THE UNDERGROUND CLIMATE

The term “climate” is used by the World Meteorological Organization (WMO) to describe the average weather conditions of a given region. “Weather conditions” mainly refer to the mean temperature and annual rainfall, as well as their seasonal variability, since these are the most readily available data and are supported by sufficiently long historical records.

This definition involves two partly arbitrary choices: the extent of the region to be considered, and the time interval over which the average is calculated. There are no strict prescriptions regarding the spatial extent, whereas for the time period WMO generally adopts a 30‑year interval, as this includes at least two cycles of solar activity, whose period is roughly 11 years.

In the underground environment, the atmosphere is negligible compared with the surrounding rock, both in terms of volume and mass; moreover, the voids (interstitial pores and fractures) are predominantly filled with water. This raises a question: can the concept of “climate” be extended to the subterranean world—or does it even make sense to speak of it?

There are, however, particular cases. In karst areas, underground spaces are largely filled with air (in continuity with the external atmosphere), and this dense network of cavities provides an important habitat for a wide variety of living organisms. It is especially with reference to these latter that it is meaningful to speak of an underground climate, as it decisively influences their metabolism.

The concept of underground climate is therefore fully justified in complex networks of cavities of karst origin, whether they are explorable by humans (caves) or not (micro‑karst cavities and discontinuities within the rock mass). With the exclusion of thermal caves, which are influenced by fluids of deep origin, the underground climate depends substantially on surface conditions. External climatic variations are transmitted into caves through the circulation of air and water. However, these changes are attenuated and delayed, depending on several factors—most notably the thermal inertia of the rock mass and the geothermal heat.

Since it is not possible to define an “underground rainfall”, in caves temperature can be considered the principal climatic parameter. While it can be useful to consider other atmospheric variables—such as relative humidity, air pressure, and airflow velocity—the features of the underground climate can often be characterised sufficiently by measuring air temperature, together with the temperature of rock and water (where present), since air temperature is strongly controlled by these two matrices.