Cold frames can be described best as miniature greenhouses. These transparent structures are usually built very low to the ground, and their purpose is to prolong the growing season for whatever plants they cover. The basic operating principle of cold frames is somewhat similar to that of greenhouses, with an important exception: the former do not utilize any other means of heating, except for sunlight.
In some areas, regular greenhouses are also called hothouses, to distinguish them from cold frames. The name is meant to indicate that the heat inside the greenhouse comes not just from sunlight, but also from heating, ventilation, and air conditioning, or some combination thereof. While these structures are suited for industrial-scale vegetable and fruit production, cold frames are better for growing these products at home, in lower quantities.
A large number of farmers usually install cold frames alongside the southern walls of greenhouses. This is done in order to provide newly-germinated seeds with a location where they can be hardened, before being replanted inside the large greenhouse. This hardening process implies exposing the seeds to harsher conditions than present inside the greenhouse, which makes them more resilient, and more capable of withstanding pests and diseases.
Their main purpose is to keep the plants inside warm during the night, or during colder day. This is achieved through filtering heat from sunlight within the structure, and then retaining that heat throughout the night. Without the cold frame, a process called convection would dissipate all the heat by the time morning arrives, and the plants would be subjected to a lot of stress. They would then have to dispense of valuable energy and resources to stave off the cold.
Homeowners who have cold frames in their yards tend to use these garden buildings to extend the growing season of whatever plants they are growing, allowing them to ripe for extended periods of time. A well set-up cold frame can produce a large volume of vegetables and fruits, enough to feed a normal family for a few months.