airfoils have been built to enhance these pressure gradients. The shape of the traditional roof also creates suction as wind blows over it. Convection Buoyancy is usually not the main effect driving windcatcher air circulation during the day. In a windless environment, a windcatcher can still function using the stack effect. The hot air, which is less dense, tends to travel upwards and escape out the top of the house via the windtower. Heating of the windtower itself can heat the air inside (making it a solar chimney), so that it rises and pulls air out of the top of the house, creating a draft. This effect can be enhanced with a heat source at the bottom of the windtower (such as humans, ~80 Watts each), but this heats the house and makes it less comfortable. A more practical technique is to cool the air as it flows down and in, using heat reservoirs and/or evaporative cooling. A takhtabush is a space similar to the ancient Roman tablinum, opening both onto a heavily shaded courtyard and onto a rear garden court (the garden side being shaded with a lattice). It is designed to capture a cross-draft. The breeze is at least partly driven by convection (since one court will generally be warmer than the other), and may also be driven by wind pressure and evaporative cooling, so the garden and courtyard are used as windcatchers. Buoyancy forces are used to cause night flushing. Night flushing (colder air) The diurnal temperature cycle means that the night air is colder than the daytime air; in arid climates, much colder. This creates appreciable buoyancy forces. Buildings may be designed to spontaneously increase ventilation at night. Courtyards in hot climates fill with cold air at night. This cold air then flows from the courtyard into adjacent rooms. The cold night air will flow in easily, as it is more dense than the rising warm air it is displacing. But in the day, the courtyard walls and awning shade it, while the air outside is heated by the sun. The cool masonry will also chill the nearby air. The courtyard air will become stably stratified, the hot air floating on top of the cold air with little mixing. The fact that the openings are at the top will trap the cool air below, though it cannot cause the temperature to drop below the nightly minimum