2D Sky Lanterns — Buoyancy Simulator

Flame

Flame heat 65%

Burn duration 18 s

Lantern

Lantern size 1.0x

Weather

Wind speed 1.0x

Click the pad or press Launch to light a lantern.

Focus Lantern — Live Readout

Internal air temp

— °C

Lift force

— N

Altitude

0 m

In flight

How it works

A sky lantern is a small hot-air balloon: a paper (or fabric) envelope open at the bottom over a small flame. The flame heats the air trapped inside the envelope. Hot air is less dense than the cool night air around it, so the envelope experiences a net upward buoyant force — exactly the principle Archimedes described for fluids, applied here to a "fluid" of air.

The lift force follows F = (ρ_ambient − ρ_hot) × V × g, where ρ is density, V is the envelope's volume and g is gravity. Density falls as temperature rises (ideal gas law: ρ ∝ 1/T at constant pressure), so a hotter flame gives a bigger density gap and more lift. The lantern only climbs once that lift exceeds the combined weight of its paper envelope, bamboo frame and the hot air itself.

Bigger lanterns benefit from a cube-square advantage: volume (and therefore lift) grows with the cube of size, while the paper and frame mass grows roughly with the square — so a larger lantern needs proportionally less heat to fly. Once the flame burns out, the trapped air cools back toward ambient, density recovers, lift collapses below weight, and the lantern sinks.

Click the launch pad to release a lantern

Sky Lanterns Buoyancy Simulator (2D)