Degree-days can be calculated by several methods. The simplest form of degree-day calculation is by the rectangle or historical method which uses simple averaging. It is less accurate, but provides adequate results and can be done by hand. Degree-days for a single day using the rectangle method can be calculated using this formula:
Other common methods of calculating degree-days are the triangular, sine wave, and cosine curve methods. All of these methods calculate degree-days as the area under the daily temperature curve. However, these are more complex and require a computer for calculation.
Most degree-day models use the sine-wave curve to calculate the number of degree-days. The sine method assumes that the daily temperature cycle takes the form of a sine wave. A sine curve is produced over a 24-hour period using the maximum and minimum temperatures from that day. Degree-days are accumulated in the area under the curve between the upper and lower thresholds.
**Accumulated degree days are represented by the area under the curve within the upper and lower thresholds.
"If one assumes that the temperature cycle is approximated by a sine wave, then 6 possible relationships can exist between the cycle and the upper and lower thresholds. That is, the temperature cycles can be (1) completely above both thresholds, (2) completely below both thresholds, (3) completely between both thresholds, (4) intercepted by the lower threshold, (5) intercepted by the upper threshold, or (6) intercepted by both thresholds. Different equations are required to compute degree days in each case, and the relationships between the maximum, minimum, and the thresholds are used to select the proper equation. The shaded area between the thresholds is denoted "heating degree days" (HDD) and the shaded area below the lower thresholds is designated "cooling degree days" (CDD)...Cooling degree days are calculated because of their possible use in prediction initiation or termination of diapause in insects or 'chilling requirements' of plants." - excerpt taken from: Allen, J.C. 1976. A Modified Sine Wave Method for Calculating Degree Days. Environ. Entomol. 5(3):388-396.
(U.T. = Upper Threshold, L.T. = Lower Threshold)
While these formulas are complex and confusing, it may be simpler to consider degree-day calculations using the sine-wave method like this:
Particular insects have different temperature bases (such as 41°F, 44°F, 48°F, etc) and all have a maximum threshold of 95°F. Therefore:
If the daily max and min temperatures are > 95,
then the degree days for that day are = 95 - lower threshold.
If the daily max and min temperatures are < lower threshold,
then the degree days for that day are = 0,
If the daily max and min temperatures are between 95 and the lower threshold,
then the degree days for that day are = daily mean - lower threshold.
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