TEHRAN– Nothing says summer like a glass of slushy lemonade. Thousands of years ago, ancient Iranians used a clever physical method to keep their drinks cool in hot, arid climates.

It may sound a bit far-fetched that ancient Persians saved ice in the middle of the desert as a smart way to cool off and store food during the summer months. In fact, they stumbled across a neat bit of physics that allowed them to create ice from water even in scorching summers.

This way of preserving ice shows people's ability to find solutions to problems with whatever material or technology they have at their disposal. It was then that the Yakhchals were found to be of great help.

The word Yakhchal stands for “ice pit.” These oddly shaped structures provided both space and conditions to store not just ice, but many types of food that would otherwise quickly spoil due to high temperatures.

On the outside, the mud-brick structure of a Yakhchal dominates the skyline with its domed shape, and on the inside, it would typically integrate an evaporation cooler system that allows ice to stay cool or even frozen while stored in underground rooms.

A typical Yakhchal would rise some 15 meters or so, and on the inside, it would contain vast spaces for storage. The evaporative cooling system in the facilities functioned as wind traps and water was supplied via qanat from nearby wells, common culvert systems in the region designed to transport water through communities and various structures.

Evaporative cooling effortlessly lowered the temperatures inside the Yakhchal, giving a cool feeling that you are actually standing in a full-size fridge.

The walls are constructed intelligently using special mortar with components such as egg whites and goat hair that provide marvelous insulation and protection from the hot desert sun.

A closer look at its mechanism reveals quite some interesting facts. At night, the Earth stays warm through the famous greenhouse effect, in which gases in the atmosphere help trap the Sun’s warmth. But on clear nights with low humidity, this effect is weaker, and objects can radiate their heat directly into space–a process called radiative cooling. The Persians found that this allowed thin layers of water in purpose-built trenches to drop to a low enough temperature to freeze –even after hot days in the desert.

Most Yakhchals have an underground square-shaped containment area, which is surmounted by a dome-shaped structure. Such trenches at the bottom were designed to collect any water coming from molten ice. Once collected, that amount of water was then refrozen during nighttime, making maximum use of the resource as well as the cold desert night temperatures. It is a repetitive process.

The water collection area had to be deep enough to stay cool, and the material from which the Yakhchal was made had to be insulating enough to keep the heat out.

Water was brought to the Yakhchal either by transporting ice directly from nearby mountains or by diverting water from an aqueduct into the Yakhchal via underground water channels called qanats.

To make lower temperatures inside, particularly during the middle of the day, they made an east-west oriented wall adjacent to some Yakhchals on the south side of the refrigerator.

Another trick used to keep the Yakhchal cool is a badgir, a type of wind-catching mechanism that catches the breeze and directs it toward the Yakhchal.

When the air came down, it was cooled by the ice and fresh air that accompanied the water in the qanat. Otherwise, badgir could be used to increase hot air and cold air to replace it. This mechanism is still used in many desert cities in modern Iran.

Once the water was frozen, it was cut into blocks to allow the water to be easily carried out of the Yakhchal.

The use of Yakhchal has ceased in modern times, and although some structures have been damaged and eroded by desert storms, many are still intact in Iran and some of its neighboring countries, as far as Tajikistan.

Over the past couple of years, tens of abandoned Yakhchals have been restored this time as travel destinations with the help of Iran’s Ministry of Cultural Heritage, Tourism, and Handicraft with the close collaboration of the local people, travel insiders, and devotees of cultural heritage.

AM