: 

National Climatic Data Center 

DATA DOCUMENTATION 

FOR 

DATA SET 9712C (DSI-9712C) 

Probability Levels for Freeze Dates and Growing Season Lengths

December 26, 2002 

National Climatic Data Center 

151 Patton Ave. 

Asheville, NC 28801-5001 USA 
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Table of Contents 

Topic Page Number

1. Abstract................................................... 3 

2. Element Names and Definitions: ............................ 4 

3. Start Date................................................. 6 

4. Stop Date.................................................. 6 

5. Coverage................................................... 6 

6. How to order data.......................................... 7 

7. Archiving Data Center. .................................... 7 

8. Technical Contact.......................................... 7 

9. Known Uncorrected Problems................................. 7 

10. Quality Statement.......................................... 7 

11. Essential Companion Data Sets.............................. 7 

12. References................................................. 8 
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1. Abstract: The computation of the above statistics was performed for 3,240 

stations in the United States (including Alaska and Hawaii) for the 1951-1980 

period. The stations were selected from the 1951-1980 Climate Normals station 

list (DSI-9641). Spring and Fall freeze date distributions from TD9712b were 

then used to produce the freeze data and growing season lengths associated 

with specified probability levels (10, 20, 30, 40, 50, 60, 70, 80, 90 percent) 

presented in this data set (TD9712c). The low temperature thresholds were the 

values of 36, 32, 28, 24, 20, and 16 degrees Fahrenheit. 

Data from the NCDC Summary of the Day (SOD) file (DSI-3200) were put through a 

vigorous quality control prior to producing the Freeze Data. The temperature 

data were put through extensive validation and interpolation procedures based 

upon the departure from the normal in conjunction with those from surrounding 

stations. As a result, the freeze data were produced from high quality, 

serially-complete station records of daily temperatures (Steurer, 1985). This 

alleviated the many possible biases or errors associated with developing 

freeze data statistics from an incomplete and lower quality data set. The 

serially complete data for the 1951-1980 period are archived in DSI-9641. 

 

All freeze dates were based upon the season August 1 through July 31 for each 

threshold temperature. Last spring dates of occurrence were chosen for the 

period August 1 of the previous year through July 31 of the selected year 

(e.g., spring season for 1961 runs from August 1, 1960 through July 31, 1961, 

except 1951 which begins on January 1). First fall dates of occurrence were 

chosen for the period August 1 of the selected year through July 31 of the 

next year (e.g., fall season for 1961 runs from August 1, 1961 through July 

31, 1962, except 1980 which ends on December 31). 

The season definition is an improvement over that previously used when the 

selection of freeze dates was performed for data from the 1931-1960 period 

(DSI-9712a). During the 1931-1960 period freeze dates were selected using a 

six month season definition ending with June 30 for the last Spring freeze and 

beginning with July 1 for the first Fall freeze. The new season definition 

coincides more closely with the annual match of temperature in which the 

warmest time of year occurs close to August 1. The change of season 

definition produces more realistic dates in the extreme northern and 

mountainous regions of the United States where temperatures frequently are at 

the threshold temperatures near the June 30 date. However, it is important to 

note that the change of season definition should have no effect on other 

stations where temperatures do not reach or exceed the preselected temperature 

during the summer. 

 

The estimation of freeze probabilities was based upon the work of H.C.S. Thom 

and R. H. Shaw which was later modified by C. K. Vestal. (See references). 

The selected probabilities were .1 through .9 in increments of .1. A date 

associated with each of the preselected probability levels was computed for 

the last spring and first fall freeze season. Similarly, the number of days 

associated with the freeze-free period was computed for each probability 

level. The probability levels represented the risk the user is willing to 

accept with regard to reaching or exceeding a certain threshold by a specific 

date or number of days. For example, suppose the .90 probability level for 

the spring season is computed to be March 1 at the 32-degree threshold. This 

means that nine times out of ten a temperature as cold as or colder than 32 

will occur later than March 1 during the spring season. For the fall season, 

the probability level represents the chance of having a temperature as cold or 

colder earlier than the computed date. The freeze-free probability level 

indicates the chance of having a longer freeze-free period than the computed 
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number of days. 

 

This data file contains: 1. Probability levels of the occurrence of freezing 

temperatures within specified thresholds. 2. Number of years in which the 

specified threshold was reached or exceeded. 3. Mean (Julian) day number of 

occurrence associated with the specified threshold. 4. Standard deviation of 

mean Julian day number of occurrences found in item 3 above. 

2. Element Names and Definitions: The data in this file are archived in a 

fixed length ASCII format. The total data volume is 3 megabytes. The data 

are sorted by the state number (ISTATE) as the primary key followed by station 

number (ISTATN), freeze season, (IFRZSN), and freeze threshold (IFRZTHdescending) as secondary keys. 

 Start End 

Element Type Width Column Column

ISTATE Integer 2 1 2 

ISTATN Integer 4 3 6 

IDIV Integer 2 7 8 

IFRZTH Integer 2 9 10 

IFRZSN Integer 1 11 11 

IPROB(1) Integer 4 12 15 

IPROB(2) Integer 4 16 19 

IPROB(3) Integer 4 20 23 

IPROB(4) Integer 4 24 27 

IPROB(5) Integer 4 28 31 

IPROB(6) Integer 4 32 35 

IPROB(7) Integer 4 36 39 

IPROB(8) Integer 4 40 43 

IPROB(9) Integer 4 44 47 

INMYR Integer 2 48 49 

RMNJL Real 4 50 53 

RSTDJL Real 5 54 58 

Element Name Element Definition______________________ 

ISTATE Characters 1-2 

 Cooperative State Code Number 

 Range 01-48, 50, and 51. 

ISTATN Characters 3-6 

 Cooperative Station Code Number 

 Range 0001-9999 

IDIV Characters 7-8 

 Cooperative Division Code Number 

 Range 01-10 

IFRZTH Characters 9-10 

 These are the threshold temperatures 

 used in determining the specific 

 probability levels of the last Spring, 

 first Fall, and growing season lengths. 

 They are expressed in Fahrenheit degrees. 

 The values are 36, 32, 28, 24, 20, and 

 16. 

IFRZSN Character 11 
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 This is the freeze season indicator code. 

 1 = Last Spring Freeze 

 2 = First Fall freeze 

 3 = Growing season length 

PROBABILITY-LVL-.10 Characters 12-15 

IPROB(1) Read Topic 21 for an explanation of 

 freeze probability levels. This data 

 field represents the 10 percent 

 probability level. Range of values in 

 this field and IPROB(2)-IPROB(9) is 

 determined by the Freeze Season Code 

 in character 11. If the freeze season 

 code is the last spring or first fall 

 freeze, the four digit value contains a 

 specific date of occurrence(month and day) 

 Range of values is 0101-1231 and 0000. 

 

 If the Freeze Season Code is growing 

 season length, the four digit value 

 represents an accumulated day number. 

 Range 0000-0365, and 9999. 

 Note: A “9999" growing season length 

 indicates > 365 days. A “0000" spring 

 and fall freeze indicates that the 

 probability of occurrence of the threshold 

 temperature is less than the indicated 

 probability. 

PROBABILITY-LVL-.20 Characters 16-19 

IPROB(2) See definition of IPROB(1) above. This 

 data field represents the 20 percent 

 probability level. Range explained in 

 characters 12-15. 

PROBABILITY-LVL-.30 Characters 20-23 

IPROB(3) See definition of IPROB(1) above. 

 This data field represents the 30 

 percent probability level. Range 

 explained in characters 12-15. 

PROBABILITY-LVL-.40 Characters 24-27 

IPROB(4) See definition of IPROB(1) above. 

 This data field represents the 40 percent 

 probability level. Range explained in 

 characters 12-15. 

PROBABILITY-LVL-.50 Characters 28-31 

IPROB(5) See definition of IPROB(1) above. 

 This data field represents the 50 percent 

 probability level. Range explained in 

 characters 12-15. 

PROBABILITY-LVL-.60 Characters 32-35 

IPROB(6) See definition of IPROB(1) above. 

 This data field represents the 60 percent 
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 probability level. Range explained in 

 characters 12-15. 

PROBABILITY-LVL-.70 Characters 36-39 

IPROB(7) See definition of IPROB(1) above. 

 This data field represents the 70 percent 

 probability level. Range explained in 

 characters 12-15. 

PROBABILITY-LVL-.80 Characters 40-43 

IPROB(8) See definition of IPROB(1) above. 

 This data field represents the 80 percent 

 probability level. Range explained in 

 characters 12-15. 

PROBABILITY-LVL.90 Characters 44-47 

IPROB(9) See definition of IPROB(1) above. 

 This data field represents the 90 percent 

 probability level. Range explained in 

 characters 12-15. 

INMYR Characters 48-49 

 This field contains 2 digits representing 

 the number of years in which the specified 

 freeze threshold was reached or exceeded. 

 Range 00-30. 

RMNJL Characters 50-53 

 This 4-digit field contains the mean 

 Julian day number of occurrences 

 associated with a specific threshold. 

 (implied 1 decimal). Season begins 

 August 1 and ends July 31 of the 

 next year. (E.g., Julian day number 

 is 1.0 for August 1 and 31.0 for 

 August 31). Range 0000-3660. 

RSTDJL Characters 54-58 

 This 5 digit field contains the standard 

 deviation of the mean Julian day number 

 of occurrences found in characters 50-53. 

 Range 00000-99999 (implied 2 decimals). 

 

3. Start Date: 19510101 

4. Stop Date: 19801231 

5. Coverage: the contiguous United States, Alaska and Hawaii 

Southernmost Latitude: 18N 

Northernmost Latitude: 65N 

Westernmost Longitude: 160W 

Easternmost Longitude: 65W 
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6. How to Order Data: 

 Ask NCDC’s Climate Services about the cost of obtaining this data set. 

 Phone: 828-271-4800 

 FAX: 828-271-4876 

 E-mail: NCDC.Orders@noaa.gov

7. Archiving Data Center: 

 National Climatic Data Center 

 Federal Building 

 151 Patton Avenue 

 Asheville, NC 28801-5001 

 Phone: (828) 271-4800. 

8. Technical Contact: 

 National Climatic Data Center 

 Federal Building 

 151 Patton Avenue 

 Asheville, NC 28801-5001 

 Phone: (828) 271-4800. 

9. Known Uncorrected Problems: There are no known uncorrected problems in 

this data set. 

10. Quality Statement: Data from the NCDC Summary of the Day (SOD) file (DSI3200) were put through a vigorous quality control prior to producing the 

Freeze Data. These temperature data were put through extensive validation and 

interpolation procedures based upon the departure from the normal in 

conjunction with those from surrounding stations. As a result, the freeze 

data were produced from high quality, serially-complete station records of 

daily temperatures. This alleviated the many possible biases or errors 

associated with developing freeze data statistics from an incomplete and lower 

quality data set. The serially complete data for the 1951-1980 period are 

archive in DSI-9641. 

 

11. Essential Companion Datasets: The use of NCDC’s Station History file 

(DSI-9767) is required in order to determine metadata on each station (name, 

location, elevation, etc.). This can be accomplished by comparing the station 

number in bytes 1 through 6 of this data set with the corresponding station 

number in the Station History data set. There is also a significant derived 

application of this data: One of the more significant applications of this 

data set is in agriculture, where late spring freezes, early fall freezes and 

short growing seasons can seriously impact plant production and crop volume. 

Various agricultural applications use these probability data for planning 

purposes. For each station, selected probability levels from this data set 

were published in the book “Freeze/Frost Data” and also used in the generation 

of freeze probabilities maps available in the Climatography of the US No. 20 

(See references). These are also available on microfiche. 

Graphs of the 32 degree Fahrenheit probability distribution of the last Spring 

and first Fall freezes are depicted with cumulative daily growing degree day 

units for bases of 40, 45, 50, 55, and 60 degrees Fahrenheit. These graphs 

are for individual climate stations for the 1951-1980 period and are available 
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on microfiche. 

12. References:

Environmental Information Summaries C-26 - Climatography of the United States 

Number 20, National Oceanic and Atmospheric Admin., NCDC, Asheville, N.C. 

Koss, W.J., Owenby, J.R., Steurer, P.M., and Ezell, D.S., 1988: Freeze/Frost 

data, Climatography of the U.S. No. 20, Supplement No. 1. Nat. Oc. And 

Atmospheric Admin., NCDC, Asheville, N.C. 186 pp. 

Steurer, P.M., 1985: Creation of a serially complete data base of high quality 

maximum and minimum temperatures. Unpublished document, NOAA, National 

Climatic Data Center, Asheville, NC, 21 pp. 

Thom, H.C.S., and Shaw, R. H., 1958: Climatological analysis of freeze data 

for Iowa. Monthly Weather Review, 86, 251-257. 

Thom, H.C.S., 1959: The distribution of freeze-date and freeze-free period of 

climatological series with freezeless years. Monthly Weather Review, 87, 136-

144. 

Vestal, C. K., 1971: First and last occurrences of low temperatures during the 

cold season. Monthly Weather Review, 99, 650-652.