By Michael Cook
Greenhouses work by the differing transparency of glass to solar and thermal radiation. Glass is transparent to most of the wavelengths of solar radiation, but is thick to the much longer (thermal infrared) wavelengths released by the plants and soil inside the greenhouse. Solar radiation is able to enter the greenhouse, where it is absorbed by the plants and heats them up. The longer thermal wavelengths emitted by the heated plants cannot get out through the glass so heat keeps building up. Plastic greenhouses, however, appear to work as well as glass ones. The plastic is not nearly as transparent to thermal infrared radiation as it is to solar radiation. This leads us to wonder why a greenhouse gets so hot on a sunny day verses a cloudy one.
Whether covered or uncovered by a greenhouse, soil naturally absorbs radiation from the sun and becomes warm. The air molecules adjacent to the soil also become warm. As air gets warmer, the molecules begin to become more active. As they bounce around, they move further apart and expand. This makes the air less dense. Meanwhile, the air above the warm air is still cool and denser than the warm air next to the soil. Because the warm air is less dense, it rises and the cooler air falls to take its place at the ground level. This are then takes in more heat from the warmed soil. This continues as the layers of warm air stack up. Without a greenhouse on a sunny day, the heated layer of air might be a mile or more thick. As the top layer gets higher in the air and further from the earth, it once again begins to cool until it is back at the same temperature that it started at and it once again begins to make it’s decent back to the earth to start the cycle over again. Also, since the warm air is stretched over such a large mass, the heat is diluted by the enormous amount of matter that must be heated.
On the contrary, a greenhouse takes away the air’s ability to grow large enough in volume to lose its heat. The heat from the soil is limited to the layer of air trapped under the glass roof. This trapped air continues to get warmer throughout the day because is a limited volume to be heated.
So why would somebody want a greenhouse? As a greenhouse traps the warm air, it also traps the evaporated water from the soil making the greenhouse warm and humid all day, creating a perfect atmosphere for plant growth. A greenhouse’s ability to heat up can also cause the plants to overheat easily. A car in the sun heats up due to the same principles as a greenhouse. For this reason, it is smart to install ventilation to keep the greenhouse at the desired temperature.
Obviously, we cannot control the amount of solar waves a greenhouse will receive every day. The amount of sunlight varies day to day and season to season. To maintain the optimal climate conditions year-round, greenhouses usually contain climate control features. Most greenhouses will contain a ventilation system so that air can be released if the greenhouse gets too warm, and some sort of heating system during cooler months to keep the greenhouse from getting too cold. Along with a ventilation system, a greenhouse should contain a plumbing system to regulate the amount the plants are watered and to keep the air humid.
Greenhouses come in various shapes and sizes with different features to accommodate any type of garden. In considering which greenhouse to buy, take into account the size and purpose of your garden. Finding the right greenhouse, whether it be a hobby greenhouse, an industrial greenhouse, a FlowerHouse Greenhouse or any other type of greenhouse, is crucial so your plants can grow to their potential.
Michael Cook is a marketer for Shedtopia. For more information about Sheds, Greenhouses, etc. visit shedtopia.com.
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