The fact that tomatoes, courgettes or cucumbers are available all year round in supermarkets is mainly due to greenhouses. Even in frosty temperatures, the greenhouse effect ensures a warm indoor climate with no heating needed; moreover, uneconomic, dry land can be utilised for agriculture. Here the Spanish province of Almeria, which has the highest concentration of greenhouses worldwide, is the most prominent example.
Greenhouse film is a science in itself: For greenhouses to work, the film naturally has to be tight so that the heat remains inside. The technical term for this is thermal convection. On the other hand, the transparent film ensures that the shortwave radiation from the sun can penetrate but is absorbed. Thus, the heat cannot escape, and this leads to a high temperature difference between the inside of the greenhouse and the surroundings.