BEFORE his opiate reverie was so rudely interrupted by the person on business from Porlock, Samuel Taylor Coleridge dreamed up the ultimate in caves. It was in Xanadu, if you remember, "where Alph, the sacred river, ran through caverns measureless to man, down to a sunless sea". Real life caves are less fantastic, but from a meteorological viewpoint, they have their own special interest.
There is reputed to be a connection, for example, between caves and thunderstorms. It has been suggested that sometimes radioactive material extracted from the rocks is deposited by subterranean waters on the walls of caves, and that this in turn leads to an ionisation of the air inside, so that it contains an abundance of electrically charged particles. Caves often "breathe out" as a result of the local fall in atmospheric pressure associated with a thunderstorm and the resultant outpouring of ionised air at the mouth provides, apparently, a preferred path for any impending stroke of lightning.
The micro climatology inside a cave is critically dependent on the number of openings it may have - whether it has multiple "mouths" or only one. Caves with more than one entrance experience drafts and convection currents, which allow the inside temperature to approach that of the world outside. Any slight difference in height between two entrances is sufficient to set up a circulation: in warm weather, the cold air inside the cave sinks, and flows out through the lower opening, white warm air is sucked in at the top; in cold wintry conditions, the reverse happens.
A cave with only a single opening, on the other hand, behaves quite differently. If it slopes downwards - a "sock" cave, as it is sometimes called - it forms a drainage pit for cold air. The air inside such a cavern is prevented from becoming completely stagnant only by the tendency, as we have seen, for the cave to "breathe" in response to external changes in barometric pressure. The temperature inside is normally found to lie somewhere between the minimum likely to occur at the entrance, and the ambient temperature of the subterranean rock.
A characteristic of this latter type of cave is the great constancy of its meteorological conditions. The temperature deep inside a one mouthed cavern may change by no more than half a degree over hundreds or even millions of years. Moreover with an abundance of underground water, and a calm, unchanging airmass, the relative humidity inside is nearly always in excess of 90 per cent: it is this saturation of the air that explains the moisture laden walls so typical of eaves.