Predicting
Temperature Trends during Freeze Nights
FP002 Quick Answer
R. L. Snyder, Biometeorology Specialist
Department of Land, Air and
Revised
Predicting when the temperature falls to a
critical
value is important for starting active frost protection methods. In
addition,
the duration of temperature below the critical value is important for
assessing
potential damage. Starting at the proper temperature is important
because it
avoids losses resulting from starting too late and it saves energy by
reducing
the operation time of the various methods.
Assuming that the predicted minimum temperature (Tp)
is correct and it will occur during the sunrise hour, then
the nighttime hourly mean temperature trend from two hours after sunset
through
the sunrise hour is estimated using equations 1 and 2.
(1)
(2)
In Eq. 1, i
=0 for
the sunset hour and i=n for
the sunrise
hour the next morning. Ti is the mean air
temperature for the
ith hour and T2
is the mean temperature during the second hour following the sunset
hour.
Starting at hour i=2, which is two
hours after
the sunset hour, the mean hourly air temperature is predicted for each
hour
through the sunrise hour the next morning. The method is illustrated in
Figure
1 (oF) and Figure 2 (oC).
Note
that the value for b depends on the temperature units. For oF, b=3.000 and for oC,
b=1.773.
It is important to remember that this method will only work during
radiation
freezes with calm, clear nights when the temperature drops because of
long wave
radiation losses from the surface. Under windy and/or cloudy
conditions, the
temperature trend cannot be predicted using the approach presented
here.
However, radiation freezes do not occur during cloudy, windy
conditions.
Generally, subtropical crops are most sensitive to freeze damage just
before
and during the harvest period when the temperature is lowest. The
sensitivity
of deciduous tree and vine crops to freezing temperature increases from
first
bloom to the small nut or fruit stages. Sensitivity is also higher when
warm
weather has preceded the frost night.
Damage to a crop
depends on how low the temperature goes and how long it is at the
minimum
temperature. Most published values for freeze damage to crops give the
T10,
T50, and T90 for 30 minutes at a given minimum air temperature. The
T10, T50,
and T90 are the air temperatures at which a 10%, 50% and 90% yield loss
is
expected when exposed to that temperature for 30 minutes. For example,
given a
T90=23oF=-5.0oC, we would expect more than 90%
yield loss
during the night of
At any given time during the night, the corresponding temperature is
the
predicted hourly mean temperature for the preceding hour. Therefore,
the
critical time for starting any active freeze protection method is one
hour
before the critical damage temperature is expected. For example, using
the
Note that this prediction method is fairly accurate, but it is not
perfect. The
observed temperatures between
Figure 1. Observed
and predicted hourly mean temperatures (oF)
during a radiation freeze on
Figure 2. Observed
and predicted hourly mean temperatures (oC) during a
radiation
freeze on
Figure 3. Predicted and
observed temperature (oF) traces
for
Figure 4. Predicted
and observed temperature (oC) traces for
When using wind machines for freeze protection, the fans should be
started
while the temperature measured at about 5 ft (1.5 m) height is above
the
critical damage temperature and before the 5-ft (1.5 m) height
temperature
falls much below the 33-ft (10 m) height temperature. There is some
evidence
that under very strong inversion (temperature increasing rapidly with
height)
conditions, air near the surface becomes stratified and stable. This
may lead
to problems mixing the air. Therefore, it is best to start when the
inversion
is not too strong. If the inversion is weak and damage is expected when
the
crop is exposed to a critical temperature Tc=30oF
(-1.1oC) for half-an-hour, then the fans should be started
one hour
prior to when the mean air temperature during the preceding hour equals
30oF
(-1.1oC). Moving the start time ahead by one hour generally
insures
that the fans will be started before damage is likely. Using Figures 1
and 2,
the mean hourly temperature was about 30oF (-1.1oC)
at
8:30 p.m. Therefore, the fans should be started at about
When using sprinklers for freeze protection,
the
sprinklers should be started and stopped when the wet-bulb temperature (Tw) is above the critical
damage
temperature (Tc). This is
discussed
in Quick Answer - FP001. The air temperature to start the sprinklers is
estimated by first measuring the dew point (Td)
temperature
(see Quick Answer - FP003). Then use Table 1 or 2 to determine the
starting
temperature by finding Tw=Tc in the top row and Td
in the left-hand column of the table. The sprinklers should be started
when the
observed temperature is at or above the corresponding air temperature (T)
listed in the table. If Td=23oF (-5.0oC)
and Tc=30oF
(-1.1oC),
then the sprinklers should be started when the air temperature is at or
above
34.0oF (1.1oC). This is because Tw=30oF
(-1.1oC) when Td=23oF (-5.0oC)
and T=34.0oF (1.1oC). Using Figures 1 and
2, the
mean hourly temperature was about 34oF (1.1oC)
for the
hour ending at 7:00 p.m. Therefore, the sprinklers should be started
about one hour
earlier at 6:00 p.m.
Table 1. Minimum turn-on and turn-off air temperatures (oF)
for sprinkler frost protection for a range of wet-bulb and dew-point
temperatures (oF)*
|
Dew-point Temperature |
Wet-bulb Temperature (oF) |
|||||||||||
|
oF |
22.0 |
23.0 |
24.0 |
25.0 |
26.0 |
27.0 |
28.0 |
29.0 |
30.0 |
31.0 |
32.0 |
|
|
32 |
|
|
|
|
|
|
|
|
|
|
32.0 |
|
|
31 |
|
|
|
|
|
|
|
|
|
31.0 |
32.7 |
|
|
30 |
|
|
|
|
|
|
|
|
30.0 |
31.7 |
33.3 |
|
|
29 |
|
|
|
|
|
|
|
29.0 |
30.6 |
32.3 |
34.0 |
|
|
28 |
|
|
|
|
|
|
28.0 |
29.6 |
31.2 |
32.9 |
34.6 |
|
|
27 |
|
|
|
|
|
27.0 |
28.6 |
30.2 |
31.8 |
33.5 |
35.2 |
|
|
26 |
|
|
|
|
26.0 |
27.6 |
29.2 |
30.8 |
32.4 |
34.0 |
35.7 |
|
|
25 |
|
|
|
25.0 |
26.5 |
28.1 |
29.7 |
31.3 |
32.9 |
34.6 |
36.3 |
|
|
24 |
|
|
24.0 |
25.5 |
27.1 |
28.6 |
30.2 |
31.8 |
33.5 |
35.1 |
36.8 |
|
|
23 |
|
23.0 |
24.5 |
26.0 |
27.6 |
29.1 |
30.7 |
32.3 |
34.0 |
35.6 |
37.3 |
|
|
22 |
22.0 |
23.5 |
25.0 |
26.5 |
28.1 |
29.6 |
31.2 |
32.8 |
34.5 |
36.1 |
37.8 |
|
|
21 |
22.5 |
24.0 |
25.5 |
27.0 |
28.5 |
30.1 |
31.7 |
33.3 |
34.9 |
36.6 |
38.2 |
|
|
20 |
22.9 |
24.4 |
25.9 |
27.4 |
29.0 |
30.6 |
32.1 |
33.7 |
35.4 |
37.0 |
38.7 |
|
|
19 |
23.4 |
24.9 |
26.4 |
27.9 |
29.4 |
31.0 |
32.6 |
34.2 |
35.8 |
37.5 |
39.1 |
|
|
18 |
23.8 |
25.3 |
26.8 |
28.3 |
29.8 |
31.4 |
33.0 |
34.6 |
36.2 |
37.9 |
39.5 |
|
|
17 |
24.2 |
25.7 |
27.2 |
28.7 |
30.2 |
31.8 |
33.4 |
35.0 |
36.6 |
38.3 |
39.9 |
|
|
16 |
24.6 |
26.1 |
27.6 |
29.1 |
30.6 |
32.2 |
33.8 |
35.4 |
37.0 |
38.7 |
40.3 |
|
|
15 |
25.0 |
26.4 |
27.9 |
29.5 |
31.0 |
32.6 |
34.2 |
35.8 |
37.4 |
39.0 |
40.7 |
|
*Select a wet-bulb temperature and dew point
temperature and
read the corresponding air temperature from the table. This table
assumes a
barometric pressure of 1013 millibars
(101.3 kPa).
Table
2. Minimum
turn-on and turn-off air
temperatures (oC) for sprinkler frost protection for a range
of
wet-bulb and dew-point temperatures (oC)*
|
Dew-point Temperature |
Wet-bulb Temperature (oC) |
|||||
|
oC |
-5.0 |
-4.0 |
-3.0 |
-2.0 |
-1.0 |
0.0 |
|
0.0 |
|
|
|
|
|
0.0 |
|
-1.0 |
|
|
|
|
-1.0 |
0.7 |
|
-2.0 |
|
|
|
-2.0 |
-0.4 |
1.3 |
|
-3.0 |
|
|
-3.0 |
-1.4 |
0.2 |
1.9 |
|
-4.0 |
|
-4.0 |
-2.5 |
-0.9 |
0.8 |
2.4 |
|
-5.0 |
-5.0 |
-3.5 |
-1.9 |
-0.4 |
1.3 |
2.9 |
|
-6.0 |
-4.5 |
-3.0 |
-1.5 |
0.1 |
1.8 |
3.4 |
|
-7.0 |
-4.1 |
-2.6 |
-1.0 |
0.6 |
2.2 |
3.9 |
|
-8.0 |
-3.6 |
-2.1 |
-0.6 |
1.0 |
2.6 |
4.3 |
|
-9.0 |
-3.3 |
-1.7 |
-0.2 |
1.4 |
3.0 |
4.7 |
*Select a wet-bulb temperature and dew point
temperature
and read the corresponding air temperature from the table. This table
assumes a
barometric pressure of 1013 millibars
(101.3 kPa).
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