temperature. This mechanism also works in windtowers. Subterranean cooling A windcatcher can also cool air by bringing into contact with cool thermal masses. These are often found underground. Below approximately 6m of depth, soil and groundwater is always at about the annual mean-average temperature (MATT) (it is this depth which is used for many ground-source heat pumps, often loosely referred to as "geothermal heat pumps" by laypeople). The thermal inertia of the soil evens out the daily and even annual temperature swings. In arid climates, the daily temperature swings are often extreme, with desert temperatures often dipping below freezing at night. Even the thermal inertia of thick masonry walls will keep a building warmer at night and cooler during the day; in hot-arid climates, thick walls with high thermal mass (adobe, stone, brick) are common (though thinner walls with high resistance against heat transmission are more modernly sometimes used). Windcatchers can thus cool by drawing air over night- or winter-cooled materials, which act as heat reservoirs. Windcatchers are also often used to ventilate lower-level indoor spaces (e.g. shabestans), which maintain frigid temperatures in the middle of the day even without windcatchers. Ice houses are traditionally used to store water frozen overnight in desert areas, or over winter in temperate areas. They may use windcatchers to circulate air into an underground or semi-underground chamber, evaporatively cooling the ice so that it melts only slowly and stays fairly dry (see lede image). At night, the windcatchers may even bring sub-freezing night air underground, helping to freeze ice. Evaporative cooling In dry climates, the evaporative cooling effect may be used by placing water at the air intake, such that the draft draws air over water and then into the house. For this reason, it is sometimes said that the fountain, in the architecture of hot, arid climates, is like the fireplace in the architecture of cold climates. Windcatchers are used for evaporative cooling in combination with a qanat, or underground canal (which also makes use of the subterranean heat reservoir described above). In this method, the open side of the tower faces away from the direction of the prevailing wind (the tower's orientation may be adjusted by directional ports at the top). When only the leeward side is left open, air is drawn upwards using the Coandă effect. This pulls air into an intake on the other side of the building. The hot