In the arid deserts of ancient Persia, temperatures frequently soar past 40 degrees Celsius, yet ancient inhabitants regularly enjoyed frozen desserts and cold drinks during the peak of summer. This feat was accomplished through a clever synergy between subterranean aqueducts called qanats and massive, cone-shaped ice houses known as yakhchals. Qanats are gently sloped underground channels that transport snowmelt and groundwater over vast distances without losing water to evaporation.
When connected to a yakhchal, the qanat provides a continuous flow of cool water into channels situated on the north side of the ice house wall. On clear winter nights, desert radiative cooling causes heat to radiate out into the clear night sky, freezing the shallow water even when ambient air temperatures remain slightly above freezing. The harvested ice was then stored inside the massive underground pit of the yakhchal.
The architectural geometry of the yakhchal's above-ground mud-brick dome was crucial for passive cooling. Built with thick walls made from heat-resistant mortar composed of sand, clay, egg whites, goat hair, and lime, the dome acted as a powerful thermal insulator. Wind catchers funneled cool air from the subterranean qanat into the storage pit while pulling warm air up and out through an aperture at the top of the dome, creating a constant evaporative cooling loop that kept ice frozen through the hottest months.