Snow igloos are built in the shape of a catenoid, which offers optimal ratios between the height and diameter of the structure to eliminate the structural tension which could otherwise cause it to implode or bulge. The stresses of snow as it ages and compresses against the igloo will not cause it to buckle because in an inverted paraboloid or catenoid the pressures are exclusively compressive.[10]
This design originates from the Central Inuit.[10] In applied mechanics, the equation for this type of structure is written y = a(cosh x/a- 1) where y is the height to any point in the surface, x is the horizontal distance to the same point, and a is a constant.[10]
If the walls are of uniform thickness and density, the maximum compressive stress at the base of a paraboloid is mathrm{S}alpha=frac{gammamathrm{d}^{2}}{24h}cdot frac{1+cosalpha+cos^{2}alpha}{(1+cosalpha)cos^{2}alpha} where d is the diameter at the base, h the height, y the unit weight of the snow, and a = arc tan 4h/d.[11]
Since stress is a force per unit area, if the walls are of uniform thickness the compressive stress is independent of wall thickness; thicker walls provide better insulation but do not strengthen the structure because of added weight.[12]
The maximum compressive stress at the base of the igloo can be obtained by multiplying S,/yd times the snow unit weight y and the mean igloo base diameter.
Igloos gradually become shorter with time due to the compressive creep of the snow.[10]