Group Title: Mimeo report - University of Florida North Florida Experiment Station ; 66-3
Title: Comparison of microweather variables under nonperforated and perforated polyethylene tobacco plant bed covers
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 Material Information
Title: Comparison of microweather variables under nonperforated and perforated polyethylene tobacco plant bed covers
Series Title: NFES mimeo rpt.
Physical Description: 6 leaves : ill. ; 28 cm.
Language: English
Creator: Dean, Charles Edgar, 1929-
Davis, D. R
North Florida Experiment Station
Publisher: North Florida Experiment Station
Place of Publication: Quincy Fla
Publication Date: 1965
 Subjects
Subject: Tobacco -- Planting -- Florida   ( lcsh )
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Includes bibliographical reference.
Statement of Responsibility: C.E. Dean and D.R. Davis.
General Note: Caption title.
 Record Information
Bibliographic ID: UF00066035
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 69661529

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NORTH FLORIDA EXPERIMENT STATION
A/F{S L C-< -Quincy, Florida /' \
October 1, 1965 1!

North Florida Station Mimeo Report NFS 66-3


COMPARISON OF MICROWEATHER VATR!ALES UNDER NONPERFORATED AND
PERFORATED POLYETHYLENE TOBACCO PLANT BED COVERS

by C. E. Dean and D. R. Davis'

Since the introduction of plastic films, many types of cover materials and styles of
covers have been used or advocated for tobacco plant beds. Solid plastic covers, made
mostly of polyethylene, are very effective in reducing the time necessary to grow tobacco
plants to transplants size. High daytime temperatures, however, may raise bed temperatures
under the covers to a point where it is detrimental to normal plant growth. To prevent
this, it is necessary to remove the covers on hot days.

In an effort to prevent heat damage to tobacco plants and to lessen the amount of
bed maintenance required, plastic covers with perforations have been developed. The
following report concerns a comparison made in 1965 of the modified microclimate of three
types of perforated covers with a solid plastic cover.

EXPERIMENTALL METHODS

Four experimental plant beds of the coldframe type, each 5 ft. 2 in. in width and
approximately 50 ft. in length, were constructed at the North Florida Experiment Station.
One coldframe was covered with clear nonperforated polyethylene plastic of 4-mil thickness,
while the other three covers were perforated with a square pattern of one quarter inch holes
every three inches each direction. One of the perforated covers was clear polyethylene
plastic of 3-mil thickness, one was a white cloth and polyethylene lamination and one was a
yellow cloth and polyethylene lamination. The sides of all four coldframes were constructed
of 1" x 10" boards. The frame work to hold the covers was constructed using a 1" x 2"
center support running the entire length of the bed and 18" above the bed surface. Wood
strips 1/4" x 2" were arched from one side across the center support to the other side
every three feet down the length of the bed to support the covers. The covers were
permanently secured down the length of one side, stretched across the top and hooked on the
other side and ends to give access for watering and other maintenance operations. Access
to the instruments, without removing the covers, was provided through a small opening in
the cover which could be effectively closed against heat loss. A three-foot walk way
separated the four coldframes which were placed in pairs, end to end.

Meteorological equipment for measuring the micrometeorological variables within and
the meteorological parameters outside the coldframes included standard Weather Bureau
instruments and the Weather Bureau's biometeorological data logging system which is housed
in a van type trailer located near the coldframes.

Weather Bureau type maximum and minimum thermometers, shielded from the rays of the
sun, were mounted on a slanting panel of wood which placed the thermometers four inches
above the bed surface near the center of each coldframe. In the same location a platinum
wire resistance temperature sensor was mounted in a well ventilated shield. A shielded
reference platinum wire aspirated resistance sensor was mounted in the open at the same

1Associate Agronomist, North Florida Experiment Station.
Advisory Agricultural Meteorologist, U. S. Weather Bureau.










level as the sensors in the coldframes. Platinum wire resistance soil sensors were placed
in the bed covered with nonperforated clear plastic and the one covered with perforated
clear plastic to measure the soil temperatures between the surface and the four inch level.
Palmer type maximum-minimum soil thermometers were placed at the 2" leVel in these same two
beds and in the bed covered with the white lamination. Information from all of the
platinum wire sensors was transmitted to the logger by cables. Readings of the maximum and
minimum air and soil thermometers were made daily about 9 a.m. The data logger recorded
information on the hour continuously.

Humidities in the test beds were sampled without removing the covers by comparing
remote telethermometer wet and dry bulb aspirated sensor readings. Sensors were perma-
nently mounted in each of the plant beds. They could be attached to the telethermometer
and a reading taken without disturbing the elements within the coldframes.

Two pyrheliometers were used to measure solar radiation, One was mounted in the
open near the beds and was used as a reference, while the second was placed in each
coldframe for one week. A totalizing anemometer was mounted near the coldframes at the
three foot level, which placed the anemometer cups a little higher than the top of the
coldframe.

RESULTS AND DISCUSSION

Table 1 shows the summary of the temperature data for the period of study which
began January 26, 1965, and ended March 12, 1965. It will be noted that there were little
differences between the means. Daily values varied considerably, but when the values for
any one coldframe were compared with another, there were both positive and negative values
and no trends were apparent. For example, maximum daily temperatures under the clear
perforated cover ranged from 3.5* higher to 5.5 lower than the maximum temperatures under
the clear nonperforated cover. In the case of the perforated white laminated cover the
range was from +6.5* to -110 and for the yellow laminated cover +19 to -3.5@.

When the daily differences in maximum temperatures between the bed covered with the
perforated clear plastic were correlated with the daily daytime air movement, a coefficient
of correlation of .073 was obtained which was not significant. This indicates that air
exchange through the 1/4 inch perforations had little effect in modifying the maximum
temperatures. The daily differences in maximum temperatures between the bed covered with
the nonperforated cover and the three beds covered with the perforated covers were
correlated with the daily radiation measurements. The coefficients were -.348, +.200 and
+.587 for the clear, white and yellow perforated covers respectively, with only the last
coefficient being significant.

Minimum temperatures under the perforated covers had much smaller deviations from
those under the nonperforated cover than did the maximum temperatures. The range of
deviation of the clear perforated cover was from +0.50 to -4.00 with 75 percent of the
deviations falling between -0.5* and -1.5. Minimum readings were higher under this cover
than the nonperforated cover on only one night. For the yellow laminated cover deviations
were very similar to those for the clear perforated cover, and again the deviations were
predominately negative. Under the white laminated cover, the deviations ranged from +2.0
to -2.5 with many nights having essentially the same reading. Here there were about as :
many positive deviations as negative.






-3-



Of particular interest in the study were the differences in the maximum temperatures,
especially on those days when air temperatures under either cover reached the 1000 mark.
The mean daily maximum temperature under the non-perforated plastic was 1.20 higher than
that under the clear perforated plastic, 0.30 lower than under the perforated white
laminated plastic and 1.80 lower than that under the perforated yellow laminated plastic.
On the warmer days when temperatures reached 1000, the temperature under the solid plastic
cover averaged 1.70 higher than that under the clear perforated cover. Readings were
frequently higher, however, under one or more of the perforated plastic covers than that
under the non-perforated cover on the hotter days and on the day when the extreme highest
temperatures were observed, the maximum reading under the solid plastic was lower than that
under either of the three perforated covers (see table 2). This was unexpected, since the
perforated plastics were introduced for the purpose of reducing the maximum daytime
temperatures in the coldframes.

These unexpected findings are partially explainable. High soil moisture was
maintained in the beds at all times. After sunup, temperatures in the beds rose very
rapidly so that the plastic covering was cooler than the hot moist air inside. As a result,
moisture condensed on the inner surface of the plastic covers. In the case of the soil
plastic, very little air exchange occurred with the cooler drier air on the outside, and
so a very heavy layer of moisture was maintained, while air exchange under the perforated
plastics decreased the condensation on the cover around the perforations. The increased
condensation under the solid cover had the effect of changing the albedo of the clear
plastic cover which in turn reduced the amount of incoming radiation. The increased
reflectivity of the cover with the reduced radiation inside the cover acted as a damper to
keep temperatures from rising much, if any, above those of the perforated plastics where
there was some air exchange but more incoming radiation to raise temperatures. Radiation
measurements both in and out of the beds showed that the incident radiation under the clear
nonperforated cover, the clear perforated cover, the white laminated cover and the yellow
laminated cover was 53%, 75%, 60% and 63%, respectively, of fully exposed conditions. These
differences in insolation within the beds could be highly significant in plant production.
The low insolation under the nonperforated plastic cover may be the parameter responsible
for abnormal plant growth previously noted in plastic covered beds by Dean 1961..


Highest temperatures were observed under the yellow and white perforated laminated
covers. The cloth laminations had the effect of reducing the radiation, but not to the
extent of the heavy condensation on the solid plastics. Additionally, the cloth lamination
caused these covers to absorb radiant energy which was reradiated, part into space and part
into the bed. This is thought to be the principal reason for the higher temperatures under
these covers. Note should be taken, however, of the fact that there was a cloth covered
plant bed just to the north of the coldframes. It is conceivable that reflected radiation
or absorbed and reradiated radiation from the cloth wall of this plant bed could have had
some influence on maximum readings in the two laminated covered coldframes since they were
nearest to the plant bed.

Diurnal Temperature Data--The mean diurnal march of temperatures under the non-
perforated cover and under the clear perforated cover along with the reference temperatures
outside the coldframes are shown in figure 1. Maximum temperatures under the plastic covers
are reached very shortly after the time of maximum solar radiation (12:38 p.m.). This is
nearly two hours before the mean maximum temperature on the reference sensor. The diurnal
march of temperatures under the perforated laminated covers followed the same pattern with
a peak about 1 p.m., as was the case in the other two coldframes.





-4-



Figure 1 also shows the degree of frost protection afforded by the two covers. The
mean degree of protection afforded by the clear nwnperforated cover and the clear perforated
cover was 4.50 and 3.3, respectively. While not shown in figure 1, the degree of pro-
tection afforded by the laminated covers was 3.40 for the white and 4,5 for the yellow.
Note that the mean low temperature occurred at 7 aim., at the reference sensor and in the
coldframes.

Two-inch soil temperature measurements were made in three of the four coldframes.
Referring again to Table 1, it will be noted that there were only small differences in soil
temperatures under the nonperforated cover and the white laminated covers Soil temperatures
under the clear perforated cover averaged nearly 2 lower on both maximum and minimum
readings, compared with the other two covers.

Humidity readings were made in the coldframes on a mostly sunny day and on a cloudy
day. On the sunny day the ambient mixing ratio in the open was 11 grams of water per
kilograms of air (g/kg), inside the clear perforated covered bed it was 15 g/kg, while the
ratio was 14 g/kg in the other three beds. Observations on the cloudy day indicated the
outside ambient mixing ratio to be 16 g/kg. Under the solid nonperforated cover the mixing
ratio was 20 g/kg which compared with 19 g/kg for the other three coldframes.

It should be noted that the aerodynamic properties of the nonperforated cover made
it difficult to secure the coVet to keep it from coming loose or tearing off tnder high wind
conditions, This problem was not experienced with perforated covets. In addition$ the
cloth and plastic layers of the laminated material had a tendency to separated thus reducing
the strength and effectiveness of this type of material for plant bed covers.

SUMMARY

Under these experimental conditions, the temperature parameters of the microclimate
of a coldframe covered with a solid plastic cover were found to differ little from those
in a coldframe covered with perforated clear plastic or cloth and plastic lamination.
Large differences were noted among the covers in the transmittance of insolation or incoming
solar radiation, with the lowest and highest values being 53 percent and 75 percent for
clear nonperforated plastic and clear perforated plastic, respectively. This was considered
to result from heavier condensation on the inner surface of the nonperforated plastic,
which increased the albedo and reduced the amount of solar radiation passing through. The
somewhat lower temperatures under the nonperforated cover on hot days can also be attributed
to this occurrence. Highest temperatures were noted in beds covered with perforated plastic
and cloth laminated materials. I1hile the cloth lamination reduced the incoming solar
radiation, the effect was not as great as was the condensation on the clear nonperforated
cover.

Soil temperatures at two inches averaged nearly 20 lower under the clear perforated
cover, compared with the nonperforated and white laminated covers. Humidity readings
showed small differences between beds on both sunny and cloudy days.

Difficulty was experienced in keeping the nonperforated cover securely fastened in
strong winds. Grommets in the laminated covers worked satisfactorily for a short time, but
later pulled out of the material. In addition, the layers of the laminated materials
tended to separate, thus reducing their effectiveness and strength.

Reference
Dean, C. E., "Plastic Covered Plant Beds for Cigar-Wrapper Tobacco", North Florida Expt.
Sta. Mimeo. Report 62-2, 1961,








TABLE 1.--SUTARY OF TEMPERATURE DATA


TEMPERATURES


NONPERFORATED
Clear


AIR


TYPE OF BED COVER
PERFORATED
Clear Laminated
White


Mean
Mean Maximum
Mean Minimum
Extreme Maximum
Extreme Minimum*

SOIL 2" DEPTH

Mean
Mean Maximum
Mean Minimum


*A 25.5" temperature occurred in
the plastic cover had blown off


this coldframe on
during the night.


the morning of February 1 but


TABLE 2.--COMPARISON


OF HIGHEST COLDFRAME TEMPERATURES


TYPE OF BED COVER
NONPERFORATED PERFORATED


Clear


February 7
February 9
February 10
February 11
February 18
February 19
February 20
February 21
February 22
February 27
February 28
March 2
March 3
March 7
March 9
March 10


101.0
107.0
104.5
97.5
104.0
101.0
101.0
107.0
99.0
108.0
107.0
100.0
112.0
101.5
107.5
100.5


Clear


97.5
103.0
101.0
96.0
103.5
101.5
97.5
104.0
99.0
104.0
104.0
101.5
112.5
102.0
106.6
101.5


Laminated
Yellow


67.3
89.2
45.5
112.0
30.0


67.1
79.1
55.0


65.9
89.5
45.3
114.0
28.5


66.3
88.0
44.6
112.5
29.5


65.3
77.7
53.0


67.9
91.0
44.8
115.5
26.0


67.5
80.9
54.1


Laminated
White


Laminated
Yellow


100.0
108.0
107.0
101.0
107.0
102.0
104.0
112.5
105.5
107.5
113.5
101.0
114.0
100.0
106.0
98.0


98.0
107.0
101.0
97.0
109.5
108.0
105.0
111.5
109.0
110.0
108.0
105.5
115.5
103.5
105.5
98.5


DRD
10/4/65
300 CC


-----1,1~--1~-~.,,----UI--UUI----







86
84
82

80
78
76
74

72
70

88
66
64

62
SO
586
56

54
52
50
48
46


24 2 4 6 8 10 12 14 16 18 20 22 24
HOUR


MARCH OF TEMPERATURES


-- -II-.--
/I \\

-__-^__ I ______







r .

___ ___




I \\
,ii \_______ \______

1 jNJN-P:RFORATED
-CLEAR-PERFORATED
RI

__ __ -^_ __ __ __ __ __


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0




F-
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F-


* *


FIG. I DIURNAL




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