UNDERGROUND
The cool of the ground
A few metres down, the temperature of the ground swings far less than that of the air, and with a lag of one or two months. It is the third lever of cooling, and in this climate it is more effective than night ventilation.
The figure, with its uncertainty
Undisturbed ground converges on the annual mean of the air — in Ragusa about sixteen and a half degrees — only below ten metres. At two or three metres the seasonal swing is attenuated but still present and shifted by one or two months: in August one expects values of the order of eighteen to twenty-one degrees, rising as depth decreases.
It is a model calculation and not a measurement on site. Since the figure is load-bearing for the sizing of the duct, it must be measured on the spot before the work is carried out, and the measurement is one of the checks provided for in the current phase.
The buried duct
The design provides for one hundred and fifty linear metres of buried duct that takes air from the north garden, runs it underground and delivers it to the rooms. The air gives up heat to the ground along the way and arrives cooler than it entered, with electricity consumption limited to the fan that moves it.
Two conditions decide whether a device of this kind works or becomes a health problem: a continuous fall with collection and discharge of condensate, and accessibility for inspection and cleaning. Both must be resolved in the detailed design, and a duct that does not resolve them is better not built.
What the Sicilian qanats teach, and what they do not
The plain of Palermo preserves a network of draining galleries of the qanat type. The oldest documented, at Danisinni and the Scibene, date from the twelfth and thirteenth centuries; others, such as the Gesuitico Alto, were dug between the sixteenth and the nineteenth. Beside the qanats the local tradition distinguishes the ingruttati, tunnels connecting wells, and the catusi, terracotta pipes.
Grafted onto the same network are the so-called scirocco chambers: vaulted underground rooms, cooled by the passage of running water in a channel, sometimes provided with skylights. Here a correction is needed that is rarely read. The type spreads in the villas of the nobility between the sixteenth and the eighteenth centuries — the oldest documented is of 1552 — reusing a hydraulic infrastructure of medieval origin. They are not an invention of the Islamic period, and whoever presents them as such is five centuries early. The exception is the case of the underground rooms of the palace of the Uscibene, of the Arab-Norman period, where a cave traversed by the water of the spring was used for the same purpose.
What these works teach, and what is worth taking up, is the combined principle: not air cooled by the ground alone, but air cooled by the ground and at the same time by the evaporation of running water along the way. The duct of the north garden can receive that addition at modest cost, and it is the point where the Sicilian precedent becomes a technical prescription instead of a quotation.
Cold kept
On the Iblei and on Etna cold was kept in neviere, pits and buried domes where the winter snow, covered with ferns and leaves, was preserved until summer: a trade documented between the seventeenth and the nineteenth centuries, with exports as far as Malta. It is not an institution of the Islamic period and is cited here for one reason only: in the same area, with the same materials, someone has already solved the problem of keeping an underground room cool for months, and his solutions can still be observed.
NOTE ON SOURCES
The dating of the Palermo qanats follows P. Todaro, G. Barbera, A. Castrorao Barba and G. Bazan, Qanats and historical irrigated landscapes in Palermo’s suburban area, in «Post-Classical Archaeologies» 10 (2020). For the scirocco chambers, E. and M. Saeli, La Camera dello Scirocco di Villa Naselli di Gela a Palermo (2022). The ground temperature values are derived by model from the climate data of Ragusa and must be confirmed by measurement on site.