Fact Sheet - Kentucky Freeze/Thaw Events 1966-1995
"A freeze/thaw event is said to have occurred if the temperature crossed the freezing point during a calendar day" (Hershfield, 1974:1). For example, a morning low temperature could be recorded at 28 F. Later that afternoon a daily high could be recorded at 37 F. This would equal one freeze/thaw event. The purpose of this research was to accumulate mean freeze/thaw data from various weather stations in Kentucky and surrounding states from 1966-1995 to determine the spatial variance of these events.
The most recent study of freeze/thaw events in Kentucky was done by Kentucky State Climatologist, Glen Conner, with data ranging from 1948 to 1973 Conner concluded in his research that an eastward trend of higher freeze/thaw means was evident.
The expectations for this research were that a similar trend still existed. This type of trend can be directly related to Kentucky's topography. The Mississippi Embayment consists of elevations less than 500 feet. Eastward, there is a gradual increase in elevation. Some elevations located in the Eastern Kentucky Coal Field are greater than 3,000 feet. Daily temperatures are recorded at five feet above the surface. Weather stations at higher elevations are more likely to record cooler temperatures for a longer period of time than stations in lower elevations.
This research can be very important to hydrologists, geologists, farmers, construction workers, or anyone involved with environmental, economical, or commercial interests. According to Grenke and Mace, mentioned on page one in David M. Hershfield's The Frequency of Freeze-Thaw Cycles', freeze/thaw events can affect the water movement in soils. This can cause surface runoff which can lead to erosion. Grenke and Mace also mention that freeze/thaw events can trigger possible flood flows in the winter and spring. These events may also affect traffic abilities.
Frost is another important factor in freeze/thaw events. Frost depends on the availability of moisture, and temperatures surrounding 32 F. Water particles that have settled in other masses will expand when they enter the frozen state, eventually breaking up its surroundings over time. This is called frost heaving. Frost heaving can destroy pavements and break up soils. This research can help forecast the first and last frost of the season. Planting and harvesting seasons may also be estimated.
The data from this research came from the Midwestern Climate Center. There were 69 weather stations chosen from Kentucky and surrounding states. These stations were chosen based on their percentage of missing data. High percentages of missing data would be invalid to this research. The data was set to a specific threshold:
1. Maximum temperatures to 33 F
2. Minimum temperatures to 32 F
3. Whole year data (as suppose to seasonal data)
4. Measured by mean counts
The threshold provided the necessary data to give total annual counts of freeze/thaw events for each weather station from 1966-1995, and average the totals into a 30 year mean. The result formed from the threshold gave the mean number of freeze/thaw events per station, per year. Once all the station's mean freeze/thaw events were accumulated, the data were put into a contour map. This helped spatially analyze the variance of these events visually.
There are many factors that could effect the temperature changes represented by the map. According to Hershfield, snow melts and flooding can cool particular areas. Albedo, the ability to reflect off a surface, can occur in high rates when dealing with snow, water, and urbanization. High albedo can raise temperatures in particular areas.
Urbanization can cause what is known as urban heat islands. This is caused when an urbanized area produces too much pollution. The pollution will generally surround the city area, acting as clouds to insulate the heat inside the island'. Temperatures inside an urban heat island can vary greatly when compared to its rural surroundings. Several factors may be involved in this variance.
The expectations of an eastward trend of higher freeze/thaw means were met. On average, Kentucky freeze/thaw events in the western division begin in October and end late March. Eastward, freeze/thaw events begin earlier and end later. Freeze/thaw events in the eastern division begin in September and end early April. The contour map visually represents this type of trend. However, the eastward trend of higher freeze/thaw means does not visually represent a smooth pattern. If omitted weather stations contained enough data, the contour map may contain a smoother pattern.
Ahrens, C. Donald, 1998, Essentials Of Meteorology, second edition, 423, 429.
Conner, Glen, 1979, Freeze Thaw Events in Kentucky, Climate Center Publication #22.
Danielson, W. Eric, Levin, James, Abrams, Elliot, 1998, Meteorology, 441-442.
Haney, C. Donald, 1997, Physiographic Diagram Of Kentucky (Figure 1).
Hershfield, M. David, 1974, The Frequency of Freeze-Thaw Cycles, 348-354.
Lutgens, K. Frederick, Tarbuck, J. Edward, 1992, fifth edition, 413.
Midwestern Climate Center, 2000, Mammoth Cave Data Sheet (Table 1).