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By Jim Nugent
District Horticulturist
Michigan State University

Are you calculating degree day accumulations and finding your values don't match the values being reported by MSU or your neighbor's electronic data collection unit? There is a logical reason! Three different methods are used to calculate degree days; i.e., 1) Averaging Method; 2) Baskerville-Emin (BE) Method; and 3) Electronic Real-time Data Collection. All methods attempt to calculate the heat accumulation above a minimum threshold temperature, often referred to the base temperature. Once both the daily maximum and minimum temperatures get above the minimum threshold temperature, (i.e., base temperature of 42degrees, 50degrees or whatever other base temperature of interest) then all methods are fairly comparable. However, differences do occur and these are accentuated in an exceptionally long period of cool spring temperatures, such as we experienced this year. Let me briefly explain:

1. Averaging Method: Easy to calculate

Degree Days(DD) = Average daily temp. - Base Temp. = (max. + min.) / 2 - Base temp.

If answer is negative, assume 0.

Example: Calculate DD Base 50 given 65 degrees max. and 40 degrees min.

Avg. = (65 + 40) / 2 = 52.5 degrees

DD base 50 = 52.5 - 50 = 2.5 degrees.

But look what happens given a maximum of 60 degrees and a minimum of 35 degrees:

Avg. = (60 + 35) / 2 = 47.5

DD base 50 = 47.5 - 50 = -2.5 = 0 degrees.

The maximum temperature was higher than the base of 50° , but no degree days were accumulated.

A grower called about the first of June reporting only about 40% of the DD₅₀ that we have recorded. Why? He was using the averaging method. It will always underestimate early season accumulation, but never have I seen it this far off!!

2. BE Method: Fits a curve (more specifically a sine curve for the mathematicians in the crowd) to the maximum and minimum temperature to simulate how the temperature varies, then calculates the area of the curve above the base temperature using calculus. Easy for you math majors! This does a better job of simulating the heat accumulation above the base temperature than does the averaging method when the minimum daily temperature is below the base (or lower threshold) temperature. To use this method, MSU Ag Meteorologist Dr. Jeff Andresen has developed a look-up chart (available by contacting your local hort agent or Jeff Andresen) and has available a software program free of charge if you have a computer. If you send Jeff a floppy disk, he'll send you the program. Let us know if you want his address. You then need only to enter the daily max and min into your computer and the machine does the rest! (Does this mean math majors will become obsolete?). This BE method is used by MSU to calculate the degree days reported for the ag weather sites around the state. When the min daily temperature exceeds the base threshold temperature, then the averaging method and the BE give similar results. To calculate your own degree days without a computer, I suggest using the BE lookup chart during the early season, then switch to the averaging method when the season gets warmer.

3. Electronic Weather Data Collection devices don't need to go through these arithmetic calculations. Instead, these devices record temperatures every few minutes. These can then be programmed to simply take the temp reading, subtract the base threshold and accumulate the readings for the day. Hence, these results will be the most accurate; however, keep in mind that virtually all of the research equating pest and plant development to degree day accumulations has been done based on the BE method of accumulation. So what does the greater accuracy mean? Probably not much at this time. However, when one begins working with the degree hour accumulations (vs degree day accumulations) necessary for simulation models such as for fireblight, then maybe it will make a difference.

Well, is this clear or clear as mud? As my former (now retired) colleague Jim Myers often said, "You may still be confused, but at least now you're confused at a higher plane."

A Microsoft Excel97 Spreadsheet for calculation of GDD's created by Bill Klein of the NW Michigan Horticultural Research Station is available to download (147k, xls).

An additional article has been written that describes how GDDs fit insect and disease phenology. To access this article click here (pdf file)

Please send any comments or suggestions regarding this site to:
Bill Klein, kleinw@msu.edu

Last Revised: 6-25-05


	Home Background & Projects Calendar Publications Staff Directory Station Videos Links Search	MAES Home \| Field Stations \| Station Home \| Publications \| Calculating Growing Degree Days Calculating Growing Degree Days By Jim Nugent District Horticulturist Michigan State University Are you calculating degree day accumulations and finding your values don't match the values being reported by MSU or your neighbor's electronic data collection unit? There is a logical reason! Three different methods are used to calculate degree days; i.e., 1) Averaging Method; 2) Baskerville-Emin (BE) Method; and 3) Electronic Real-time Data Collection. All methods attempt to calculate the heat accumulation above a minimum threshold temperature, often referred to the base temperature. Once both the daily maximum and minimum temperatures get above the minimum threshold temperature, (i.e., base temperature of 42degrees, 50degrees or whatever other base temperature of interest) then all methods are fairly comparable. However, differences do occur and these are accentuated in an exceptionally long period of cool spring temperatures, such as we experienced this year. Let me briefly explain: 1. Averaging Method: Easy to calculate Degree Days(DD) = Average daily temp. - Base Temp. = (max. + min.) / 2 - Base temp. If answer is negative, assume 0. Example: Calculate DD Base 50 given 65 degrees max. and 40 degrees min. Avg. = (65 + 40) / 2 = 52.5 degrees DD base 50 = 52.5 - 50 = 2.5 degrees. But look what happens given a maximum of 60 degrees and a minimum of 35 degrees: Avg. = (60 + 35) / 2 = 47.5 DD base 50 = 47.5 - 50 = -2.5 = 0 degrees. The maximum temperature was higher than the base of 50° , but no degree days were accumulated. A grower called about the first of June reporting only about 40% of the DD₅₀ that we have recorded. Why? He was using the averaging method. It will always underestimate early season accumulation, but never have I seen it this far off!! 2. BE Method: Fits a curve (more specifically a sine curve for the mathematicians in the crowd) to the maximum and minimum temperature to simulate how the temperature varies, then calculates the area of the curve above the base temperature using calculus. Easy for you math majors! This does a better job of simulating the heat accumulation above the base temperature than does the averaging method when the minimum daily temperature is below the base (or lower threshold) temperature. To use this method, MSU Ag Meteorologist Dr. Jeff Andresen has developed a look-up chart (available by contacting your local hort agent or Jeff Andresen) and has available a software program free of charge if you have a computer. If you send Jeff a floppy disk, he'll send you the program. Let us know if you want his address. You then need only to enter the daily max and min into your computer and the machine does the rest! (Does this mean math majors will become obsolete?). This BE method is used by MSU to calculate the degree days reported for the ag weather sites around the state. When the min daily temperature exceeds the base threshold temperature, then the averaging method and the BE give similar results. To calculate your own degree days without a computer, I suggest using the BE lookup chart during the early season, then switch to the averaging method when the season gets warmer. 3. Electronic Weather Data Collection devices don't need to go through these arithmetic calculations. Instead, these devices record temperatures every few minutes. These can then be programmed to simply take the temp reading, subtract the base threshold and accumulate the readings for the day. Hence, these results will be the most accurate; however, keep in mind that virtually all of the research equating pest and plant development to degree day accumulations has been done based on the BE method of accumulation. So what does the greater accuracy mean? Probably not much at this time. However, when one begins working with the degree hour accumulations (vs degree day accumulations) necessary for simulation models such as for fireblight, then maybe it will make a difference. Well, is this clear or clear as mud? As my former (now retired) colleague Jim Myers often said, "You may still be confused, but at least now you're confused at a higher plane." A Microsoft Excel97 Spreadsheet for calculation of GDD's created by Bill Klein of the NW Michigan Horticultural Research Station is available to download (147k, xls). An additional article has been written that describes how GDDs fit insect and disease phenology. To access this article click here (pdf file) Please send any comments or suggestions regarding this site to: Bill Klein, kleinw@msu.edu Last Revised: 6-25-05
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