A greenhouse may be likened to a miniature Earth with an atmosphere defined by glass rather than layers of air. Both the Earth and the earthen floor of the greenhouse are heated by radiant energy reflected back into the atmosphere (or greenhouse interior) as heat.
Significant differences exist, however, in how the greenhouse maintains a hospitable atmosphere for its inhabitants. The glass sides and roof of the greenhouse admit most wavelengths of solar radiation. The tint in the glass largely reflects the longer wavelengths at the infrared end of the spectrum that are produced by the warmed earth and growing plants inside.
Unlike the atmosphere, though, greenhouses also provide a barrier to warmed air as it rises from the ground inside in a process called convection. So, in addition to their use of radiant energy, greenhouses also conserve it by limiting convection.
A Balancing Act
Because a greenhouse is such a small system, it requires controls that manage how much radiant energy to admit and how to circulate heat.
Small greenhouses use black netting or nylon over portions of the structure to limit incoming radiation. Larger structures may have sections painted with whitewash. Many permanent structures have windows with latches at the top of the structure that can be opened using devices similar to awning cranks. These movable glass panels allow warm air to convect out and cool external air to "drop" in as needed.
Seasonal greenhouses may use covers or removable covers on each end to allow air circulation. Fans keep air circulating much as they do in a heated or air conditioned building, mixing air and forcing warm air down out of the rafters.
During cool evenings, fans are not used; cooler air sits near the glass and heat radiates from the ground, keeping plants warm. Fans also manage humidity levels, lessening blossom end rot and other fungus problems.
Natural weather events and climatological cycles complicate the greenhouse operator's task. Cloudy or stormy days will not provide as much incoming energy as sunny days. Wind and rain or snow draw off heat from the surface of the glass or plastic covering.
Changes of seasons, in addition to temperature changes, bring changes in the angle of sunlight, altering the intensity of solar radiation. In some climates, additional heat and lighting equipment may be required to keep greenhouse atmospheres consistent, especially in the late days of winter when nurseries need more light and "bottom heat" for early starts of plants. A sunny winter day may "heat" a greenhouse to dangerous levels faster than a cloudy summer day because the exterior temperature has a minor effect on the interior atmosphere; it is the solar radiation that heats the surfaces and gets plants growing.
The science of greenhouse-keeping consists of mixing air and managing the mechanics to provide a consistent atmosphere for growing plants.