Fans are the most common devices used in alternative anti-freeze systems to irrigation. The fan is a device comprising a rotating blade placed at a certain height from the ground (reference value about 10 m) put there to collect the hottest air at the height of the fan and push it downwards, counteracting the layering and thermal inversion that characterises late frost episodes. The response of this defence system increases in accordance with the intensity of the layering present and the difference between the ground temperature and the temperature at the height of the fan. It is not very effective if the freezing is caused by advection where there is no thermal inversion, and may even increase the harm due to remixing of cold air. The area protected by a fan is indicated by the manufacturers of these machines as being 2-3 ha each, making it likely that the range of action is between 100 and 140 m from the fan shaft. There are mobile – and therefore transportable – versions on sale, versions with related hot air production by burners, air that is directed downwards, increasing the protection effect. In areas close to housing, a problem with this technique is the level of noise produced for applications if the system has not been adequately sound-proofed (engine) or if the rotation of the fan is not silent enough.