The greenhouse is an area isolated from the rest of the environment by a transparent cover that allows maximum transmission of sunlight, through which the plants' photosynthetic process takes place.
The radiation reaches the greenhouse directly from the sun and is diffused back from several reflecting celestial bodies throughout the heavens (clouds, dust etc.).
Not all the radiation that reaches the covering sheets reaches the plants. Depending on the properties of the cover material and the angle of ray's impact on the cover, some of the radiation is bounced back into the sky, and some is absorbed in the material itself.
The light that passes into the greenhouse is absorbed in the plants, the soil, and parts of the structure itself and warms them (since the energy contained in the absorbed radiation turns to heat.) The air in the greenhouse also comes in contact with the heated areas, is heated by these, and the result is an increase in the general temperature in the greenhouse.
Covering materials in greenhouses usually block long-wave radiation (light emitted from bodies having low temperatures.) Therefore, the heat that is emitted from the heated areas is absorbed in the cover, causing it, in turn, to warm as well.
This conversion of energy from radiation to heat in a covered area is commonly called "the It should be remembered that the amount of energy available to the plants inside the greenhouse depends upon many variables:
First and foremost, the amount of solar energy outside: Outside energy is affected by geographical location (latitude), time of year, time of day, celestial brightness, etc.
Outside energy, upon entering the structures, is somewhat reduced due to the physical properties of the cover, the geometrical shape of the structure and its position in relation to the north (which determines the angle between sunlight and the cover surface, in turn affecting the transmitting radiation.) Add to that the mutual shading of the structure's skeletal parts, shading from support systems that are installed above the plants and so forth (and therefore the amount of sunlight inside the structure will be lower than outside of it).
This process (the greenhouse effect) enables reaching a much higher temperature inside the greenhouse than outside of it. In a low-temperature environment, this will naturally be considered a positive thing. In areas where temperatures are sometimes lower than freezing point, plants in the open either die or yield nothing.
Cultivating under cover is therefore crucial to achieving any produce at all. However, when and where the outside temperature is naturally high (such as summer in Israel), the temperature inside the greenhouse will be extremely high. In order to avoid these extreme temperatures, special measures should be taken, such as ventilation, cooling, shading, etc.
In point of fact, cultivating crops in a greenhouse is far more complex. Aside from influences on the temperature, due to water vaporization from the plants, there is an increase in humidity compared to outside conditions. In this context, the increased humidity can be regarded as positive and beneficial within certain boundaries. However, if humidity rises too high and for long periods of time (when the greenhouse is closed and unventilated), there is a high probability of damages.