Hand Calculating
Degree Days
By R.L. Snyder, Biometeorologist
Atmospheric
Science, University of California
Copyright - Regents of the University of California
Created – November 1, 2001
Last Revision –
Introduction
Development of many
organisms is dependent on temperature. All
farmers know that crops and pests develop faster in warmer than in cooler years.
However, there is not necessarily a yield or quality benefit in cool or
warm seasons. Organisms simply grow
or develop faster when the air temperature is warmer.
When temperatures are
higher, organisms develop faster. However, they are exposed to the greater heat for fewer days
and the net accumulation of heat required for development is about the same as
for organisms grown under cooler conditions for more days.
This accumulation of heat is called “physiological time” and ºD
are a measure of physiological time. One
oD is defined as one degree above a threshold temperature (TL)
during 24 hours.
Regardless of the
temperature range for growth, the number of oD to develop is
about the same, whereas the number of days to develop is greatly different.
Clearly, oD provides a better predictive tool for
development. Even with fluctuating temperature, it is the total heat
accumulation within an optimal range that determines the time to complete
development.
Hand
Calculating Degree Days
When a threshold temperature is below the minimum
temperature, the degree-days are simply calculated as the mean minus the
threshold temperature. If the
threshold is above the maximum temperature, then there are zero degree-days.
However, the calculation is more complicated for the single sine wave
method, which is used in California, when the threshold falls between the
minimum and maximum temperatures.
The calculation of oD
with microcomputers is relatively simple, but not everyone uses computers.
As an alternative, Snyder (1998) presented a method to calculate oD
using the table given below for the case when the threshold temperature falls
between the daily maximum and minimum temperature. The results are the same as using the single sine method (Zalom
et al., 1983). To use the table,
simply calculate the daily temperature amplitude (α):
and the ratio (R):
Find the value for N
corresponding to R in the table and multiply N by the temperature
amplitude (α) to calculate ºD.
Again, when the threshold is lower than the minimum temperature, ºD are
calculated as the difference between the mean and threshold temperatures.
Degree-day hand calculation table.
Calculate a value for R and select the corresponding N
value from the table. Multiply N
by the daily temperature amplitude α
|
R |
N |
R |
N |
R |
N |
|
0.00 |
1.00 |
0.34 |
0.495 |
0.67 |
0.167 |
|
0.01 |
0.981 |
0.35 |
0.483 |
0.68 |
0.159 |
|
0.02 |
0.962 |
0.36 |
0.471 |
0.69 |
0.152 |
|
0.03 |
0.944 |
0.37 |
0.459 |
0.70 |
0.144 |
|
0.04 |
0.927 |
0.38 |
0.448 |
0.71 |
0.137 |
|
0.05 |
0.910 |
0.39 |
0.436 |
0.72 |
0.130 |
|
0.06 |
0.893 |
0.40 |
0.425 |
0.73 |
0.123 |
|
0.07 |
0.876 |
0.41 |
0.413 |
0.74 |
0.116 |
|
0.08 |
0.859 |
0.42 |
0.402 |
0.75 |
0.109 |
|
0.09 |
0.843 |
0.43 |
0.391 |
0.76 |
0.102 |
|
0.10 |
0.827 |
0.44 |
0.381 |
0.77 |
0.096 |
|
0.11 |
0.811 |
0.45 |
0.370 |
0.78 |
0.090 |
|
0.12 |
0.796 |
0.46 |
0.359 |
0.79 |
0.084 |
|
0.13 |
0.780 |
0.47 |
0.349 |
0.80 |
0.078 |
|
0.14 |
0.765 |
0.48 |
0.339 |
0.81 |
0.072 |
|
0.15 |
0.750 |
0.49 |
0.328 |
0.82 |
0.066 |
|
0.16 |
0.735 |
0.50 |
0.318 |
0.83 |
0.061 |
|
0.17 |
0.721 |
0.51 |
0.308 |
0.84 |
0.055 |
|
0.18 |
0.706 |
0.52 |
0.299 |
0.85 |
0.050 |
|
0.19 |
0.692 |
0.53 |
0.289 |
0.86 |
0.045 |
|
0.20 |
0.678 |
0.54 |
0.279 |
0.87 |
0.040 |
|
0.21 |
0.664 |
0.55 |
0.27 |
0.88 |
0.036 |
|
0.22 |
0.650 |
0.56 |
0.261 |
0.89 |
0.031 |
|
0.23 |
0.636 |
0.57 |
0.251 |
0.90 |
0.027 |
|
0.24 |
0.622 |
0.58 |
0.242 |
0.91 |
0.023 |
|
0.25 |
0.609 |
0.59 |
0.233 |
0.92 |
0.019 |
|
0.26 |
0.596 |
0.60 |
0.225 |
0.93 |
0.016 |
|
0.27 |
0.583 |
0.61 |
0.216 |
0.94 |
0.013 |
|
0.28 |
0.570 |
0.62 |
0.208 |
0.95 |
0.010 |
|
0.29 |
0.557 |
0.63 |
0.199 |
0.96 |
0.007 |
|
0.30 |
0.554 |
0.64 |
0.191 |
0.97 |
0.004 |
|
0.31 |
0.532 |
065 |
0.183 |
0.98 |
0.002 |
|
0.32 |
0.519 |
0.66 |
0.175 |
0.99 |
0.001 |
|
0.33 |
0.507 |
|
|
1.00 |
0.000 |
References
Snyder, R.L.
1985. Hand calculating degree-days.
J. Agric. & For. Meteorol. 35:353-358.
Zalom, F.G., P.B.
Goodell, L.T. Wilson, W.W. Barnett, and W.J. Bentley.
1983.
Degree-days: The calculation and use of heat units in pest management.
UC DANR Leaflet 21373.