Geothermal heat pumps extract heat either from groundwater (hydrothermal) or from the ground (geothermal). In the former case, heat is extracted through an open-loop system (open loop), in the latter, closed-loop probes are used. Air source heat pumps (aerothermal), on the other hand, extract heat from the outside air. Geothermal heat pumps are not affected by external climatic conditions since the temperature of the water table or ground is almost constant throughout the year, and therefore maintain constant and very high efficiencies. Air source heat pumps, due to the fluctuation of the outside temperature during the year, are less advisable in cold climates (with temperatures close to zero) as they reduce their efficiency in such conditions.