Journal of Cave and Karst Studies - ISSN 0146-9517
Volume 60 Number 1: 27-32 - April 1998

A publication of the National Speleological Society

Air Temperature and Relative Humidity Study: Torgac Cave, New Mexico
Jeffrey Forbes


Torgac Cave, located in south-central New Mexico is remarkable for at least three reasons: its extraordinary gypsum speleothems, its large bat population, and the unusually cold temperatures within the cave. It has been proposed that the presence of the bats and the speleothems may be related to the anomalous temperatures observed in the cave. This paper reports the results of a five week study of cave air temperature and relative humidity conducted during January-February 1995.

Several different techniques were employed to measure the temperature and relative humidity (RH) of the cave atmosphere. A sling psychrometer was used to make spot measurements of temperature and RH at 11 locations throughout the cave on two successive dates. In addition, hourly measurements of air temperature and RH at a single location were recorded over a five week period using a capacitance probe and data logger.

A novel technique for inferring RH was also attempted in which kaolinite clay samples were placed at different locations throughout the cave, and at different elevations above the cave floor at a single location, and allowed to equilibrate with the cave air. Subsequent laboratory analysis of the water activity and gravimetric water content of the clay samples following equilibration allowed the time-averaged RH to be inferred, and these data were used to reconstruct the vertical RH profile of the cave atmosphere.

Cave air temperatures measured with the sling psychrometer ranged from 5.4 to 11.1ºC, while RH values ranged between 47% and 89%. The driest conditions were encountered close to the entrance, where cold, dense winter air sinks into the main entrance, drying the cave passages below as it warms. The data logger and capacitance probe recorded subtle, but distinct diurnal temperature and RH fluctuations in the Tray Room located some 75 m from the nearest entrance. These measurements suggest that the cave generally becomes progressively drier throughout the winter months, then again becomes wetter during the summer, both in response to inflow of surface water from summer thunderstorms, and stable thermal stratification of the cave atmosphere during the warmer months, which prevents the density-driven influx of outside air.

Values of RH inferred from laboratory analysis of clay samples placed in the cave correlated well with those measured directly using the sling psychrometer or capacitance probe. Analysis of the clay samples indicated the existence of large vertical variations in time-averaged RH from floor to ceiling of the cave passage. Based on these preliminary results, the clay equilibration method could prove useful in other studies of humidity stratification of cave air.

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