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The effect of perforation on temperature conditions in plastic tunnels
J. agric. Engng Res. (1972) 17,172-177
The Effect of Perforation on Temperature Conditions in Plastic Tunnels GUNNAR GUTIORMSEN*
The effect of plastic tunnels on air temperatures was investigated with a perforated area of 0'65, 1'3, 2'6, 3'9, 5·2 and 6·5 % under varying weather conditions as defined by cloud-cover observations. The diameter of the openings was 10 ern. The effect on temperature was calculated for day-time maxima, night-time minima, 24-h range, average and daily sequence of temperature. Perforation resulted in a marked reduction in the amount of day-time heat and in maximum day-time temperatures, though minimum night-time temperatures were lowered only as much as l aC. The use of adequately ventilated tunnels together with an extended period of covering towards harvesting seems to imply considerable advantages over conventional methods of cultivation.
1. Introduction The positive effect of plastic tunnels on plant growth and development depends upon the extent to which their use is conducive to an improvement in the natural microclimate. But the high maximum temperatures which may ensue involve a considerable risk of impeding growth. In April, maximum air temperatures owing to high in-coming radiation can be about 40°C in closed tunnels.' Minimum night-time temperatures inside the tunnels differ little from outside temperatures, which can be below O°C in the beginning of the growing season. Plants in plastic tunnels can therefore be exposed to extreme temperature variations in the course of a 24-h period. Businger" has formulated the heat loss (cal 1- 2t-1) in greenhouses under certain conditions owing to ventilation, as follows: Hven=C a l ' V/A w ' S(Tin-Tex) + L· V/A w ' S(qln-qex) Cal is the heat capacity of the air (call- 3°C-l), and Vand Aware the volume and the surface of the greenhouse respectively. Tin and T ex are the internal and external air temperatures. S is the number of times the volume of air is replaced per unit of time. L is the heat of vaporization (cal crrr"), and q is the specific humidity. V/A w is less for plastic tunnels than for ordinary greenhouses. A greater S is therefore necessary if the coefficient of heat transfer under otherwise equal conditions is to be the same as in ordinary greenhouses. It is difficult to measure ventilation in plastic tunnels and greenhouses because inward radiation and wind velocity are always variable. In previous investigations, Shadbolt et al. 3 found that the effect of ordinary ventilation by lifting one side of the cover was greater than that of ventilation through perforations in the plastic. The amount of perforation is of course the crucial point. Preliminary studies showed that relatively large openings were necessary in order to keep maximum day-time temperatures down to a reasonable level in full sunshine. There seemed to be a good chance of improving current techniques of cultivation by means of well ventilated tunnels; the efficacy of perforations has therefore been investigated more in detail. The effect of ventilation is of less interest for tunnels under conditions of cloudy weather and prevailing high winds. It is more important to investigate the effects of ventilation in relation to maximum day-time temperatures and minimum night-time temperatures in clear (i.e. cloudless) weather when there is apt to be little wind. Thus it seemed expedient to base a definition of outside climate on a factor of in-coming radiation. As a relatively clear relationship between • State Experimental Station Landv ik, N-4890 Grimstad. Norway
172
173
G. GUTTORMSEN
observations of cloud-cover and the efficacy of plastic tunnels has previously been found, 1 outside climate has, in the present investigation, been described by means of ordinary meteorological observations of cloud-cover.
2. Methods The investigations were carried out at the State Experimental Station at Landvik, near Grimstad, Norway (58° 20' N., 8° 31' E.) in September, 1968. The site was flat, at an altitude of 7 m and at a distance of about 20 m from the permanent meteorological station. The tunnels were ranged in a north-south direction, each one being 1 x 3 m. The area of plastic was 1·2 m2/linear m of tunnel. The height and width of the tunnels and the position of the perforations is shown in Fig. 1. The openings were burnt out, each one having a diameter of 10 em (0,65 % opening equalling 1 perforation/linear metre of the tunnel).
jC~:\ I--
A
-;
Fig. 1. Plastic (PE) tunnels used in the experiments. (A=100 em, B=30 em, C=10 em, D=E=40 em)
Temperatures were measured every 30 min by means of copper-constatan thermocouples connected to an electronic punch-tape recorder. The thermocouples were placed in the middle of the tunnels, 10 em above the ground, under U-shaped shields of perforated aluminium. Records were kept for 3 parallel tunnels. The results were expressed as the increase in the maximum day-time temperature (Tin-Tex max day), the differences in temperature at the night-time minimum (Tin- Tex min nilhl), the 24-hour temperature range in the tunnels (L1 Tin = Tin max -Tin min), the average temperature increase in a 24-h period, and the effect on the temperature sequence throughout a 24-h period. ('t'ln=temperature in the tunnels; Tex=temperature in plots without plastic covers.) Cloud-cover was registered at the meteorological station at 8a.m., 1p.m. and 7p.m., the average of these 3 observations being taken as the average cloud-cover for the 24-h period. A more detailed description of the method appears elsewhere.'
3. Results Fig. 2 shows the correlation coefficients between cloud-cover and the effect of the tunnels on maximum day-time temperature and the 24-h fluctuation of temperature in the tunnels respectively. Calculations of coefficients were Tex gave lower correlation coefficients than ones based on Tin alone. The correlation between cloud-cover observations and the effect of the tunnels was distinctly reduced by the perforation of the tunnels, though all the correlation coefficients in Fig. 2 are statistically significant. It is reasonable to assume that the poorer correlation between cloud-cover and the effect of the tunnels with a greater area of perforation is owing to ventilation, i.e. a transfer of heat in addition to that resulting from radiation. The effect of the tunnels (with various areas of perforation and varying degrees of cloud-cover) on maximum day-time and minimum night-time temperatures is shown in Fig. 3, while the 24-h temperature range appears in Fig. 4. The deviation from the linearity is not statistically significant for perforated tunnels, whereas the regression was curvilinear for closed tunnels when calculations included day-time maxima. Perforation was accompanied by a reduction in minimum night-time temperatures never in excess of c. 1°C. The correlation between cloud-cover observations and the effect of perforated tunnels on night-time minima was not significant.
174
TEMPERATURE CONDITIONS IN PLASTIC TUNNELS lin-;r;x max' cloudiness
-(}90 -0,80 -+ -0,70 I-
*** **
+ +
-0,60 I-
+ -0,50 I-
+ +
+
t:.Tin. cloudiness -090 I-0·80 1-+ -0,70
ll;Ht
+
+
-
+
+
+
** +
-0'60 -0,50 I
I
I
00·5 1·3
2·6
3·9
5·2
6'5
Area of the perforations (%)
Fig. 2. Coefficients of curvilinear correlation between cloud-cover as an independent variable and the increase in the maximum day-time temperature, and the diurnal range of temperatures in the tunnels. The significances are indicated by vertical arrows ***=P
18
a
16
14 '65 ~ ~
/2
10
1·3
:J
2 '"E 6 '"
I-
4 2 lin
a
-r;
0
min night
~
- 2 ~--:-::---'::::-:~-:!-:-:-':-:-'::-::L---,-L_L--L..J
o
10 20
30 40 50 60 70 80 90 100 % cloud - cover
Fig. 3. The effect ofperforated plastic tunnels on the maximum (day) and minimum (night) temperatures as a function of cloud-cover. The area of the perforations (%) is indicated by figures
0'65% Y=I5·4336-0·0550X-0·0004X2 2'6% Y= 8·5825+0·0328X-0·0008X2 6'5% Y= 51881+0·0130X-0·0005X2
The effect of ventilation was greatest when the effect of the tunnel was at a maximum (i.e. when the temperature gradient was greatest). Perforation (6,5 %reduced the effect of the tunnel on the maximum day-time temperature by c. lOoC in clear weather and c. 4°C in cloudy weather (as compared to a closed tunnel). Fig. 3 also reveals that, for closed tunnels, clouding resulted in a reduction of the effect of the tunnel equivalent to a 6·5 % opening in clear weather. Because of the reduction in the maximum day-time temperature, the 24-h temperature range was also reduced by as much as lOoC (Fig. 4).
175
G. GUTTORMSEN
sr; 30
0
28
·65
26 24
1·3
:;>
26
e
22 3'9
2. 20 52 ~
~ E
~
6·5
18 16
14 12
I
10 80
10 20 30 40
5~ 6~
70
~o
10
9
160
% cloud - cover
Fig. 4. The relation between the diurnal range oftemperatures in perforated plastic tunnels and cloud-cover. The area of the perforations (%) is indicated by figures 0'65% Y=30·1217 -0·1299X-0·0005X' 2'6% Y=23·4138-0·0382X-0·00lOX' 6'5% Y=20·4576-0·0514X-0·0008X'
The effect of the tunnels on the temperature sequence throughout a 24-h period in 3 different types of weather (according to cloud-cover observations) is shown in Fig. 5. On the average, a 6·5 % perforation area reduced day-time maxima by c. 9°, 7° and 3°C for the 3 types of weather respectively. Fig. 5 illustrates how the effect of ventilation depends upon weather conditions, and that perforation results in a reduction of day-time temperatures, whereas night-time minima remain almost unchanged. The effect of the tunnels on the 24-h mean temperature as a function of the area of perforation is given in Fig. 6. Perforation (6'5 %) reduced the effect of the tunnels by 4°C, 2·5°C and 2·5°C for the three types of weather respectively. The effect of ventilation/opening diminished with an increase in the number of openings, and there was only a slight increase in the effect of ventilation when the area of the perforations was increased to more than 3·9 %(6 openings/linear m).
4. Discussion One might suppose that because closed tunnels act as a barrier to air-circulation, perforating the tunnels would lead to great air-circulation and would thus reduce the danger of night frost on clear nights. The results of this investigation do not confirm this supposition. The reason may be the stillness of the air just before dawn, as shown by Geiger." If the air is motionless just when the danger of frost is greatest, the perforations will not affect the distribution of temperatures, and the danger of night frost is not reduced. Perforation of the tunnels naturally also promotes factors which lower night-time minima. The heat of evaporation released by condensation has a stabilizing effect on the temperature and will counteract extremely low night-time temperatures. Under otherwise equal conditions, a drying-out of the air owing to ventilation will therefore increase the likelihood of frost. A similar effect is seen also in the heat loss by ventilation during the day and in the reduction of the heat-conductivity of the soil owing to drying out. The net result of these conditions was that the minimum night-time temperature in the perforated tunnels was lowered by as much as 1°C. Less condensation was registered in perforated tunnels, but it is not clear in which direction this influenced the balance of warmth.
176
TEMPERATURE CONDITIONS IN PLASTIC TUNNELS
Tin- Tex 16
14
Sunrise
Sunset
H
H
12 10
8 6
4
2 0
u
0
7;"n-~x
~
.
:l
17"'83% Cloud-cover
<; 10
'" E Q.
H
8
~
6 4
2 0 7in-~x
83"'100% Cloud-cover
6
o
H
4
2 ol====~
-224 02 04 06 08
10
12 14
16 18 20 22
24
Time (h)
Fig. 5. Diurnal sequence of the effect of the tunnels on temperatures. The figures indicate the area of the perforations
co
6
uS o
o
0"65 1"3
2"6
3"9
S"2
6-S
Area of the petoronons (%)
Fig. 6. The effect ofplastic tunnels on the diurnal average temperatures as a function ofarea of the perforations. The daily means of cloud-cover for each group of days are indicated by figures
177
G. GUTTORMSEN
In practice, the reduction in the cumulative amount of day-time heat and in the extreme daytime temperatures which resulted from perforation is far more important than the slight reduction in protection against frost. Data from later investigations (unpublished) show that under Norwegian climatic conditions, perforation (2'6-5'2 % opening) from the beginning of the growing season early in April can lead to a considerable reduction in growth compared with perforation at a later date. In practice, the perforations should be made when outside temperatures in clear weather rise to c. 15°C (equivalent to c. 30°C in the tunnels, according to Fig. 3). Adequate ventilation makes it possible to extend the period of covering until nearer the harvesting; i.e. tunnels can be used to advantage after outside temperatures have usually passed their optimal point for the cultivation of plants. An extended period of covering with properly perforated tunnels should be advantageous with more nearly optimal temperatures (because of the lessened risk of heat damage), and with higher temperatures during the chilly periods that may occur later in the growing season. For climatic conditions in Denmark, Aslyng" found a close connection between the reduction in wind velocity owing to shelter and the reduction in potential evapotranspiration. One may reasonably assume that despite higher temperatures, perforated tunnels will in many instances improve the water balance in the plants because of the reduced wind velocity. REFERENCES 1
Guttormsen, G. (1972). The effect ofplastic tunnels on air and soil temperatures in relation to observations of cloud-cover (In press)
2
3
4 S
Businger, J. A. (1966). The glasshouse (greenhouse) climate. In Physics of Plant Environment (WijK, W. R. van, ed.) pp, 277-318. North-Holland Publishing, Amsterdam Shadbolt, C. A.; Me Coy, O. D.; Whiting, F. L. (1962). The microclimate of plastic shelters used for vegetable production. HiIIgardia 32,251 Geiger, R. (1966). The Climate near the Ground p. 42. Harvard University Press, Cambridge, Mass. Aslyng, H. C. (1958). Shelter and its effect on climate and water balance. Oikos 9, 282
The Effect of Perforation on Temperature Conditions in Plastic Tunnels GUNNAR GUTIORMSEN*
The effect of plastic tunnels on air temperatures was investigated with a perforated area of 0'65, 1'3, 2'6, 3'9, 5·2 and 6·5 % under varying weather conditions as defined by cloud-cover observations. The diameter of the openings was 10 ern. The effect on temperature was calculated for day-time maxima, night-time minima, 24-h range, average and daily sequence of temperature. Perforation resulted in a marked reduction in the amount of day-time heat and in maximum day-time temperatures, though minimum night-time temperatures were lowered only as much as l aC. The use of adequately ventilated tunnels together with an extended period of covering towards harvesting seems to imply considerable advantages over conventional methods of cultivation.
1. Introduction The positive effect of plastic tunnels on plant growth and development depends upon the extent to which their use is conducive to an improvement in the natural microclimate. But the high maximum temperatures which may ensue involve a considerable risk of impeding growth. In April, maximum air temperatures owing to high in-coming radiation can be about 40°C in closed tunnels.' Minimum night-time temperatures inside the tunnels differ little from outside temperatures, which can be below O°C in the beginning of the growing season. Plants in plastic tunnels can therefore be exposed to extreme temperature variations in the course of a 24-h period. Businger" has formulated the heat loss (cal 1- 2t-1) in greenhouses under certain conditions owing to ventilation, as follows: Hven=C a l ' V/A w ' S(Tin-Tex) + L· V/A w ' S(qln-qex) Cal is the heat capacity of the air (call- 3°C-l), and Vand Aware the volume and the surface of the greenhouse respectively. Tin and T ex are the internal and external air temperatures. S is the number of times the volume of air is replaced per unit of time. L is the heat of vaporization (cal crrr"), and q is the specific humidity. V/A w is less for plastic tunnels than for ordinary greenhouses. A greater S is therefore necessary if the coefficient of heat transfer under otherwise equal conditions is to be the same as in ordinary greenhouses. It is difficult to measure ventilation in plastic tunnels and greenhouses because inward radiation and wind velocity are always variable. In previous investigations, Shadbolt et al. 3 found that the effect of ordinary ventilation by lifting one side of the cover was greater than that of ventilation through perforations in the plastic. The amount of perforation is of course the crucial point. Preliminary studies showed that relatively large openings were necessary in order to keep maximum day-time temperatures down to a reasonable level in full sunshine. There seemed to be a good chance of improving current techniques of cultivation by means of well ventilated tunnels; the efficacy of perforations has therefore been investigated more in detail. The effect of ventilation is of less interest for tunnels under conditions of cloudy weather and prevailing high winds. It is more important to investigate the effects of ventilation in relation to maximum day-time temperatures and minimum night-time temperatures in clear (i.e. cloudless) weather when there is apt to be little wind. Thus it seemed expedient to base a definition of outside climate on a factor of in-coming radiation. As a relatively clear relationship between • State Experimental Station Landv ik, N-4890 Grimstad. Norway
172
173
G. GUTTORMSEN
observations of cloud-cover and the efficacy of plastic tunnels has previously been found, 1 outside climate has, in the present investigation, been described by means of ordinary meteorological observations of cloud-cover.
2. Methods The investigations were carried out at the State Experimental Station at Landvik, near Grimstad, Norway (58° 20' N., 8° 31' E.) in September, 1968. The site was flat, at an altitude of 7 m and at a distance of about 20 m from the permanent meteorological station. The tunnels were ranged in a north-south direction, each one being 1 x 3 m. The area of plastic was 1·2 m2/linear m of tunnel. The height and width of the tunnels and the position of the perforations is shown in Fig. 1. The openings were burnt out, each one having a diameter of 10 em (0,65 % opening equalling 1 perforation/linear metre of the tunnel).
jC~:\ I--
A
-;
Fig. 1. Plastic (PE) tunnels used in the experiments. (A=100 em, B=30 em, C=10 em, D=E=40 em)
Temperatures were measured every 30 min by means of copper-constatan thermocouples connected to an electronic punch-tape recorder. The thermocouples were placed in the middle of the tunnels, 10 em above the ground, under U-shaped shields of perforated aluminium. Records were kept for 3 parallel tunnels. The results were expressed as the increase in the maximum day-time temperature (Tin-Tex max day), the differences in temperature at the night-time minimum (Tin- Tex min nilhl), the 24-hour temperature range in the tunnels (L1 Tin = Tin max -Tin min), the average temperature increase in a 24-h period, and the effect on the temperature sequence throughout a 24-h period. ('t'ln=temperature in the tunnels; Tex=temperature in plots without plastic covers.) Cloud-cover was registered at the meteorological station at 8a.m., 1p.m. and 7p.m., the average of these 3 observations being taken as the average cloud-cover for the 24-h period. A more detailed description of the method appears elsewhere.'
3. Results Fig. 2 shows the correlation coefficients between cloud-cover and the effect of the tunnels on maximum day-time temperature and the 24-h fluctuation of temperature in the tunnels respectively. Calculations of coefficients were Tex gave lower correlation coefficients than ones based on Tin alone. The correlation between cloud-cover observations and the effect of the tunnels was distinctly reduced by the perforation of the tunnels, though all the correlation coefficients in Fig. 2 are statistically significant. It is reasonable to assume that the poorer correlation between cloud-cover and the effect of the tunnels with a greater area of perforation is owing to ventilation, i.e. a transfer of heat in addition to that resulting from radiation. The effect of the tunnels (with various areas of perforation and varying degrees of cloud-cover) on maximum day-time and minimum night-time temperatures is shown in Fig. 3, while the 24-h temperature range appears in Fig. 4. The deviation from the linearity is not statistically significant for perforated tunnels, whereas the regression was curvilinear for closed tunnels when calculations included day-time maxima. Perforation was accompanied by a reduction in minimum night-time temperatures never in excess of c. 1°C. The correlation between cloud-cover observations and the effect of perforated tunnels on night-time minima was not significant.
174
TEMPERATURE CONDITIONS IN PLASTIC TUNNELS lin-;r;x max' cloudiness
-(}90 -0,80 -+ -0,70 I-
*** **
+ +
-0,60 I-
+ -0,50 I-
+ +
+
t:.Tin. cloudiness -090 I-0·80 1-+ -0,70
ll;Ht
+
+
-
+
+
+
** +
-0'60 -0,50 I
I
I
00·5 1·3
2·6
3·9
5·2
6'5
Area of the perforations (%)
Fig. 2. Coefficients of curvilinear correlation between cloud-cover as an independent variable and the increase in the maximum day-time temperature, and the diurnal range of temperatures in the tunnels. The significances are indicated by vertical arrows ***=P
18
a
16
14 '65 ~ ~
/2
10
1·3
:J
2 '"E 6 '"
I-
4 2 lin
a
-r;
0
min night
~
- 2 ~--:-::---'::::-:~-:!-:-:-':-:-'::-::L---,-L_L--L..J
o
10 20
30 40 50 60 70 80 90 100 % cloud - cover
Fig. 3. The effect ofperforated plastic tunnels on the maximum (day) and minimum (night) temperatures as a function of cloud-cover. The area of the perforations (%) is indicated by figures
0'65% Y=I5·4336-0·0550X-0·0004X2 2'6% Y= 8·5825+0·0328X-0·0008X2 6'5% Y= 51881+0·0130X-0·0005X2
The effect of ventilation was greatest when the effect of the tunnel was at a maximum (i.e. when the temperature gradient was greatest). Perforation (6,5 %reduced the effect of the tunnel on the maximum day-time temperature by c. lOoC in clear weather and c. 4°C in cloudy weather (as compared to a closed tunnel). Fig. 3 also reveals that, for closed tunnels, clouding resulted in a reduction of the effect of the tunnel equivalent to a 6·5 % opening in clear weather. Because of the reduction in the maximum day-time temperature, the 24-h temperature range was also reduced by as much as lOoC (Fig. 4).
175
G. GUTTORMSEN
sr; 30
0
28
·65
26 24
1·3
:;>
26
e
22 3'9
2. 20 52 ~
~ E
~
6·5
18 16
14 12
I
10 80
10 20 30 40
5~ 6~
70
~o
10
9
160
% cloud - cover
Fig. 4. The relation between the diurnal range oftemperatures in perforated plastic tunnels and cloud-cover. The area of the perforations (%) is indicated by figures 0'65% Y=30·1217 -0·1299X-0·0005X' 2'6% Y=23·4138-0·0382X-0·00lOX' 6'5% Y=20·4576-0·0514X-0·0008X'
The effect of the tunnels on the temperature sequence throughout a 24-h period in 3 different types of weather (according to cloud-cover observations) is shown in Fig. 5. On the average, a 6·5 % perforation area reduced day-time maxima by c. 9°, 7° and 3°C for the 3 types of weather respectively. Fig. 5 illustrates how the effect of ventilation depends upon weather conditions, and that perforation results in a reduction of day-time temperatures, whereas night-time minima remain almost unchanged. The effect of the tunnels on the 24-h mean temperature as a function of the area of perforation is given in Fig. 6. Perforation (6'5 %) reduced the effect of the tunnels by 4°C, 2·5°C and 2·5°C for the three types of weather respectively. The effect of ventilation/opening diminished with an increase in the number of openings, and there was only a slight increase in the effect of ventilation when the area of the perforations was increased to more than 3·9 %(6 openings/linear m).
4. Discussion One might suppose that because closed tunnels act as a barrier to air-circulation, perforating the tunnels would lead to great air-circulation and would thus reduce the danger of night frost on clear nights. The results of this investigation do not confirm this supposition. The reason may be the stillness of the air just before dawn, as shown by Geiger." If the air is motionless just when the danger of frost is greatest, the perforations will not affect the distribution of temperatures, and the danger of night frost is not reduced. Perforation of the tunnels naturally also promotes factors which lower night-time minima. The heat of evaporation released by condensation has a stabilizing effect on the temperature and will counteract extremely low night-time temperatures. Under otherwise equal conditions, a drying-out of the air owing to ventilation will therefore increase the likelihood of frost. A similar effect is seen also in the heat loss by ventilation during the day and in the reduction of the heat-conductivity of the soil owing to drying out. The net result of these conditions was that the minimum night-time temperature in the perforated tunnels was lowered by as much as 1°C. Less condensation was registered in perforated tunnels, but it is not clear in which direction this influenced the balance of warmth.
176
TEMPERATURE CONDITIONS IN PLASTIC TUNNELS
Tin- Tex 16
14
Sunrise
Sunset
H
H
12 10
8 6
4
2 0
u
0
7;"n-~x
~
.
:l
17"'83% Cloud-cover
<; 10
'" E Q.
H
8
~
6 4
2 0 7in-~x
83"'100% Cloud-cover
6
o
H
4
2 ol====~
-224 02 04 06 08
10
12 14
16 18 20 22
24
Time (h)
Fig. 5. Diurnal sequence of the effect of the tunnels on temperatures. The figures indicate the area of the perforations
co
6
uS o
o
0"65 1"3
2"6
3"9
S"2
6-S
Area of the petoronons (%)
Fig. 6. The effect ofplastic tunnels on the diurnal average temperatures as a function ofarea of the perforations. The daily means of cloud-cover for each group of days are indicated by figures
177
G. GUTTORMSEN
In practice, the reduction in the cumulative amount of day-time heat and in the extreme daytime temperatures which resulted from perforation is far more important than the slight reduction in protection against frost. Data from later investigations (unpublished) show that under Norwegian climatic conditions, perforation (2'6-5'2 % opening) from the beginning of the growing season early in April can lead to a considerable reduction in growth compared with perforation at a later date. In practice, the perforations should be made when outside temperatures in clear weather rise to c. 15°C (equivalent to c. 30°C in the tunnels, according to Fig. 3). Adequate ventilation makes it possible to extend the period of covering until nearer the harvesting; i.e. tunnels can be used to advantage after outside temperatures have usually passed their optimal point for the cultivation of plants. An extended period of covering with properly perforated tunnels should be advantageous with more nearly optimal temperatures (because of the lessened risk of heat damage), and with higher temperatures during the chilly periods that may occur later in the growing season. For climatic conditions in Denmark, Aslyng" found a close connection between the reduction in wind velocity owing to shelter and the reduction in potential evapotranspiration. One may reasonably assume that despite higher temperatures, perforated tunnels will in many instances improve the water balance in the plants because of the reduced wind velocity. REFERENCES 1
Guttormsen, G. (1972). The effect ofplastic tunnels on air and soil temperatures in relation to observations of cloud-cover (In press)
2
3
4 S
Businger, J. A. (1966). The glasshouse (greenhouse) climate. In Physics of Plant Environment (WijK, W. R. van, ed.) pp, 277-318. North-Holland Publishing, Amsterdam Shadbolt, C. A.; Me Coy, O. D.; Whiting, F. L. (1962). The microclimate of plastic shelters used for vegetable production. HiIIgardia 32,251 Geiger, R. (1966). The Climate near the Ground p. 42. Harvard University Press, Cambridge, Mass. Aslyng, H. C. (1958). Shelter and its effect on climate and water balance. Oikos 9, 282