Evidence of daily and seasonal inversions of airflow in Petites Dales cave, Normandy, France

A detailed study of the microclimate of Petites Dales karst cave with single-entrance is presented in this paper. Two years of cave monitoring investigate temperature, weight of water vapour and airflow distributions outside and inside the cavity to determine the global behaviour of the karst cave. In addition, daily measurements of temperature and airflow demonstrate clearly the stratification of the air and inversions of the airflow during day-time. The cave air is intensively exchanged with external air during the years, with two main modes: (i) airflow enters from the ceiling during summer period and (ii) airflow enters from the floor during winter. At the entrance, an Sshaped velocity profile was observed and far from the entrance, convection loops were observed. During the other period of the year, the airflows may switch from top to bottom or conversely. The thermodynamic conditions of switching are discussed in this paper. This study provided for the first time evidence of daily airflow inversion. Study of cave climates is required in the comprehension of cave flora and fauna, thermodynamic conditions for karst processes underground, hydrogeological features of speleothems, human impact when caves are visited. Key words: cave, microclimate, karst, airflow, temperature, variability.  Dnevni in sezonski obrati smeri zracnega toka v jami Petites Dales, Normandija, Francija V clanku porocamo o podrobni mikroklimatski studiji v jami Petites Dales. V jami z enim samim vhodom smo dve leti opazovali temperaturo, vlago in porazdelitev zracnih tokov. Podrobne dnevne meritve temperature in zracnega toka kažejo na stratifikacijo in dnevni obrat smeri toka. Poleti zunanji zrak vdira v jamo pri stropu, pozimi pa pri tleh. Pri tem se pri vhodu oblikuje profil hitrosti zraka v obliki crke S, globlje v jami pa konvekcijska zanka. V vmesnih obdobjih lahko pride do preklapljanja iz dotoka pod stropom v dotok pri dnu in obratno, kar v clanku obravnavamo z vidika termodinamike. V tej raziskavi smo prvic pojasnili dnevne obrate smeri zraka v primerljivih jamah. Studije jamske mikroklime so pomembne za razumevanje jamskega ekosistema, procesov v jamah ter clovekovega vpliva nanje. Kljucne besede: jama, mikroklima, kras, zracni tok, temperatura, spremenljivost.