Diagnosing the Airflow Mechanism of Blowhole Cave, Edwards County, Texas

A Quantitative Test of the Thermal-Chimney Model

Abstract

In late summer 1976, I recorded outside air temperature and signed entrance airflow at hourly intervals during five consecutive days at Blowhole Cave, Edwards County, Texas, with a return visit two weeks later that captured a continuous overnight cycle. The dataset comprises 96 hourly samples taken with a vane anemometer at a 0.139 m. (1.5 ft.) entrance restriction, with cave-air temperature independently measured at 22.2 °C (72 °F). I test the two dominant cave-ventilation hypotheses against these data. The buoyancy-driven thermal-chimney model fits the signed flow with a Pearson correlation of r = +0.55 against sign(ΔT) ∣ΔT∣ , with negligible correlation against dT/dt, and yields an effective chimney height Δh ≈ 49 m (159 ft) from a single-parameter regression. The flow reverses sign precisely when outside temperature crosses cave temperature, a parameter-free prediction of the chimney model that no other mechanism reproduces. The 102 m (336 ft) of vertical relief surveyed inside the cave is more than sufficient to account for the observed flow. The barometric / large-volume hypothesis is rejected on six independent grounds, including a hidden-volume requirement of 1.6 Å~ 107 m., or roughly 1,100 times the surveyed cave volume. The result confirms the qualitative field interpretation recorded by the 1976 Project Deep survey team. The analysis extends the original treatment in Passmore (2023) with signed-flow analysis, USGS 3DEP topographic verification, and contextualization against the post-2020 cave-ventilation literature.