What's the temperature in the plastic shed?

The temperature in the greenhouse and its changes have the following characteristics:

(1) has obvious seasonal differences. Chart 19 is the monthly average temperature change chart inside and outside the greenhouse measured by Tianjin Vegetable Research Institute. As can be seen from the figure, the temperature level and its changes in the greenhouse can be divided into the following stages: ① 10/mid-October to mid-February 165438. In mid-June165438+1October, the outdoor temperature is lower than 0℃, the indoor temperature is lower than 10.0℃, and the ten-day average temperature is maintained at (3.2 0.8)℃. At this time, although the highest temperature can be maintained at around 20.0℃, the lowest temperature is only 3.0 ~ 6.0℃, even falling below 0℃, resulting in frost.

Figure 19 Monthly average temperature inside and outside the greenhouse

② The period from the end of June 65438+February to the end of June 65438+ 10 is the lowest temperature in the greenhouse in a year, with the average temperature of ten days being (-3.0 0.8)℃ and the greenhouse temperature being lower than 0℃ for a long time. (3) From early February to mid-February, the temperature in the shed began to rise, and the ten-day average temperature was higher than 0℃, but still lower than 10℃. At this time, the highest temperature in the shed can reach 15.0℃, but the lowest temperature is still below 3.0℃. (4) From late February to mid-March, the temperature in the shed rose rapidly, with the ten-day average temperature reaching over 65438 00.0℃, with an average of (65438 02.0 2.0)℃, the highest temperature reaching 38.0℃, but the lowest temperature was still low. ⑤ From late March to late April, the ten-day average temperature in the shed has reached about 20.0℃, and the highest temperature is 40.0℃. At this time, the temperature outside the shed is changeable, and once the cold air strikes, it is easy to have a "temperature reversal phenomenon". The greenhouse temperature is lower than the outdoor temperature, lower than 0℃, and frost occurs. ⑥ From early May to late September, the ten-day average temperature in the shed is (24.4 3.0)℃, and the highest temperature is 50.0 ~ 60.0℃, so it is necessary to ventilate and cool down in time. It is hot outside in July and August, and it is also hot and humid in the shed. It is necessary to ventilate and reduce the temperature and humidity. Under the condition of adequate ventilation, the temperature inside the shed is 2.0 ~ 4.0℃ lower than that outside the shed due to the cover of the ceiling film, and the temperature difference between inside and outside the shed is not obvious from August to September. ⑦ 10/early October 10 to1early October165438, the shed temperature dropped, and the ten-day average temperature was (14.7 4.0)℃. 10 in June, reduce ventilation and pay attention to heat preservation. If it is done well, the temperature outside the shed will be below -5.0℃ in early winter.

(2) The criteria for dividing the four seasons in the greenhouse are: average weather temperature ≥22.0℃, average maximum temperature ≥28.0℃ in ten days, and minimum temperature ≥15.0℃ in summer; Winter average temperature ≤ 10.0℃, ten-day average maximum temperature ≤ 17.0℃, and minimum temperature ≤4.0℃. In the middle is spring and autumn. According to the above standards, the four seasons division of several horticultural facilities in Beijing is shown in Table 14. It can be seen that the spring in the greenhouse is advanced and the autumn is postponed.

Table 14 Seasonal Division of Plastic Greenhouse in Beijing

(3) The daily temperature changes dramatically, with temperature reversal. Fig. 20 is a graph of daily temperature changes inside and outside the greenhouse. As can be seen from the figure, the daily temperature change trend inside and outside the greenhouse is consistent, but the temperature difference between day and night in the greenhouse is large. The lowest temperature appears before sunrise 1 ~ 2 hours, which is a little later or at the same time as that outside the shed. The duration is short, and the temperature in the shed rises quickly. The highest temperature appears at 12: 00 ~ 14: 00, which is earlier than or at the same time outside the shed. After 14: 00, the shed temperature dropped. The temperature difference between day and night in the shed is from the end of 12 to the middle of February the following year, 10.0 ~ 15.0℃, and from March to September it is 20.0 ~ 30.0℃, and the temperature difference in sunny days is much greater than that in cloudy days. The warming effect of greenhouse is related to weather conditions, such as measured in Xi 'an from April 1979 to April 13 and 15 for three consecutive days. On cloudy days, the average daily temperature in the greenhouse is 3.0℃ higher than that outside, the lowest temperature is 2.0℃ higher and the highest temperature is 5.0℃ higher. Cloudy days correspond to: 14.0℃, 4.0℃,18.0℃; Sunny days are 20.5℃, 1 1.5℃, 35.0℃. Humidification in sunny days is obviously higher than that in cloudy days.

Fig. 20 Variation curve of day and night temperature inside and outside the greenhouse

The phenomenon of "temperature inversion" in greenhouse is a big problem in greenhouse production. The so-called "temperature reversal phenomenon" is the phenomenon that the lowest temperature inside the shed is lower than the lowest temperature outside the shed. The phenomenon of inversion in spring is obvious, which is the most harmful. At this time, although the lowest temperature in the shed is generally higher than or close to that outside the shed, the lowest temperature difference is only 0.0 ~ 0.2℃. When cold air invaded south, the lowest temperature in the shed was lower than that outside the shed 1.0 ~ 2.0℃ on the first sunny and breezy night after the northerly wind, which caused the newly planted seedlings to freeze or die. The temperature reversal starts at night 10 and ends when the temperature in the shed rises after sunrise. This phenomenon is most obvious in cloudy days with northerly winds, but at night the wind stops and the weather turns fine, and the temperature in the shed is the lowest. In late spring and early summer, because the temperature rises, even if the temperature reverses, it will not harm crops; The temperature reversal in autumn is not obvious. Therefore, from the perspective of greenhouse production, keeping warm in early spring is the most important. You can cover grass around the greenhouse, cover multiple layers in the greenhouse, and heat it in the greenhouse if necessary.

(4) The horizontal distribution of temperature in greenhouse is uneven, and there is a "marginal effect". Figure 2 1 is the data measured in Tianjin Vegetable Research Institute in 1978, and the curve in the figure represents the isoline of 1.0 meters high. As can be seen from the picture, the temperature in the central and south-central regions is higher in both day and night. During the day, the temperature is the lowest in the north-central part, and it is lower in the northwest corner and southeast corner at night. The horizontal distribution trend of daily average temperature in the shed is more consistent with that in the daytime. It is higher in the middle and south, and the edge temperature is lower, especially in the north.

Figure 2 1 Horizontal distribution of air temperature in greenhouse (thin solid line in the figure is isotherm, and temperature value in the figure is℃)

In the north of China, especially in the northeast, due to the uneven distribution of temperature in the greenhouse, a low temperature zone appears at the edge of the greenhouse at 1 ~ 2 meters, which is 2.0 ~ 3.0℃ lower than the central temperature of the greenhouse. This is called marginal effect. Covering the west of the greenhouse with a layer of grass at night can alleviate the influence of low temperature.