Day 0Outside 0 °CCave air 0 °CStack ΔP 0 PaFlow 0 m³/sstillFPS 0

Drag to orbit, wheel or pinch to zoom. Streaks show the air moving through the cave (cool blue to warm orange by air temperature). Warm cave air rises in winter and leaves by the upper entrance; in summer the cold, dense cave air sinks out of the lower entrance and the flow reverses. The glowing ring marks the squeeze.

Cave Chimney Effect: Stack Airflow Between Two Entrances

A cave with a low and a high entrance breathes by itself: the temperature difference between the cave air and the outside sets up a pressure difference, and air flows through the chambers and squeezes. Change the weather, the height, the squeeze and the cave volume, or let the seasons run. Schematic cut-away, not a real cave survey.

Weather

Cave

Time and display

Live readouts

Air speed in each part of the path

Continuity: the same volume flow Q passes every section, so the air speed is Q / A, high in the squeeze and slow in the big rooms.

Flow vs outside temperature

Temperature history

Independent numerical checks

Controls

Drag / one finger: orbit; wheel / pinch: zoom.
Outside temperature is a slider; switch on Seasons to let a one-year cycle with day-night swings drive it (the slider then follows).
Squeeze area narrows or opens the tightest passage; the ring glows brighter when the air there is faster.
Pressure trend adds the separate barometric breathing readout (not drawn).
Quality: High is the default on every device; Low (also ?q=low) uses fewer air streaks and no mist.

The science

Stack (chimney) effect. Air is an ideal gas, ρ = p / (R T). Two entrances at heights differing by H are joined by the cave. In hydrostatic equilibrium the pressure at the top is p(H) = p₀ exp(−g H / (R T)) for each air column, so the pressure the cave air exerts at the upper entrance minus the outside pressure there is ΔP = p₀ [ e^{−gH/(R T_in)} − e^{−gH/(R T_out)} ] ≈ (ρ_out − ρ_in) g H. Warmer, lighter cave air (winter) gives ΔP > 0 and the flow leaves at the top; colder, denser cave air (summer) reverses it. At T_out = T_in the flow stops.

Flow through the path. The flow passes through sections in series (entrance, rooms, squeeze, climb, shaft), each with a loss coefficient K_i and cross-section A_i: ΔP = ½ ρ Q² Σ K_i / A_i², so Q ∝ √ΔP, and the speed in section i is v_i = Q / A_i.

The cave warms and cools. Cave-air temperature relaxes toward the rock temperature with a wall time constant τ and is replaced by outside air at the rate Q/V: dT_in/dt = (Q/V)(T_out − T_in) + (T_rock − T_in)/τ. A strong winter draught therefore cools the cave and weakens itself (negative feedback).

Barometric breathing. Independently, a falling outside pressure makes a closed cave of volume V exhale Q = −(V/p) dp/dt.

Simplifications and model values. Steady flow in a one-dimensional path, isothermal air columns, dry air, no humidity or CO₂, no wind pressure on the entrances, lumped cave-air temperature, and round-number loss coefficients, section areas, volume and wall time constant. The drawing is schematic and its scale is set by the height slider. Mist at the exit is a visual hint, not a computed cloud. Models built with Blender, scene by Three.js.