📄 earth_science

Climate in a Single-Entrance Cave Controlled by Buoyancy-Driven Counterflow and Thermal Inertia

This study demonstrates that ventilation in single-entrance karst caves is governed by buoyancy-driven bidirectional counterflow during cold periods and is significantly modulated by the thermal inertia of the surrounding rock and sediments, which sustains airflow and alters responses to subsequent cooling events.

Original authors: Franci Gabrovšek, Vanessa E. Johnston, Nenad Buzjak, Dalibor Paar, Matej Blatnik, Andrea Martín Pérez

Published 2026-08-27

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Franci Gabrovšek, Vanessa E. Johnston, Nenad Buzjak, Dalibor Paar, Matej Blatnik, Andrea Martín Pérez

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep underground, in the quiet dark of limestone caves, the air does not sit still. It moves, driven by the same invisible forces that make hot air rise and cold air sink. These subterranean chambers are not sealed tombs but open systems connected to the world above, where the temperature of the outside atmosphere constantly battles the temperature of the rock deep inside. This exchange is vital. The air that circulates through these tunnels carries heat and gases, shaping the chemical weathering of the stone and sustaining unique ecosystems that have evolved in the dark. For decades, scientists understood how caves with multiple openings at different heights ventilate, a process similar to a chimney where air flows in one door and out another. But caves with only a single entrance present a different puzzle. In these one-way doors, air cannot simply flow through; it must enter and leave through the same opening, creating a complex internal struggle between incoming cold air and outgoing warm air.

A team of researchers set out to solve this puzzle by studying Košelevka Cave in Slovenia, a single-entrance cave that slopes downward for sixty meters before opening into a series of chambers. The cave is not just a geological curiosity; its walls are covered in delicate, fibrous white formations called moonmilk, whose creation may depend on the specific climate inside. To understand the air's behavior, the team installed sensors to track temperature and wind speed at various heights and depths, recording data over several years from autumn 2021 through the winter of 2025 and into 2026. They also used simple smoke sticks to visualize the direction of the air, watching how the smoke drifted at different levels within the passage. What they found was a dynamic system where the cave breathes in a very specific way, governed not just by the temperature outside, but by the thermal memory of the rock itself.

The study revealed that this single-entrance cave operates like a two-lane highway for air, but only during the cold months. When the outside temperature drops below a specific threshold of about 7.5 degrees Celsius, dense, cold air from the surface rushes into the cave along the floor. Simultaneously, the warmer, lighter air inside the cave rises and escapes along the ceiling. This creates a counterflow, a bidirectional stream where cold air moves in at the bottom and warm air moves out at the top. The researchers observed that this exchange is not a simple on-off switch. When the outside air is cold enough to trigger this flow, the speed of the wind does not increase in a straight line as the temperature drops. Instead, the airflow speeds up quickly at first, then settles into a steady plateau where the wind speed stays relatively constant even as the outside air gets much colder. Only when the temperature difference becomes extreme does the wind pick up speed again.

This behavior is controlled by the cave's thermal inertia, a concept that describes how slowly the rock and sediments heat up or cool down. During the winter, the incoming cold air chills the floor and the lower walls of the cave. This cooling of the rock is crucial because it maintains the temperature difference needed to keep the air moving. Even if the outside temperature rises slightly above the threshold that usually stops the flow, the air continues to circulate for a time. This happens because the rock and the cave floor remain colder than the air above them, sustaining the buoyancy that drives the counterflow. The system only comes to a complete standstill once the entire mass of rock and sediment has warmed up enough to match the outside air, a process that can take days. The researchers noted that the air layers mix significantly near the middle of the passage, creating a turbulent zone where the cold floor air and warm ceiling air exchange heat, further influencing the flow.

The findings challenge the idea that cave ventilation is a simple reaction to the weather outside. Instead, the cave acts as a massive thermal battery, storing cold in its rock and releasing it to drive airflow long after the initial trigger has passed. The study showed that the strength of the wind depends on the history of the cave's temperature, not just the current weather. In the early part of the winter, when the rock is still relatively warm, the airflow is stronger for a given temperature difference than it is later in the season, when the rock has been thoroughly chilled. This suggests that the cave's internal climate is a complex dance between the immediate push of the outside air and the slow, lingering pull of the cooled stone. By mapping these patterns, the researchers have provided a clearer picture of how single-entrance caves breathe, revealing that the air inside is a product of a continuous, turbulent exchange between the atmosphere and the deep, slow-moving thermal mass of the earth.

Technical Summary: Climate in a Single-Entrance Cave Controlled by Buoyancy-Driven Counterflow and Thermal Inertia

Problem Statement
Karst caves represent unique subsurface environments where climate is governed by the complex interplay of advective heat transport, convective exchange at the rock–air interface, and conductive heat transport within the surrounding rock. While airflow is a primary driver of heat and CO₂ exchange between the underground and the atmosphere, the physical mechanisms controlling ventilation in caves with specific morphologies—particularly single-entrance systems—remain poorly understood. Unlike multi-entrance caves where the "chimney effect" drives symmetric ventilation, single-entrance caves require bidirectional counterflow (air entering and leaving through the same passage). The specific dynamics of buoyancy-driven counterflow, the role of thermal inertia in sustaining ventilation, and the non-linear relationship between buoyancy forcing and airflow velocity in these systems are not fully characterized.

Methodology
The study focuses on Košelevka Cave, a 130-meter-long, single-entrance cave in the Dinaric karst of Slovenia, characterized by a dominant down-sloping passage connected to chambers. The research employed a multi-year observational campaign (autumn 2021–spring 2023 and autumn 2025–winter 2026) utilizing:

Distributed Temperature Sensing: HOBO Mx 400 sensors placed at various locations along the floor, ceiling, and internal chambers, as well as at different vertical heights within the main passage to capture stratification.
Airflow Measurement: Ultrasonic anemometers (Gill) positioned at different heights (0.25 m and 0.75 m above the floor) to measure mean and maximum velocities at high temporal resolution (0.25–1 Hz).
Qualitative Flow Visualization: Smoke stick observations to determine flow direction and layering at specific elevations.
Environmental Monitoring: Comparison of cave data with external meteorological stations (Logatec and Zaplana) to correlate internal dynamics with external temperature ( $T_{out}$ ).
Data Analysis: Time-series analysis of temperature gradients ( $\Delta T$ ) and airflow velocities, calculation of dimensionless numbers (Grashof, Reynolds, and Interfacial Froude numbers), and examination of seasonal evolution patterns.

Key Results

Switch Temperature ( $T_{sw}$ ) and Ventilation Regimes: The cave exhibits a distinct "switch temperature" of approximately 7.5 °C. Ventilation is active only when external temperatures fall below this threshold. When $T_{out} > T_{sw}$ , airflow diminishes to near zero, and cave temperatures asymptotically approach equilibrium.
Bidirectional Counterflow: During cold periods, a stable counterflow is established: dense external air descends along the floor (downflow), while warmer, less dense cave air ascends along the ceiling (upflow). Smoke observations confirmed this stratification, with flow reversal occurring in the middle of the passage cross-section.
Non-Linear Airflow Response: Airflow velocity exhibits a non-monotonic, sigmoidal dependence on buoyancy forcing (temperature difference).
- Velocity Plateau: A distinct "velocity plateau" is observed where airflow increases only weakly despite increasing temperature differences ( $\Delta T$ ). This suggests a regime approaching internal hydraulic control, where additional buoyancy is accommodated by enhanced turbulent entrainment and changes in layer thickness rather than increased velocity.
- Seasonal Hysteresis: For the same temperature difference, airflow is stronger in the early cold season (autumn) than in late winter/spring. This indicates that the cooling of the surrounding rock and sediments over the winter season alters the system's response, allowing cold air to penetrate deeper with less warming.
Thermal Inertia and Sustained Flow: Ventilation is not solely driven by the instantaneous external temperature difference. The cooling of the surrounding rock and cave sediments during cold-air intrusions creates a thermal disequilibrium that sustains the buoyancy driving the counterflow even after external temperatures rise slightly. The system remains ventilated until the internal rock and air mass reach thermal equilibrium.
Turbulent Mixing: Vertical temperature profiles indicate significant turbulent mixing, particularly near the interface between the upflow and downflow layers (approximately 1 meter above the floor). The flow is characterized by high Reynolds numbers ( $Re \sim 10^4 - 10^5$ ) and Grashof numbers ( $Gr \sim 10^9 - 10^{10}$ ), confirming a fully turbulent, buoyancy-dominated exchange flow operating near internal hydraulic criticality (Interfacial Froude number $Fr_i \approx 0.8 - 1.0$ ).

Significance and Claims
The paper claims to provide a physically consistent interpretation of natural ventilation in single-entrance caves, demonstrating that these systems are controlled not only by the instantaneous temperature difference between the cave and the atmosphere but significantly by the thermal inertia of the surrounding rock and sediment mass.

Key contributions include:

Mechanism Elucidation: The identification of thermal inertia as a critical factor that prolongs ventilation and alters airflow response to subsequent cooling events, distinguishing single-entrance cave dynamics from simple chimney-effect models.
Hydraulic Control: The proposal that the observed "velocity plateau" reflects a regime approaching internal hydraulic control, where the flow is constrained by the geometry and mixing dynamics rather than solely by the driving buoyancy.
Coupled Dynamics: The demonstration that airflow, heat transfer, and buoyancy are dynamically coupled in these systems, with the density field evolving continuously due to heat exchange between air, rock, and sediments.

The authors conclude that while the study is limited to a qualitative description of observations in a specific cave, it highlights the richness of processes in confined subsurface environments. They posit that caves serve as natural laboratories for studying the interaction between buoyancy-driven flow, turbulent mixing, and heat transport, and that these findings may apply to other poorly documented karst systems with similar morphologies. The paper does not propose specific new applications but suggests that a quantitative model capable of reproducing these dynamics is a necessary future step.

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