Page 20-Corn Talk, Lancaster Farming, Saturday, April 17,1993
Growing Degree Days Swayed By Observation Time
Art DeGaetano
Northeast Regional
Climate Center
Department of
Soil, Crop
And Atmospheric
Sciences, New York
State College
Of Agriculture
And Life Sciences
Cornell
Growing degree day (GDD)
accumulations are widely used
to evaluate crop varieties and
manage their cultivation and
harvest. Although calculating
GDD accumulations is relative
ly straightforward, differences
in the time of day that ther
mometers are checked for daily
maximum and minimum temp
eratures can lead to significant
differences in GDD accumula
tions over weekly or longer
periods.
Simulations using hourly
temperature data indicate that
the time required to reach a
given GDD threshold value
during the growing season
often varied by two weeks or
more solely due to observation
time differences. Generally,
thermometers are observed for
daily temperature extremes at
one of three times, either in the
morning (7 a.m.-8 a.m.), the
early evening (4 p.m.-7 p.m.),
or at midnight, and cover the
preceding 24 hours.
Whereas temperature read
ings taken at midnight tend to
be similar to those taken at the
most common observation
hours, 7 a.m. and 8 a.m., aver
age temperatures calculated
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from afternoon observations
tend to be warmer. This warm
bias results because these
observation hours coincide
with the normal time of maxi
mum temperature occurrence.
If a warm afternoon is fol
lowed by a day with cooler
temperatures, the maximum
temperature observation taken
during the warm afternoon will
likely represent the maximum
temperature of both the current
and subsequent 24-hour per
iods because, once reset, the
thermometer will remain at or
near the maximum tempera
ture. (These thermometers
can’t be reset to lower than the
current air temperature.) In
effect, this maximum tempera
ture is recorded twice. Thus,
sites that make observations
about the time of the daily max
imum temperature tend to accu
mulate GDD at a faster rate
than stations taking morning or
midnight observations.
In contrast, GDD calculated
from daily temperature obser
vations taken near the time of
the daily minimum temperature
occurrence (generally 5 a.m.-6
a.m.) tend to accumulate at a
relatively slow rate, because the
minimum temperature on a par
ticularly cold morning is often
recorded as the minimum for
two successive days. Since
maximum and minimum temp
eratures can occur at any time
of the day, a certain degree of
observation time bias is asso
ciated with each observation
hour. However, when com
pared to GDD calculated using
the average of 24 hourly temp
erature values, this bias tends to
be small for observations taken
at 8 a.m.
A correction method has
been developed by the North
east Regional Climate Center
for use in the northeastern
United States. Using the proce
dure, weekly base SO GDD,
86-50 GDD, and Canadian com
heat unit totals based on any
observation hour can be stan
dardized to an 8 a.m. observa
tion time. For each month, cor
rection factors representing the
average difference between the
weekly GDD accumulation cal
culated using a given observa
tion hour and that of an 8 a.m.
observation schedule are used table l
Weekly Correction Factors GDDSO (86-50 GDD)
May June July Aug.
-3 (0) -3 (-3) -3 (-3) -4 (-4)
-6 (-3) -5 (-4) -5 (-5) -6 (-5)
-2 (-1) -2 (-1) -2 (-2) -2 (-2)
7 (9) 8 (8) 6 (5) 6 (5)
4 (6) 4 (5) 3 (3) 2 (2)
2(4) 2(3) 1(1) 1(1)
Observation Hour
Midnight
6:00 a.m.
7:00 a.m.
4:00 p.m.
6:00 p.m.
7:00 p.m.
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as a basis for standardization.
Correction factors for several
hours arc given in Table 1.
To standardize the weekly
ODD accumulation for a given
hour, the tabulated correction
factor is simply subtracted from
the ODD total.
For example, assume 140
GDDSO have accumulated dur
ing a week in August using an 6
a.m. observation. To standar
dize this total to that of an 8 a.m.
observation, -6 GDDSO should
be subtracted (actually adding 6
in this case) from the 6 a.m. val
ue, giving a standardized total
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of 146 GDDSO,
Similarly, if 100 GDDSO
have accumulated during a
week in June based on a 4 p.m.
observation, 8 GDDSO should
be subtracted from this value to
arrive at the standardized total
of 92 GDDSO. Additional
refinements to the standardiza
tion procedure are also
possible.
An additional correction is
also included for GDD totals
calculated from late afternoon
or evening temperature obser
vations. In these cases, the
maximum temperatures
(Turn to Pag* 21)
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Sept.
-4 (-3)
-6 (-4)
-3 (-2)
7(7)
3(3)
1 (1)