Sky Lanterns: The Physics of Buoyancy

What Are Sky Lanterns?

Sky lanterns, also known as Kongming lanterns or floating lanterns, are lightweight paper structures that can rise into the air when heated. They have been used for centuries in various cultural and military contexts.

The key to their flight lies in the difference between the density of hot air inside the lantern and the cooler ambient air outside.

How Do Sky Lanterns Fly?

When a flame is placed under the paper lantern, it heats up the enclosed air. As the air warms, its density decreases, making it lighter than the surrounding cool air.

This creates an upward buoyant force that can lift the lantern if it exceeds the weight of the lantern itself.

live demo · related simulation● LIVE

Buoyancy and Density

The principle behind sky lantern flight is based on Archimedes' principle, which states that a body immersed in a fluid experiences an upward buoyant force equal to the weight of the displaced fluid.

In this case, the displaced fluid is air, and the buoyant force must be greater than the lantern's weight for it to rise.

Factors Affecting Flight

Several factors can influence whether a sky lantern will fly. These include the temperature difference between the heated air inside the lantern and the ambient air, the size of the lantern (which affects its surface area and thus the buoyant force), and wind speed.

A larger flame or longer burn duration increases the temperature difference, potentially increasing lift.

Frequently asked questions

Why do sky lanterns use paper?

Paper is lightweight and flexible, allowing it to form a thin, strong envelope that can hold heated air. It also burns easily, making it convenient for the flame source.

Can sky lanterns be used in any weather conditions?

Sky lanterns are typically launched on calm nights with little wind to ensure they rise and do not drift uncontrollably. Strong winds can cause them to land unpredictably or even crash into structures.

Are sky lanterns environmentally friendly?

While traditional paper lanterns may pose a fire hazard, modern versions are often made from biodegradable materials and launched in controlled environments to minimize environmental impact. However, they can still cause littering issues if not properly disposed of.

How do sky lanterns help in military operations?

Historically, sky lanterns were used for signaling and communication purposes during the night. They could be launched to indicate positions or send messages over long distances, making them valuable tools in ancient warfare.

Try it live

Everything above runs in your browser — open Sky Lanterns Buoyancy Simulator (2D) and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.

▶ Open Sky Lanterns Buoyancy Simulator (2D) simulation