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6. The sun factor in air movement
Air
movement by convection
The
courtyard house
The
takhtabush
Traditional city layout and climate
Under some circumstances, judicious architectural designs can be used to take advantage of the sun factor as a driving force for maintaining air movement. Generally, this technique is applied where large areas are available and is based on the principle of convection.
Warm air is less dense than cool air and therefore will rise in an environment of cool air. This movement is called convection and can lead to the phenomenon called the stack effect. As the warm air rises, it must be replaced by cooler air from the surroundings. If a heat source exists below the initial pocket of warm air, the cooler air replacing it will also be warmed and will rise. Using a continuous heat source, a steady flow of air is generated. In vernacular architecture, this effect has been exploited to produce small areas with cool breezes, using the ground heated by the sun as the heat source. As long as a large volume of cooler air is available and is unaffected by heat from the sun, the hotter the sun heats the ground, the stronger will be the breeze.
The relatively static cooling system used in a courtyard house can provide the basis for understanding modifications that can generate air movement by convection. In hot dry zones, air temperature drops considerably after sunset from re-radiation to the night sky. The air is relatively free of water vapor that would reflect the heat or infrared radiation back toward the ground, as occurs in warm humid regions.
To enhance thermal comfort, this phenomenon has been used in the architectural design of houses by employing the courtyard concept.
Nature is hostile at ground level in these zones, especially in the deserts. People learned to close their houses to the outside and open them inwardly onto internal courtyards called sahn, which are open to the sky. This arrangement provides drops in air temperature of 10-20 C° (18-36 F°) at night. This might explain why the lunar crescent as a symbol of the night sky is so meaningful to Arab people and ultimately to all Muslims, to the point of appearing on the flags of eight predominantly Muslim nations.
As evening advances, the warm air of the courtyard, which was heated directly by the sun and indirectly by the warm buildings, rises and is gradually replaced by the already cooled night air from above. This cool air accumulates in the courtyard in laminar layers and seeps into the surrounding rooms, cooling them. In the morning, the air of the courtyard, which is shaded by its four walls, and the surrounding rooms heat slowly and remain cool until late in the day when the sun shines directly into the courtyard. The warm wind passing above the house during the day does not enter the courtyard but merely creates eddies inside, unless baffles have been installed to deflect the airflow. In this way, the courtyard serves as a reservoir of coolness. The courtyard concept is universally applied in the traditional architecture of countries in hot arid regions stretching from Iran in the East to the shores of the Atlantic Ocean in the West, and in both rural and urban housing design. Examples from Egypt, Tunisia, and Iraq are shown in figures 69, 70, and 71, respectively. Figure 72 shows a view of the courtyard of the AsSuhaymi house in Cairo, illustrating the pleasant atmosphere that can be created within the courtyard and the arrangement of the surrounding spaces, some with mashrabiya-filled openings.
Modifications of the courtyard concept have been developed to ensure a steady flow of air by convection. The vernacular architecture of the Arab house includes an element called the takhtabush, a type of loggia. This is a covered outdoor sitting area at ground level, located between the courtyard and the back garden, opening completely onto the courtyard and through a mashrabiya onto the back garden. Since the back garden is larger and thus less shaded than the courtyard, air heats up more readily there than in the courtyard. The heated air rising in the back garden draws cool air from the courtyard through the takhtabush, creating a cool draft, as in the As-Suhaymi house and the Qã'a of Muhib Ash-Shãf'i Al-Muwaqqi in Cairo, shown in figures 73 and 74. A similar arrangement can be found in the tablinum of the ancient Roman villas of Pompeii.
This concept can be used in the town plan of a village or a residential sector from which automobile traffic is excluded, to provide a cool and agreeable meeting place for the inhabitants. In this case, the takhtabush can be set between two squares, one larger than the other. The larger square is on the leeward side to help in creating drafts by pressure differential. This design is illustrated in the village of Bans, Egypt, shown in figure 75.
The people of a village or a residential quarter often gather in certain agreeable places, in addition to parks, which were created unintentionally by the configuration of the buildings. Some of these places are well oriented to receive sunlight and are protected against wind, places elderly people would choose in the winter. Other places are shaded from the sun, have elements like the takhtabush to produce drafts, and are sought in the summer. It is important that the architect note this need and, based on a scientific understanding of the situation, consciously create agreeable public places that reintroduce human scale and aesthetics to townscapes.
Traditional city layout and climate
As climate is a dominant factor in traditional town planning, a marked uniformity in urbanization is found in all hot arid zones. The layouts of almost all traditional cities in the area are characterized by two features: narrow winding streets, and large open courtyards and internal gardens.
Typically, large courtyards serving as reservoirs of cool, fresh air dominate a city plan, as seen in examples from Marrakech, Morocco; Tunis, Tunisia; and Damascus, Syria; shown in figures 76, 77, and 78, respectively. At first sight, this arrangement appears far superior to the gridiron layout with wide boulevards of Washington, D.C., shown in figure 79, that is often held up as a model for city planning, even in hot arid climates. The narrow meandering streets with closed vistas perform the same function as a courtyard. They retain any cool air that may be deposited during the night from being swept out by the first puff of wind as would occur in a gridiron plan with wide boulevards. But to objectively judge this matter, a comprehensive comparison between the two design concepts is required, based on measurements of the open courtyards, internal gardens, and external streets and squares, and their corresponding air quality and temperatures.
While traditional layouts may not be designed to accommodate motor traffic, solutions to this problem exist. One alternative is to encircle the residential quarter with a ring road for cars, with cul-desac streets branching off into the interior as suggested by the Radburn Plan. Another solution is the Dynopolis concept launched by Doxiadis, which assumes that the characteristic traditional layout can be preserved within the quarter.
With regard to a gridiron town plan, buildings crowded in the city center affect wind movement in that quarter, creating eddies and lowering the wind velocity by friction and change of direction. Research in the Federal Republic of Germany showed that mean wind speed dropped from 5.1 to 3.1 m/s (16.7 to 10 ft/s) in one German town as it grew in size and expanded. In Detroit, Michigan, the wind speed dropped from 6.5 to 3.8 m/s (21.3 to 12.5 ft/s) over a period of 20 years. And in Stuttgart, Federal Republic of Germany, the number of days in which the wind was stagnant increased from 1% in 1894 to 20% in 1923. It can be concluded that, when buildings are crowded into a small area, the wind velocity decreases markedly. Wind above the city is affected by three factors: (1) high winds, (2) microclimatic winds which are affected by the topography and the configuration of the city, and (3) the wind movement created by the city itself.
As the solar warming process is greatest at the center of a city, the hot air of this sector rises by convection and is replaced by air from the other quarters. When the city plan is a gridiron pattern with wide, straight streets, hot air laden with dust and fumes from automobile traffic collects from the surrounding quarters and from the industrial areas and forms a dome of polluted air above the city center. This phenomenon can be seen at night by observing the reflection of city lights on floating particles of dust suspended in the air, which take on the colors of advertisement illumination. However, if the architect must adopt a gridiron street pattern with wide avenues, then sufficient green areas should be spread over the geographical area in order to redistribute the heat evenly within the city and avoid its concentration in the center.