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

Abstract

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 zračnega toka v jami Petites Dales, Normandija, Francija

V članku poročamo o podrobni mikroklimatski študiji v jami Petites Dales. V jami z enim samim vhodom smo dve leti opazovali temperaturo, vlago in porazdelitev zračnih tokov. Podrobne dnevne meritve temperature in zračnega 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 črke 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 članku obravnavamo z vidika termodinamike. V tej raziskavi smo prvič pojasnili dnevne obrate smeri zraka v primerljivih jamah. Študije jamske mikroklime so pomembne za razumevanje jamskega ekosistema, procesov v jamah ter človekovega vpliva nanje.

Ključne besede: jama, mikroklima, kras, zračni tok, temperatura, spremenljivost.