Fact Sheet - Kentucky's Hail Distribution
Hail is a dangerous and damaging weather phenomenon whenever and wherever it occurs. It is formed only in thunderstorms, as a result of convective lift. A small ice pellet is lifted into the middle of upper portions of a cumulonimbus cloud (a thunderstorm cloud), where it collides with supercooled water. This water freezes, layer by layer, causing the ice pellet to grow in size. Eventually the ice pellet becomes too heavy and cannot remain in the air. It falls out of the cloud, and if it doesn't melt by the time it hits the ground, it's known as hail (Carlson 1999). Hail is not a common sight for many Kentuckians. Even so, significant hail events occur in all regions of the state. The record hail event for Kentucky occurred in November of 1967. On this date, hail up to a half inch in diameter accumulated near Summer Shade in Metcalfe County. The accumulation of hail on the ground was up to six inches in some spots (Conner 1993). While this is an extreme example of Kentucky's hail occurrences, the amount of large hail events (hail with a diameter of 0.75 inches or greater, as specified by the Storm Prediction Center) varies greatly by county across the state.
Hail is the result of two general weather conditions. First, hail may be caused by meso-scale thunderstorms that are usually frontal and widespread. Second, physiographic features, large bodies of water, and localized convection can develop localized thunderstorms that can be conducive to hail (Changnon 1999). Because of this, one would expect Eastern Kentucky to receive fewer hail events, because the mountains inhibit severe thunderstorms from forming. A map of total hail events by county (was created using archived data from the Storm Prediction Center from 1980-1995. Another map, which normalized these data, was created to show the number of hail events per one hundred square miles for the given time period. Both maps show a similar pattern, in which the eastern counties reported considerably fewer hail events in the sixteen-year period. The number of western counties (with less relief) had significantly more hail events recorded. While the different physiographies of Kentucky play an important role in the amount of hail received, another factor seems to be involved.
Much like the high wind events in Kentucky, the counties with frequent hail events had a correlation to the population of the county. Boone, Daviess, Fayette, Jefferson, Oldham, and Warren counties all had a higher number of large hail events reported. Each of these counties has a higher population as well. So why do counties with a higher population tend to have more hail events on record? The answer lies, once again, in the reporting. In rural, less-populated counties, hail is more likely to go unreported or even unnoticed, when structural or crop damage does not occur. When hail falls in a more populated county, however, it's more likely to cause damage and to be reported.
The hail maps show a much more apparent west to east distribution of hail than the high wind events, even though population remained a large factor. Kentucky's thunderstorm frequency is greatest from May through August, usually peaking in July (Changery 1981). Therefore, one could expect a higher frequency of hail events during this four-month period as well. The counties in Western and North-Central Kentucky reported much more hail than the south-central and eastern counties. In recent years, more and more hail events have been reported in Kentucky. This trend is apparent throughout the United States, with the number of reported hail events rising sharply in the past thirty years (Carlson 1999). While the maps provide a good idea of hail distribution throughout the state, an increased use of NEXRAD and an improved weather observation network will surely provide more details into Kentucky's hail events.
Carlson, Christy. "Hail Climatology". Internet. http://www.ianr.unl.edu/snrs/ amet498/carlson/whatishail.html. (May 24, 2000).
Changery, M.J. "National Thunderstorm Frequencies for the Contiguous United States". National Climatic Center. 1981.
Changnon, Stanley A. "Long-term Fluctuations in Hail Indices in the United States". Journal of Climate. Volume 38. 1999.
Conner, Glen. "Climatic Extremes in Kentucky". Kentucky Climate Center. June 1993.
Storm Prediction Center. "Historical Hail Data". Internet. http://www.spc.noaa.gov/archive/hail/ (May 24, 2000).
U.S. Census Bureau. Population of Kentucky Counties. 1990.