An igloo, a shelter built from frozen water, can safely contain a heat source, like a lamp or a small fire, without immediately melting into a puddle. This snow house functions not despite its material but because of it. The survival of the structure relies on a precise balance of physics, material science, and design. The secret lies in the unique properties of compacted snow, the geometry of the dome, and a deliberate process of internal self-glazing that protects the walls from the heat.
The Insulating Power of Snow
The reason an igloo does not melt quickly is the insulating quality of the snow itself. Igloos are constructed from blocks of hard, wind-compressed snow, which is primarily composed of air. Snow is not solid ice; it is a matrix of ice crystals that trap an immense volume of air, often up to 95% of its total volume. Air is a poor conductor of heat, and these trapped air pockets drastically lower the snow’s thermal conductivity. The snow walls act like a thick, porous blanket, slowing the transfer of heat from the interior to the frigid exterior environment. This thermal barrier is effective enough that the interior temperature can be maintained well above freezing in the upper dome.
Heat Dynamics and Dome Architecture
The geometry of the igloo’s dome is not merely structural but also thermally functional, managing the movement of warm air inside. The half-sphere shape is structurally sound, distributing weight evenly and efficiently, and is ideally suited for controlling convection. Hot air, which is less dense, naturally rises toward the ceiling where it pools, creating a thermal stratification within the shelter. This keeps the hottest air concentrated at the top of the dome, away from the lower walls and the occupants. The dome’s design also allows cold, dense air to sink toward the floor and collect in a dug-out area near the entrance known as a “cold sink.” This tiered interior space ensures the living area remains in the warmer air layer, preventing the cold air from mixing with the main volume of heated air.
Sintering: The Inner Layer
When an igloo is first occupied and a heat source is introduced, a process of surface hardening begins. The internal heat, whether from a lamp, fire, or body warmth, causes the innermost layer of snow crystals to partially melt. This meltwater, now denser than the surrounding snow, quickly refreezes due to the thermal gradient and the cold mass of the wall behind it. This melt-refreeze cycle, often called sintering or glazing, creates a thin, dense, protective layer of ice on the interior surface. This ice glaze acts as a structural reinforcement and moisture barrier, preventing the deeper snow from becoming saturated and eroding. The hardened inner shell resists further melting and strengthens the entire structure, allowing it to withstand prolonged occupation.
Managing Internal Temperature and Ventilation
The presence of a heat source inside an igloo requires careful management to ensure both safety and structural integrity. Traditional Arctic inhabitants often used a qulliq, a small oil lamp, rather than a large open fire, as the primary source of supplemental heat. These lamps provide a consistent, contained flame that is easier to regulate than a bonfire. Ventilation is necessary to manage the byproducts of combustion and human occupancy. A small hole is deliberately placed in the ceiling to allow for the escape of smoke, excess heat, and humid air. This ventilation also prevents the dangerous buildup of carbon monoxide gas from the burning lamp. By allowing hot, moist air to vent, the system maintains the thermal balance, preventing the inner surface from overheating or causing structural collapse.