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Degree Days
Temperature is a key factor contributing to the develop-
ment of plants, insects and plant diseases. Degree Days
are a way to quantify the amount of heat that is available,
which is a function of the time the temperature is within a
given temperature range. For example, if the base temper-
ature is determined to be 40 degrees and the actual tem-
perature is 41 degrees for 24 consecutive hours, one De-
gree Day is said to have accumulated (41 – 40 = 1 degree
for 24 hours or 1 day). Degree Days indicate the develop-
mental stage of a pest generation. This allows for more
precise pesticide recommendations.
The logger’s firmware calculates Degree Days using the
integral method. Degree Day values are calculated at 15
minute intervals to produce Degree Quarter-Hours (DQH),
which are then summed over a full day. DQH are calcu-
lated as follows:
DQH = T
avg
- T
base
Where T
avg
is the average temperature over the 15-minute
interval and T
base
is the base temperature. If the average
temperature is greater than the upper limit of the tempera-
ture range, the upper temperature limit is used instead of
the average temperature when calculating DQH. If the
average temperature is less than the base temperature,
DQH is set equal to zero for that interval.
Chill Hours
Chill hours are calculated as the amount of time spent be-
low a base temperature. Chill hours accumulations are
used to estimate dormancy for tree fruit.
Degree Days/Chill
Hours
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