The climate underground
Going down deep in the Earth’s crust, the temperatures increase gradually (about 3°C every 100 m): this phenomenon is well-known in mines, where very high temperatures are reached in the deep, and where the galleries have to be ventilated to create conditions that allow the survival of the miners. One might think that it is the same inside caves: actually, due to the strong air circulation always present within caves, especially if they belong to complex, highly developed systems with a great drop, the temperature inside the cave is practically constant the whole year round and is approximately equal to the average annual external temperature with the exception of the part close to the entrances, where the temperature is closely connected to the external one. On the Lombard Prealps, for example, temperature is about 8-10° C for the caves that have entrances at an altitude of about 1000-1200 m, and drops to 2-3 ° C for caves at altitudes of about 2000-2300 m.
Water temperature inside the cave is in equilibrium with that of the air and is also constant the whole year round. Karst springs can therefore be distinguished from other springs for their constant temperature and for the fact that it is usually higher than that of external streams in winter and lower in summer. This is not the case, of course, for caves fed by waters coming from the deep, that are generally warmer (at times even too warm, as in many hydrothermal springs!).