the Bootheel, with its lowlands, the average first fall frost occurs during the last week of October or as late as the first week of November in the extreme southeastern tip of the state, he said. Local terrain is another factor in determining fall's first frost. Temperatures can be highly variable within small distances due to topography. Minimum temperatures can vary as much as 10 degrees over a short distance from the bottom of a valley to a nearby hilltop. Cool air, being denser than warm air, moves down the slopes of hills, accumulating in valleys. Low lying areas, such as river bottoms, will likely be colder than their surroundings on clear, calm nights. Date: 10/4/07
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