A Statistical Analysis of the Frost/Freeze Climatology of Eastern Kentucky

 
Farmers and gardeners are always interested in the average dates of the first and last frost and freeze for an area. This information allows them to plant the proper crops for their area and to know when to plan for planting and harvesting of their crops. These dates vary depending on location and topography, and in some cases they can vary a great deal. The varied terrain of eastern Kentucky causes quite a bit of variation in the average frost/freeze dates and in the article below, we will describe the climatology of the first and last frost and freeze of eastern Kentucky using a statistical approach.
Methodology

Seasonal dates of the first 36ºF, 32ºF and 28ºF temperatures were analyzed using a percentile rank analysis for several surface observing sites over east Kentucky. The observing sites include first-order reporting stations, such as the National Weather Service Office in Jackson and the London/Corbin Airport. They also include several long-running cooperative weather observing stations. 

Stations Used in Frost/Freeze Climatology Study (Year Records Began for Station)
Stations Used in Frost/Freeze Climatology Study

The first/last 36ºF temperature was used to approximate the date of first/last frost for the growing season, 32ºF being the date of first/last freeze and 28ºF approximating the date of first/last hard freeze for the growing season.

How to Read the Frost/Freeze Climatology Box & Whisker Charts

Box and whisker diagrams were employed to view the data.  An example of the diagram format is shown at left.  The shaded region of each graph (i.e., "the box") shows the middle 50% of the range of first and/or last frost/freeze dates. The top of the box is the 25th percentile and the bottom is the 75th percentile. The solid line within the box shows the median (or 50th percentile) first and/or last frost/freeze date for that location. The lines extending upward and downward from the box (i.e., "the whiskers") reach to the 10th and 90th percentile of the data distribution. That is, only 10% of the data lies above and below the ends of the whiskers.  Lastly, the "x" indicates the extreme (record) freeze date during the period of record for that particular location. The intent of the box and whisker format is to give a comprehensive view of the range of first and last frost/freeze dates at individual sites, and facilitate comparison between different locations. For more information on this format please see Box and Whisker Plots for Local Climate Datasets: Interpretation and Creation using Excel 2007/2010 by Peter Banacos.

Finally the data was organized and graphed according to physiographic region. Click here for more information on the physiographic regions of the National Weather Service Jackson forecast area from the Kentucky Geological Survey.
Variation in frost/freeze dates due to topography

Much of the eastern third of Kentucky, known as the Eastern Coal Fields physiographic region, is characterized by rugged, steep terrain. While elevation differences between valley bottom and adjacent ridge tops does not usually exceed 1000 feet, the steepness of the slope creates deep, sheltered valleys, especially east of Interstate 75. These deep valleys experience earlier sunset/later sunrise times than broader valleys. This allows an inversion to form quickly keeping the valleys sheltered from the mean wind, maximizing cooling. Valleys containing larger rivers and creeks in the Coal Fields are usually broader and less sheltered. Fog often forms in the river valleys modifying temperatures and producing a later first frost/freeze date than the deeper valleys in the fall, and a corresponding earlier last frost/freeze date in the spring.  Finally, with the cold air pooling in the valley bottoms, a warm layer of air often forms at ridge top level.  This is known as the thermal belt and it causes a much later first frost/freeze date to be found on the ridges in the fall, and an earlier last frost/freeze date in the spring, as evident by the NWS Jackson data.  It is these topographic variations in location that create the main differences in frost/freeze climatology for eastern Kentucky, not necessarily how far north or south a station lies.  This is evident by the fact that Stearns and Ashland have a similar frost/freeze climatology, despite Stearns being located in southern Kentucky near the Tennessee border and Ashland being located near the Ohio River in northeast Kentucky.

Idealized illustration of cold pool and thermal belt structure typically found during clear, calm nights in the mountainous terrain of east Kentucky.  Also illustrated is an example of temperature modification due to a body of water, in this case a small lake.

The other major physiographic regions of east Kentucky, the eastern Bluegrass or “Knobs” region has less rugged terrain when compared to the Coal Fields, and less variation in temperature from location to location.  Based on the comparison below, stations in the eastern Bluegrass tend to have a similar frost/freeze climatology than stations in the Coal Field Valleys.

Spring Frost Climatology

The median date of last 36ºF or colder temperature is used in this study as a proxy for the last frost of spring.  During the 30 year period from 1981 to 2010, the median date of last frost in the Coal Field valley and eastern Bluegrass regions runs from April 28th to May 3rd.  In the more sheltered valleys away from rivers and larger bodies of water, the median date of last frost can run from May 3rd to as late as May 14th.  However, on the ridges in the Coal Field region the median date of last frost is as early as April 18th.  Last frosts have been observed as late as June 22nd at the Stearns and Ashland stations, but more consistently have occurred around June 1st at the majority of Coal Field valley and eastern Bluegrass stations. The earliest date of last frost is in early April at the majority of the locations used in this study.

Statistics on the Last 36ºF or Colder Temperature of the Spring
click on image for more detail

Spring Freeze Climatology

During the 30 year period from 1981 to 2010, the typical last freeze of spring in the Coal Field valley and eastern Bluegrass regions runs from April 13th to April 20th.  In the more sheltered valleys away from rivers and larger bodies of water, the median date of last freeze can run from April 20th to as late as May 3rd.  However, on the ridges in the Coal Field region the median date of last freeze occurs as early as April 9th.  Last freezes have occurred as late as the second week of June, but more typically occur in late May at the majority of stations. Last freezes have occurred as early as the last week of March for most valley and eastern Bluegrass locations.

Statistics on the Last 32ºF or Colder Temperature of the Spring
click on image for more detail

During the 30 year period from 1981 to 2010, the typical last killing freeze of spring, defined here as the last 28ºF or colder temperature, in the Coal Field valley and eastern Bluegrass regions runs from April 1st to April 9th.  In the more sheltered valleys away from rivers and larger bodies of water, the median date of last killing freeze can run from April 9th to as late as April 20th.  However, on the ridges in the Coal Field region the median date of last killing freeze occurs as early as the last day of March. Killing freezes have occurred as late as the last week of May at West Liberty and Ashland.  However,  the remaining stations in the study show a last killing freeze occurring during the second week of May.  Last killing freezes have occurred as early as the last week of February or first week of March for most valley and eastern Bluegrass locations.

Statistics of the Last 28ºF of Colder Temperature of the Spring
click on image for more detail

Individual Spring Station Statistics (click on images for more detail)

 

Deep Coal Field Valleys
West Liberty 3NW Ashland
West Liberty Ashland

Stearns 2S
Stearns


 
Coal Field Valleys
Williamsburg Monticello
Williamsburg Monticello
   
Mount Vernon Somerset 2N
Mount Vernon Somerset
   
London Corbin Airport Barbourville
London Barbourville
   
Paintsville 1E Farmers
Paintsville Farmers

Baxter
Baxter
 

 
Eastern Bluegrass
Mt. Sterling 5N Maysville
Mt. Sterling Maysville
 

 
Coal Field Ridges
NWS Jackson
National Weather Service Office near Jackson
 

Fall Frost Climatology

The median date of first 36ºF or colder temperature is used in this study as a proxy for the first fall frost of the year.  During the 30 year period from 1981 to 2010, the median date of first frost in the Coal Field valley and eastern Bluegrass regions runs from October 7th to October 17th.  In the more sheltered valleys away from rivers and larger bodies of water, the median date of first frost can run from September 28th to as late as October 6th.  However, on the ridges in the Coal Field region the median date of first frost is as early as October 17th.  It is noteworthy to point out that the ridgetop location of NWS Jackson and the valley locations of Baxter and Paintsville have very similar median dates of first frost.  Since Baxter and Paintsville lie on rivers, this fact illustrates how the warm rivers modify the temperatures at nearby locations in early fall.  First frosts have been observed as early as late August at the Farmers and Ashland stations in northern Kentucky, but more consistently have occurred as early as mid September at the majority of stations. The latest first frost has occurred as late as mid November.

Statistics on the First 36ºF or Colder Temperature of the Fall
click on image for more detail

Fall Freeze Climatology

During the 30 year period from 1981 to 2010, the typical first freeze of fall in the Coal Field valley and eastern Bluegrass regions runs from October 24th to October 17th.  In the more sheltered valleys away from rivers and larger bodies of water, the median date of first freeze can run from October 6th to as late as October 16th.  However, on the ridges in the Coal Field region the median date of last freeze occurs as late as the last week of October.  First freezes have occurred as early as mid September and as late as mid November in the valleys and late November on the ridge tops.

Statistics on the First 32ºF or Colder Temperature of the Fall
click on image for more detail

During the 30 year period from 1981 to 2010, the typical first killing freeze of fall, defined here as the first 28ºF or colder temperature, in the Coal Field valley and eastern Bluegrass regions runs from October 26th to as late as November 7th.  In the more sheltered valleys away from rivers and larger bodies of water, the median date of first killing freeze can run from as early as October 19th to as late as October 25th.  However, on the ridges in the Coal Field region the median date of first killing freeze occurs around the 12th of November. Killing freezes have occurred as early late September in the northeast Coal Field valleys and eastern Bluegrass, and as late as early to mid December in the Coal Field valleys.

/images/jkl/climate/Freeze/FreezeChart28.jpg
click on image for more detail

Individual Fall Station Statistics (click on images for more detail)

 

Deep Coal Field Valleys
West Liberty 3NW Ashland
West Liberty Ashland

Stearns 2S
Stearns


 
Coal Field Valleys
Williamsburg Monticello
Williamsburg Monticello
   
Mount Vernon Somerset 2N
Mount Vernon Somerset
   
London Corbin Airport Barbourville
London Barbourville
   
Paintsville 1E Farmers
Paintsville Farmers

Baxter
Baxter
 

 
Eastern Bluegrass
Mt. Sterling 5N Maysville
Mt. Sterling Maysville
 

 
Coal Field Ridges
NWS Jackson
National Weather Service Office near Jackson
 
 

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