What is "polytunnel"

n. A tunnel made of polyethylene, used to grow plants that require a higher temperature and/or humidity than that which is available in the environment.

A polytunnel (also known as a polyhouse, hoop greenhouse or hoophouse, or high tunnel) is a tunnel made of polyethylene, usually semi-circular, square or elongated in shape. The interior heats up because incoming solar radiation from the sun warms plants, soil, and other things inside the building faster than heat can escape the structure. Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall. Temperature, humidity and ventilation can be controlled by equipment fixed in the polytunnel. Polytunnels are mainly used in temperate regions in similar ways to glass greenhouses and cloches ( row covers).